text:
Challenge Help
This page contains help and information on building model solar cars and boats as well as lists some common suppliers of car and boat gear. More information will be added as it becomes available.
For further help of any kind please refer to the website's CONTACT page and the TMSC will be more than happy to answer any challenge related questions (Tasmanian or national).
Model Solar Car Help, 2010
Model Solar Car Design Overview
Car Design Hints, 2011
Model Solar Car Simulator
Model Solar Car Book
Model Solar Boat Help
Solar Panel Anomalies
Low Shunt Resistance Solar Cells
The Physics of Solar Cells
Solar Panel Cracking
Motor datasheets
Faulhaber 2224
Faulhaber 2232
Faulhaber 2233
Maxon RE-max 21
Maxon RE-max 24
Maxon RE 25
Solar Freaks Forum
Solar Freaks
This site has been up and running for the last few years now and is a great place to discuss and seek help with regards to the solar challenge.
Past Example Cars
Some of these have been amongst the fastest cars ever seen at state or national level.
2008 AIMSCC Demo Car Autocad Files
This car was run around the track on several occasions at the national event in Hobart in 2008 and in that time set an unofficial 15.75s lap record in 107% sun. The fastest lap by a car competing at the event that year was recorded at 16.18s in 122% sun.
2009 Test Car
This car was built to comply with the 2009 regulations and has been run at a number of Tas state and national events since then as an example car. It ranks as being one of the fastest cars ever built and has clocked times of 16.11s in 91% sun and 16.43s in 79.5% sun in demo runs at the 2009 nationals in Melbourne. 2 of the videos below show it in action against the eventual 5th placegetter from Girrawheen SHS that year and the St Paul's car which ought to have finished in the top 4.
In 2010 it also recorded a 15.51 second lap time in 108% sun at the nationals in Fremantle. An onboard video clip of this lap can be seen below amongst the example car videos.
2010 Demo Lego Sprint Car
A member of the committee constructed and raced a car made up entirely of Lego and Lego Technic (excluding solar panel, wiring, switch and motor) to help demonstrate how simple it can be to build a working car. Some photos of the car can be seen below.
Example car videos
(L-R: 2008 demo car, 2009 test car, 2010 Lego Sprint Car, 2009 test car (on board))
Suppliers of Components and Materials
Solar Challenge Catalogues
Below are a few catalogues from some of the more popular suppliers of parts and materials for the model solar car and boat challenges.
2012 Scorpio Solar Catalogue
2012 Scorpio Technology Catalogue
CAM Art, Craft and Technology - Old Solar Catalogue
RI Gear Solar Car Price List
While most challenge-specific items from Scorpio Technology are listed in their Solar Catalogue, there is also some relevant gear included in their Technnology Catalogue.
For example, the electric motors in the Technnology Catalogue are all priced at less than $5 and so are well suited for use in the junior boat and sprint car events.
The catalogue also includes a selection of low cost encapsulated solar modules. These are in a similar encapsulation style to the Engelec modules that John Jeffery once sold, although they appear to be a little less susceptible to the cell-cracking that has been discussed in the "Solar Panel Cracking" pdf listed further up the page.
Of these, the cheapest option is the $6.88 Panel #8 (1.4W @ 2V) where 7 of these (around $50) are suitable for a car (makes a 14V Vmp panel). This should produce almost 10W (providing the 1.4W that has been stated is correct) and is around a third of the cost of 2 of the Scorpio #29 car or #26 boat panels.
Although the voltage would be getting a little low for cars running with electronics, 6 x #8 Panels (12V) could even be used on cars opting to compete without any at an event. On the other hand, 8 x #8 Panels could be used to increase panel voltage to 16V and cells then masked over to limit the power to 10W.
As can be seen in their old solar catalogue above, CAM Art, Craft & Technology appear to have been selling the same 4-cell Engelec modules that the TMSC's John Jeffery once did. The supplier has however recently removed the item from their online shopping system and this suggests that the modules may now no longer be available.
CAM also sell a range of other gear including a selection of solar motors as well as components from RI Gear (although these look to be more expensive than ordering directly from RI).
While inexpensive hobby motors and motors recovered from VCRs can be used to successfully power a model solar car or boat, their performance is inferior to the high quality motors used in the most competitive vehicles.
Two such high quality motor manufacturers are Faulhaber and Maxon which are based in Germany and Switzerland.
Since first being released by Faulhaber in 2005, virtually all car teams have been using the Faulhaber 2232 6V motor and it is the TMSC's recommended motor of choice. This motor is similar to the previously used 2233 Faulhaber model but is fitted with rare earth magnets and has a lower armature resistance. The 2232 is also superior to the previously used 2224 model.
The datasheets of these three motors can be downloaded from further up the page and the full list of Faulhaber's product range can be viewed on the company's website.
30-31 out of the 32 national finalists have used the Faulhaber 2232 6V motor at the last three Australian-International Model Solar Car Challenges. The odd cars out typically used a Maxon 118740 motor. This motor has some similar characteristics to the 2232, but is over twice the weight and cost and has yet to prove itself on the race track. It however has a very low terminal resistance and simulations based on real world dynamometer testing have shown that it could potentially outperform the 2232 in high sunlight conditions. The datasheet of this motor is also downloadable above (RE 25).
A couple of other Maxon datasheets are also listed. The (RE-max 24) datasheet contains specifications on the 220404 9V maxon motor which actually has a max efficiency that is 2% higher than that of the 2232. This motor however also possesses some other slightly less desirable characteristics for model solar car racing and this is the reason why the 2232 is still the motor of choice. This being said, the 220404 is still largely unproven out on the track and dynomometer testing has shown there to be very little difference between it and the Faulhaber.
The remaining Maxon datasheet (RE-max 21) is of the motor (221011) that was used by the winner and third place at the 2005 national car event (technically the 221024 through-shaft version was used but they are essentially the same motor). Like the Faulhaber 2224 & 2233 motors, this motor is now also considered to be inferior to the 2232.
(L-R: Faulhaber 2233 4.5V, Faulhaber 2232 6V, Maxon 220404 9V, Maxon 221011 9V)
After 2005, the top 4 cars in 2006 - 2011 have all used the Faulhaber 2232. The 2232 has also been commonly used by boats competing in the secondary/senior boat division where motors costing more than $5 RRP are permitted.
Fauhaber motors are distributed to specialised resellers across Australia such as Scorpio Technology or CAM Art, Craft and Technology by Erntec Australia but can also be purchased from Erntec directly.
They have also recently become available at SolarMPPT.com which is run by Mr. Tony Bazouni, designer and destributor of the Easymax and Automax electronic units used by many car challenge competitors. Mr. Bazouni ordered in a small number of Faulhaber 2232's directly from a supplier in Germany late last year as a trial run and is again looking at this option for in 2012. Buying from Germany in bulk could see individual motor costs to teams drop to around $80-$90 (including postage) and this would be almost half the price of what places like Scorpio, Erntec and CAM Art, Craft & Tech have been selling them for of late. As such, SolarMPPT.com would be the TMSC's recommended supplier of 2232 motors in 2012 (once they become available).
Maxon motors are distributed across Australia by Maxon Motor Australia and contact details as well as the company's complete product range can be found on the Maxon website.
Motors Continued - Advanced Boats and Sprint Cars
As mentioned a little earlier on in this section on motors, the 2232 6V is also commonly seen in the secondary/senior boat division where there is no limit on motor cost. This motor is well-matched to the output of a light weight Scorpio boat panel (see solar panel section) and will allow for near maximum power transfer between the solar panel and motor shaft if the boat has been set up correctly. The characteristics of the motor also permit a direct-drive connection between the motor and propeller shafts to be used.
For optimal performance, the correct solar panel configuration (series or parallel) and propeller (pitch and/or size) must then simply be selected to best suit the prevailing weather conditions. Knowing which setup to use and when will largely be based on findings and results obtained from boat testing that the team has carried out. As such, more testing will generally result in better boat performance.
Another similarly priced motor that teams competing in the advanced boat division may like to consider is the Maxon Re-max 221010 6V (see the RE-max 21 datasheet up the page). This motor has the same max efficiency as the Faulhaber but is around 20g lighter (which can be quite significant in model solar boat racing). Due to its higher rpm and lower torque constant, this motor would then require a slightly smaller propeller to that of the 2232 (for any particular set of weather conditions).
Although both the 2232 6V and 221010 6V are very good motors, they come at a somewhat hefty price and will each set teams back a considerable amount of money. This can be a little on the expensive side for many schools, particularly if several boats are being built, and so a few lower cost options may like to be sought out.
The Maxon RE-max 221020 2V and RE-max 214896 4.5V motors probably fit into this lower cost category and the datasheets for both can be downloaded below.
Maxon 221010 2V
Maxon 214896 4.5V
(L-R: Maxon Re-max 221020 2V, Maxon Re-max 214896 4.5V)
A limited stock of the first of these motors can be found at SolarMPPT.com for $49.99. Upon inspection of the datasheet it can be seen that this motor has a rather low nominal voltage (2V) with a high speed constant and low terminal resistance. It also has a max efficiency of 82% which is a good 5% less than that of the 2232 and 221010.
For these characteristics to best suit a Scorpio boat panel the panel will most likely need to be kept in the parallel configuration at all times (as this will ensure that the motor speed is kept low enough to suit a direct drive motor-to-prop-shaft arrangement and also keep the motor in a more efficient operating range).
In series the motor would alternatively be required to reach very high rotational speeds for max power transfer from the solar panel, and this could potentially overspeed and destroy the motor. Even if this isn't to be the case, the motor may not run near its peak efficiency and will also very probably require some form of speed reduction between the motor and the propeller shafts (so that a suitable prop can be used).
The second RE-max 214896 4.5V motor is also currently in limited supply and can be sourced from Jun Guo, a former competitor in the solar car challenge and now member of the WA Committee. Jun acquired a number of these motors towards the end of 2011 from overseas and is now looking to distribute them to teams across Australia for $25 each.
Weighing just 26g, this Maxon is smaller and lighter than the previously discussed 2232 (62g), 221010 (42g) and 221020 (43g) motors. It also possesses a max efficiency of 87% and its characteristics perhaps make it better suited to a Scorpio panel than the 221010.
At 2340rpm/V this motor however still has a rather high speed constant and so a gearbox may be needed to reduce the rotational speed of the propeller shaft when arranging the solar panel in the series (this panel configuration is required in higher sunlight conditions for the 214896 to operate near peak efficiency).
In lower sunlights where the solar panel current and motor resistive losses are much lower, switching the panel to parallel and using the correct propeller should see an improvement in boat performance.
Although a gear box may be required, the particular planetary gear heads that were originally fitted to the shipment of motors that Jun ordered in contained a much too large reduction ratio. Planetary gear boxes are also less efficient than single stage gearing and so Jun removed these apart from the 10 tooth steel gear that was fitted to the motor shaft (as seen in the above image). This is a 0.3 mod gear and has been left in place largely due to the fact that it has been soldered/welded to the shaft and is difficult to remove.
As such, unless teams are going to go to the trouble of trying to remove the gear, the motor will need to have some additional gearing fitted. Despite requiring a little extra effort, this shouldn't be beyond the ability of those competing in the advanced boat division and Jun has put together a $10 kit of bearings, gears and shafts to assist teams. An example of the kind of gear box that can be constructed using this kit is depicted below.
(Maxon 214896 4.5V motor fitted with an example gear box)
For further details, contact Jun at firstname.lastname@example.com.
Even though Maxon Australia have been known to do special deals on certain stock in the past, probably the other main option for teams looking around for low cost Maxon and Faulhaber motors would be to carry out a thorough internet search (where overseas suppliers and places like Ebay sometimes sell these types of motors at a reduced cost).
Be however wary that many of these motors are generally not well-suited to the Scorpio solar panels that most boat teams use and so getting hold of one doesn't necessarily mean that it will be any good for a model solar boat.
This being said, the TMSC has found several cheap and suitable high quality motors at various stages over the past few years. For example, an Ebay seller based in the United States was selling the same RE-max 214896 4.5V motor discussed above for just $15 in mid 2011.
Any of these lower cost options will more than likely also interest teams and students competing in the Tasmanian model solar sprint car challenge as the use of a high quality motor will undoubtedly improve car performance. This event allows motors up to $50 to be used and so either one of the 221020 or 214896 motors would fit in under the upper cost limit.
Motors Continued - Junior Boats
In order to comply with the new regulations for 2012, teams competing in the junior boat division this year must use a motor that costs no more than $5 RRP. This new lower cost limit has been introduced to ensure that all teams are restricted to hobby motor use only. This guarantees an even level of competition unlike in previous years where the limit had been set to $20 and some teams were able to source high quality motors for less than this.
Hobby motors can be sourced from many different places and most teams ought to be able to find something at a local hobby or electronics supplier. This being said, suppliers of the solar challenge such as Scorpio Technology and CAM Art, Craft and Technology in Victoria tend to offer a wider range to select from and so should be considered. For example, Scorpio Technology list no fewer than 8 electric motors in their Technology Catalogue.
Given their low cost, the TMSC would recommend purchasing several different types of hobby motors and then testing them in a range of weather conditions to determine the top performers.
Another option may be to use a Grand Wing Servo-Tech (GWS) LPS gear box. These are a light weight indoor RC plane gear box but are also suitable for use in the model solar boat challenge, particularly the junior division.
(GWS LPS gear box and motor with 6:1 speed reduction gearing)
3 of the top 4 junior boats, including the winner, had a GWS gear box or similar installed at the 2011 national event. Similarly, the winner of the 2010 junior division also used a GWS system.
Although the motor used in the GWS gear box is not as efficient as a precision coreless motor such as a Maxon or Faulhaber (max eff. of around 65% compared to 85%), it is well matched with a Scorpio panel and will operate at close to max efficiency at max panel power in higher sunlights when the panel is connected in series and the correct propeller used (a 35-40mm dual bladed prop may be a good starting point for testing). Due to the reduction gearing, the gear box generates a higher prop shaft torque than what a standard hobby motors produces, allowing for a larger and more efficient propeller to be used.
For best performance in low sunlights the Scorpio panel will need to be switched to a parallel configuration and the correct propeller chosen for max power transfer. Testing both panel configurations and various prop sizes will help determine the best setup at intermediate sunlights.
Combined, the gear box and motor weigh approx. 21 grams which is lighter than many hobby motors on their own.
Due to the high level of shaft torque, the coupling between the gear box and propeller shaft will probably require some special attention in order to eliminate slippage. Gluing the commonly used flexible coupling to the gear box shaft may suffice.
The gear boxes + motors are available from Jun Guo in W.A. for $15 each ($5 motor and $10 gear box) + p/h. To order or for further details please contact Jun by email at email@example.com
Solar Modules & Panels
There are a large range of solar modules and panels out there on the market. The following suppliers are just a few of the more common ones used by teams competing in the model solar challenge.
(L-R: Engelec 4-cell modules (25 & 28mm), Futurlec 1V 400mA module, Scorpio #8 module, Scorpio #26 panel, CAM/Old Dick Smith module)
Scorpio Technology
Scorpio Technology are perhaps the solar challenge's most popular supplier of solar panels and sell a sizeable range. Of this range there are 2 different types of panels, with the first type being a lightweight fiberglass encapsulated panel and the second type a set of heavier resin encapsulated solar modules.
Unless boat teams are willing to search around and source out very high efficiency bare solar cells and make up their own custom-made lightweight panel, Scorpio's lightweight fiberglass panels are easily the best commercially available panels out there for both the Junior and Senior boat events. The solar cells used in these panels have a good efficiency and therefore produce a high amount of power for the allowable 350 sq cm area. At the same time the panels are also extremely lightweight and only weigh around 50g. These panels (#26 in the solar catalogue) come in two models where the only difference between the 2 is that they are categorised depending on whether they produce more or less power than a 5.8W threshold. The higher power panels then cost a little extra due to their use of slightly higher efficiency cells (which produce a little extra current and therefore power).
Over the last few years, the lightweight Scorpio panels have also commonly been used by teams competing in the car event. Here, 2 panels are necessary in order to obtain a voltage high enough for an electronics unit to be used (ie 14V is achieved from 2 x 7V 2011 Scorpio #26 boat panels connected in series). Given that the panel weight + ballast is determined by a formula for the car event depending on the power of the panel, the low weight Scorpio panels may (depending on the design of the car) allow more ballast to be carried further down in the vehicle. This can be seen as an advantage as it will improve its stability around corners and make it less likely to roll over due to a lower centre of gravity. The second advantage of these panels is that they do not suffer from temperature related cell-cracking as has been detailed in the "Solar Panel Cracking" document further up the page.
Perhaps the major drawback of these panels is that they are relatively fragile. This is ok for the boat event if handled with care, but cars can at times have spectacular crashes which can pose a risk. As such, these panels are typically mounted onto a backing of some sort in the car event. A good heat conductor such as thin aluminium sheet is commonly used as this increases the thermal capacity of the panel meaning that the solar cells take longer to heat up in the sun.
Scorpio also lists a new 4.9W #29 car panel in their Solar Catalogue. The TMSC Committee has been told that this is a heavier, resin-encapsulated panel but the exact make up of the encapsulating compound is unknown at this stage. As such, teams would be advised to contact Scorpio and enquire about them. If the encapsulation is indeed made up of the same hard compound seen on the Engelecs modules then there is a very real possibility that these panels may suffer the same cell-cracking fate (as discussed in the "Solar Panel Cracking" pdf).
At $80-$90 per panel, Scorpio's #26 & #29 panels can be rather expensive when purchasing 2 of these for a single car. Another option to explore may be some of the panels that have been included in the Technology Catalogue. The TMSC's pick of these would have to be the Panel #8 modules (1.4W @ 2V Vmp). As already discussed under the Supplier Catalogue heading above, these panels are very low cost and a reasonable alternative for teams not wanting to break the budget. Just be wary of cell cracking as these are also hard encapsulated. This being said, a sample has been recently tested and appears to be less susceptible to fracturing than the Engelecs. The modules also have solder tabs that minimise the heat applied to the cells when soldering. Future Scorpio batches may one day include a soft encapsulation which would make these an excellent low cost panel to use.
CAM Art, Craft & Technology
CAM supply teams with a variety of solar challenge gear as detailed by their old solar catalogue above and online shopping system (under the solar product aisles).
The old catalogue also includes what appears to be the same 4 cell modules (often termed "Engelecs" by teams and many of the committee) that were previously sold by the TMSC's Mr. John Jeffery. These have however recently been removed from their website as so may no longer be available.
On a side note - While the solar cells used in the Engelec modules are good quality, the hard epoxy encapsulation that acts as a protective coating can often cause the cells to fracture due to varying thermal expansion rates. This can then lower the output of the panel and potentially affect solar car performance. As a result, a different type of panel may be more suitable, particularly if the intension is there for it to be used over several years of competition.
These panels were also relatively expensive in comparison with the Scorpio #8 panels, especially when considering that they have a lower output power (and current). On the other hand this means that a lower power panel can be achieved (at the same voltage) without having to mask or cover over any of the cell area.
8 - 10 of these modules were/are typically used on a car and these generally produce between 6 and 9W depending on the number of modules and whether 25mm or 28mm wide cells are used in the manufacturing process.
Perhaps a safer option from a cell cracking standpoint may be CAM's 1.5V 590mA solar modules (listed on the CAM website). These appear to be very similar to the modules that Dick Smith Electronics once sold and were in common use by many solar car teams several years ago. If indeed the same, these modules carry a soft encapsulation compound allowing the cells to freely expand and contract with changing temperatures and therefore eliminate cell cracking.
At the time when these modules were popular the actual cells themselves often lacked in quality compared to the newer Engelecs or Scorpio panels due to being made from factory seconds. As a result, panel fill factors were often very average. Whether these new CAM modules are now using updated and higher quality cells is unknown to the TMSC committee.
Again, these modules are reasonably expensive and a minimum of 10 modules would be recommended for use on a solar car running on electronics.
Be aware that Dick Smith are now selling a new 1.5V solar module. While these may be similar to the old style or CAM modules output-wise, they are physically much larger dimension-wise where the cell area occupies a smaller percentage of the overall module size. As a result, panels made from these modules can get quite large even though they may not produce any more power than a smaller panel that has been assembled using some other type of module.
Finally, CAM also sell a 2V hard encapsulated module similar to the one offered by Scorpio Technology but at a considerably higher cost.
Kite Magic
Kite Magic are based in Sydney and run by NSW Sunsprint Coordinator, Mr. Michael Richards. Michael offers teams a 2V hard encapsulation module similar to the CAM and Scorpio #8 panels. Although these are also somewhat more expensive than those that Scorpio sell, Kite Magic not only provides teams with modules to make their own panels but will also make up panels according to your requirements and specifications.
2 cars from WA used panels made from 6 of these modules (approx. 12V Vmp) at the 2011 national event in Hobart. While this number may be well suited on a car without electronics, the voltage is perhaps a little on the low side for a car running with an electronics unit on board (where a 7 module panel may give better performance).
Futurlec is a global online electronics superstore and offers customers a generous range of solar panels at a reasonable cost. The company has an Australian office in NSW and also a .au website (although not as many solar panels appear to be listed on there).
While these panels are yet to be seen on a car competing at the Tasmanian or national events, several samples have been purchased by the TMSC and these appear physically sound. Unfortunately they however also possess the same hard encapsulation compound that is present on the Engelec, Scorpio #8, 2V CAM and Kite Magic modules mentioned previously.
Futurlec has both a solar cell and solar panel page. On the first of these, the cells that are perhaps most suitable to the solar challenge are the 1.0V, 415mA modules that they have listed. These are essentially equivalent to half an Engelec module where at least 14 would be recommended when making up a panel for the AIMSC solar car challenge. 16 modules may however be a better option and at $3.90 US each that's around $60 for a low power (around 6.5W) 16V car panel.
Another option may be to consider the panels listed on the site's solar panel page. As with the Scorpio #26 & #29 car and boat panels, 2 (or maybe even 3) of these need to be connected together in order to generate an output suitable for a model solar car running with electronics. Two example arrangements might be to connect 3 of the 5V 650mA panels in series to produce 15V @ 650mA or to connect 2 of the 18V 280mA panels in parallel to produce 18V @ 560mA. Both of these options cost about half as much as a similar panel made up from 2 of Scorpio's #29 car panels.
Teams looking at very low cost options may like to consider this 10W panel from Ebay.
These panels have been brought to the attention of the TMSC by an interstate colleague and cost just $32.00 with free postage. They have a stated output of 18V @ 0.56A and this is suitable for a model solar car running with an electronics unit. The cells are also glass protected and do not suffer from cracking due to changes in panel temperature (unlike many of the hard encapsulated modules that have been mentioned previously).
The major downside of these panels probably resides in the fact that they are quite heavy and weigh just under 1kg (although removal of the aluminium framing will see the weight reduced down to around 800g). Without frame, this will just fit under the 850g that a 10W panel + ballast is required to weigh when running without an electronics system.
The panel still easily fits in under the 1600g that is required when an electronics unit is used, but will inevitably affect vehicle cornering stability if mounted too high above the track on the car. As such, teams deciding to use this panel would be advised to come up with a design that will keep the panel nice and low on the car to make it as stable as possible.
Note that these panels are a new 2012 model and are somewhat different to the 10W panels sold at Jaycar and other electronics suppliers in recent years. These Jaycar panels appear to have some cells with different sized areas within their series strings and testing carried out by Mr. Ian Gardner from Victoria has shown their quality to be relatively inconsistent (with some panels giving much better results than others).
Technology Education Centre
Based in South Australia, the Technology Education Centre is another supplier that solar challenge teams have sourced solar panels and other gear from in the past (including in 2011 where 2 cars competing at the national event used panels made up from Tech Ed solar modules).
Tech Ed sell a similar 1V hard epoxy encapsulated module to the Futurlec ones listed further up the page but at a much higher cost ($15.95 AUD vs $3.90 USD). This being said, the modules do output slightly more current (and power) and may arguably also be manufactured to a higher quality.
Tech Ed also supplies a 350 sq cm solar panel. Weighing in at 170g this however does not compare with the lightweight (50g) panels that Scorpio is selling and so would be considered an inferior choice if wanting to maximise performance in the boat challenge. Furthermore, Scorpio panels use higher efficiency solar cells and therefore produce more output power.
On the other hand, the panels look to be more robust and can be arranged into a 12V @ 0.4A configuration which is able to be coupled with an electronics unit to put together a very low powered (just under 5W) car. In comparison, a single 7V Scorpio panel does not produce the required voltage to run a standard electronics system.
If 2 panels happen to be used as has been suggested on the Tech Ed website, and these are connected in an all-series configuration, be aware that the combination of the two will most likely cause the open circuit voltage to exceed 25V. This will lead to power measurement at scrutineering being carried out as per regulation 8.10 in the car specifications and will almost certainly lead to the team being at a disadvantage as basically all solar challenge panels register a fill factor of less than 0.8. A safer bet may be to connect these panels in series-parallel and produce an output of close to 12V @ 0.8A (although this does leave the voltage a little on the low side for a car running with an electronics unit onboard).
Bare Solar Cells
Teams willing to challenge themselves may also like to consider making up their own panels from bare solar cells. While this can be a delicate and time consuming process, it adds another dimension to the car building process and also gives teams more flexibilty when wanting to produce a panel with a specific power and/or voltage/current output.
For example, a single 7V #26 boat panel from Scorpio Technology does not produce enough voltage to run a typical electronics unit like an Easy or Automax. Using bare solar cells, an ultra lightweight 350 sq cm custom-made boat panel can however then be assembled with a higher voltage to solve this problem. This would allow a senior division boat to run with an electronics unit and almost certainly improve its performance if the boat hasn't quite been set up perfectly for the prevailing weather conditions.
Another example may be when a team wishes to run their solar car on a very low power panel (say 5W) but at the same time be able to have the option of switching between electronics and no electronics. Here, a 5W panel without electronics is only required to weigh 100g which cannot be achieved if using a panel made from any of the hard encapsulated solar modules listed above. A Scorpio boat panel (50g) would be light enough although it doesn't produce the voltage needed by an electronics unit for it to function (as mentioned in the previous example). In this scenario a panel made from bare cells would again be a solution.
While good quality and low cost solar cell suppliers can be found quite easily online on various sites like Ebay, most of these sell large 5 x 5 inch or 6 x 6 inch cells which produce several amps of current. This is too great for a model solar car or boat and so the cells need to be cut down to a size where the output is more suitable.
If teams are willing to experiment with cell-cutting techniques then this is certainly an option. Below are however a few suppliers that sell pre-cut solar cells meaning that no cutting is necessary.
(L-R: Plastecs WB-40 400 mA 1 x 2 inch bare cell, Silicon Solar P-Maxx-400mA bare cell)
Silicon Solar
Silicon Solar are an american company that sell a variety of solar cells that have been cut down to different sizes. The ones to go for here are probably the 400mA cells which the link above will take you to. These are by far the cheapest Silicon Solar option at $0.70 USD each and around 25 cells will get you to the 350 sq cm cell area limit in the boat event. This panel will then produce around 5W @ 12.5V and 400mA. While this is not quite as powerful as the Scorpio boat panels (where the solar cells are more efficient), the voltage is however easily high enough to run a standard electronics unit (which can offer an advantage if the boat hasn't been set up properly).
Be aware that while the 25 cells needed for a boat panel @ $0.70 USD each will come to a total of less than $20, the company's shipping costs via their online shopping system are outrageously expensive. To this end, teams would be advised to contact the company by email and try to negotiate a different shipping method at a reduced cost and then make an order through the sales team. Purchasing enough cells for 2 or 3 panels at once would also help reduce shipping costs.
These cells are probably some of the only ones being sold on Ebay that have been cut down to useable size for the model solar challenge. While the cells are listed as being B-grade and the precise quality unknown to the TMSC, they are another very low cost option for teams at $20 for 45 cells (plus $5 postage from the US). At this kind of cost it might almost be worth ordering in a set just to see how they perform.
Note that these cells have been cut down to 1 x 3 inches (2.5 x 7.5 cm) and so only 18 could be used on a 350 sq cm boat panel. This will give around a 9V panel (which is most likely too low for a standard electronics unit to function) and so these cells are only really applicable to the car event. Any boat panel made from them would be inferior to the Scorpio ones as the cells are less efficient (meaning that they output less power for the same cell area).
Somewhere between 28 and 36 of these cells would probably be recommended for a car, where 36 cells ought to give teams close to a 9W panel (as stated by the Ebay seller) with an 18V 500mA output.
Some other suppliers are Everbright Solar and Plastecs but these tend to be much more expensive.
Everbright sell the same sized 1 x 3 inch cells as the Ebay seller above but at a considerably higher cost and with a minimum order quantity of 100 cells while Plastecs sell a smaller 400 mA 1 x 2 inch cell (WB-40) which is also rather costly.
Over the last decade and a bit, electronics systems in the AIMSC have gone from being almost non-existent to becoming an integral part of modern model solar car racing. Such is the advantage that an electronics unit can bring, one has now been needed over the last 6 - 7 years of competition in order to remain competitive at the top level. In fact, all top 4 placegetters at the nationals since 2005 have used some form of electronics system.
Why an electronics system improves the performance of a car is quite simple. It maximises the power delivered to the motor from the solar panel at all times.
See, a solar panel's power output is heavily dependent on the electrical load placed on it. That is, it will only put out a maximum power at an ideal load. Since a solar car motor is a variable load, and the load will change significantly over the course of a solar car race, the panel will therefore only ever experience this ideal load for a short period of time while racing (if at all). For the rest of the time the load is non-ideal and a power loss exists. This ultimately results in a loss of overall car performance, even if the correct motor and gearing has been selected.
The secret in the electronics unit is that it continuously loads the solar panel at an ideal load. This then enables it to recover maximum power at all times (minus the losses of the electronics system which are minor).
Through some clever electronics, the power from the panel is then transformed to suit the motor load. In other words, at the start of a race when the motor voltage is low (motor voltage is proportional to rotational speed), the electronics deliver a higher current to the motor. This in turn generates more motor torque (motor torque is proportional to the current flowing through it) and leads to better car acceleration (an electronics system is most influential at the start of a model solar car race).
Since the solar panel's maximum power is maintained (minus the electronics losses), and power equals voltage multiplied by current (P = VI), the motor current then starts to drop as the car accelerates and there is an increase in motor voltage (remember that motor voltage is proportional to rotational speed). This trend will then continue until the car reaches a top speed (at which a voltage-current balance depending on the gearing of the car is obtained).
Of course, nothing is free in life and, as mentioned earlier, an electronics system will inevitably have some losses (these will vary somewhat depending on what is being delivered to the motor). This power loss is however minor compared to the advantage that is offered.
Not only does an electronics system serve as an advantage in any given race, it can also significantly simplify model solar car racing.
Without such a system, solar panel configuration and car gearing must be changed to suit the weather conditions (if car performance is to be maximised). This can often be difficult to get right, especially on days where the sun intensity is all over the place.
The addition of an electronics system to a car however rarely sees the need for a change in gearing. Here, the same gearing used at full sun can also give acceptable performance in very overcast conditions. In fact, numerous past national event winners and placegetters have gone through entire competitions without having to make a single gear change (even with variable sun conditions being present).
In recent times there have been several different electronics systems available to teams for purchase. Below is some information on all those used at the last few nationals and where to get them from.
As already covered in the motor section up the page, SolarMPPT.com is run by Mr. Tony Bazouni, designer and distributor of the Easymax III and Automax electronic systems. Both these units have been designed specifically for the model solar challenge and are highly efficient as well as very compact due to the use of surface mount technology.
The Automax is essentially an upgrade of the Easymax where the Automax automatically tracks the solar panel's maximum power point (MPP) continuously. This means that the electronics will always be set up for maximum power transfer between the solar panel and motor, no matter the type of panel being used, its temperature or the sun level.
In contrast, the Easymax requires a button to be manually pressed in order for the unit to lock onto the solar panel's MPP. Once initialised, the unit will then retain this setting until the button is pressed again and the unit re-set.
Since the maximum power point is different for different solar panels and also changes with varying sun intensities and panel temperatures, the Easymax will need to be set and re-set every so often when there is a dramatic change in panel temperature (or sun level) from the conditions that were present at the time of the previous button press.
Although some teams re-set their unit before almost every race, weather conditions (or panel temperatures) are often not variable enough to need readjustment after the initial setting. As such, teams can often go through an entire competition without needing to touch the Easymax after first setup.
Due to its auto-tracking nature and therefore ease of use, the Automax (which first became available in 2010) has become more and more popular in the last couple of years. In fact, 90% of teams that competed in the 2011 national event used this unit.
Documentation for the Automax and Easymax III can downloaded below and both units are available for purchase at SolarMPPT for $90 and $50 respectively.
Automax Documentation
Easymax Documentation
Given that both units work more or less identically when the Easymax has been set up properly, teams looking at keeping costs to a minimum may like to consider using the cheaper of the two systems.
Engineered Electronics
Prior to the release of the Easy & Automax, the most popular electronics unit amongst solar car teams was probably the Engelec Max 4 (for which the documentation can be downloaded below).
Engelec Max 4 Documentation
This unit was designed by the TMSC's Mr. John Jeffery and originated during the late 90's and early 2000's while John's sons were still racing. John then started selling units once they had finished competing.
The Engelec unit has featured in many top cars over the years and was used by Tasmania's Hobart College team that won the 2005 & 2006 national events.
Like the Easymax, the Engelec Max 4 needs to be manually set. This process is however a little more involved as a multimeter is required and a potentiometer adjusted with a screwdriver (rather than a simple button press being needed).
At this stage John is uncertain whether he will continue with his production of the Max 4. Either way, he may still have some units left in stock and so teams would be advised to contact him if interested. The Max 4 can also be found on CAM's online shopping system (albeit at a higher cost).
Scorpio Technology
Scorpio Technology also sell 2 different electronics system kits where these must be self assembled and soldered together. Like the Engelec Max 4 these then also need to be set up manually using a multimeter and adjusting a potentiometer.
Despite not being as efficient as the Automax, Easymax and Max 4 units, the Scorpio systems are very cheap (less than $10) and perhaps an option worth considering for car teams content on putting together a good (but not necessarily event-winning) vehicle. The key difference between the 2 Scorpio units is that one is a high voltage and the other a low voltage version. Here, the high voltage kit is aimed at solar cars and the low voltage version intended for the advanced solar boats (where this is the only commercially available system that is suitable for use in combination with a single 7V Scorpio boat panel).
Other Electronics Systems and Information
One other system that has been seen at the nationals over the years and is perhaps worth a mention is the BHHS V4.2 MPPT. This unit is however only available to Box Hill High School teams and not for commercial sale. Like the Automax, this is also an auto-tracking unit and testing has shown the 2 to be virtually identical in performance.
(Top L-R: Automax, Easymax III, Engelec Max 4, Scorpio low voltage unit. Bottom: BHHS V4.2 unit)
Due to the ease of use of these electronics systems (especially the auto-tracking units), and not having to change solar panel configuration or car gear ratios, the AIMSC committee has sadly seen a slow decline in team knowledge surrounding panel configurations, motor operation, gearing, etc.
This has prompted the committee to take action and come up with a way in which to encourage teams to once again explore running without electronics (hopefully boosting student learning in the process).
The committee eventually decided that the best way to achieve this was by offering a considerable weight reduction to cars running without an electronics system on board.
As such, there are now two ballasting formulas (as seen in section 8.13 of the 2012 car regulations), one each for cars running with and without electronics. These have been carefully arranged in such a way that intends to give some advantage to non-electronic cars (providing they have been set up correctly for the conditions). The extent of this advantage will vary with different sun intensities and range from being quite small in high sun to perhaps 2-3 seconds in low sun.
As an example in the difference of solar panel weights, a 10W car using an electronics system will be required to carry a 1600g solar array + ballast in 2012. The same car would however only be required to carry an 850g solar array + ballast (close to half the weight) if the team opted to run without electronics.
In summary, the TMSC strongly encourages all car teams to make an electronics system available to themselves to start with. Even highly experienced teams wishing to try their hand at no electronics would be advised to keep a unit handy since running without can be basically impossible to get right in the wrong kind of weather conditions.
Still keeping an electronics unit at hand as a safety net, teams may then like to begin looking into racing without electronics once they have gained some challenge experience and developed a better understanding of their car (as well as achieved sound operation with an electronics unit on board). A good amount of gear ratio and solar panel configuration testing at various sun levels will then be critical if car performance wants to be optimised.
Car wiring and solar panel configurations
Seeing as motors, solar panels and electronics units have now all been discussed to some degree, let's take a closer look at how they actually connect together in a model solar car.
Although some teams seem to turn up to events having wiring and connectors all over the place, model solar car wiring is in fact quite simple really. This particularly holds true when an electronics system is used and an example wiring diagram using either an Automax or Easymax III from SolarMPPT can be found below.
Here, connectors called Micro Deans plugs are needed for connection with the input and output of the Automax and Easymax III and SolarMPPT will supply a set of these with the purchase of a unit. These are fantastic little plugs and, if possible, their use is highly recommended wherever a connection needs to be made. To this end, they have also been used to connect up the remainder of the circuit above (ie between the panel and the switch/electronics).
Most local RC Hobby stores will stock these and for those that don't they can be purchased from SolarMPPT (or many other places online).
Also notice that two different coloured Deans connectors (red and black) have been used in the example. This is to match the input and output plugs of the SolarMPPT electronics units as one is red and the other black.
Although not necessary, colour coding will make circuits easier to follow and faster to connect up properly under stressful situations during an event.
Colour coding wiring according to polarity is also useful for keeping things as simple as possible. Use multicore wire and don't go too large or small with the gauge.
Something like the figure 8 red and black twin conductor low voltage DC power cable that Dick Smith Electronics sell is ideal for model solar car use. Some stores may still sell this by the metre although 10 metre rolls can be purchased for around $10 if this isn't the case.
Finally, try and keep wiring as short as reasonably possible. Excessive wiring will just add weight to the car and also result in an increase in power loss due to higher wire resistance.
The switch that has been included in the above circuit is a simple single-pole-double-throw on-on toggle switch available from Jaycar. It comes in two versions, a mini and sub-mini, and functions as a regular single-pole-single-throw on/off switch if the third pin is neglected.
Use shrink tubing (some is supplied with the Deans plugs but you may need to get some extra in which case an electronics supplier like Jaycar should be able to help you out) to insulate the switch and plug terminals after soldering to prevent the possibility of any wires shorting.
Some teams also elect to include what is called an inhibit switch in their circuit. This is designed to rest up against a closed start gate at the start of a model solar car race and cut off the power to the motor. When opened, the switch is then released and power to the motor activated.
Although this has the ability to protect the motor from over heating and prevent excessive wheel spin at the start gate, the switch has not been included in the example circuit as it can be another area of possible complication.
Such a switch probably isn't worth the hassle as long as teams keep a cover on their solar panel at the start gate until moments before it is opened.
Before moving on, dummy loads for the purpose of setting up an electronics system ought to perhaps also be discussed. Although automatic tracking units like the Automax and Box Hill HS version 4.2 do not require one of these, others like the Easymax III, Engelec Max4 or Scorpio ones may find one useful.
See, to set up one of these other units a constant load needs to be placed on the output. While this can be done by locking up the drive wheel and stalling the motor, doing this in bright sunlight could see the motor overheat.
To prevent this from happening, a dummy load made from resistors with the same/similar value as the motor winding resistance can instead be used (where this mimics the motor at stall where the terminal resistance of a 2232 6V Faulhaber is around 0.9 Ohms). This is then temporarily connected to the output of the electronics in place of the motor as seen in the following image.
Note that not just any resistor or set of resistors can be used here. Most seen on circuit boards offer too high of a resistance and, if not, are typically rated at only 0.25 or 0.5 Watts meaning that they will overheat rather quickly and most likely fail if the electronics system is connected to say a 10W solar panel.
A higher wattage resistance with a power rating equal to or greater than the solar panel needs to be used here and an example of a 1 Ohm 5W dummy load setup is presented below.
This particular setup is aimed for use with a lower power solar panel although it has also been used to set up systems with 7 or 8W panels. This connects to the output of an Easymax III via the Micro Deans plug and, once plugged in, the unit is then ready to be set with a manual button press (as has been discussed in the electronics section up the page).
Engelec and Scorpio units need to be set a little differently and use screwdriver to adjust a potentiometer rather than have a button pressed. So as the user has some form of useful feedback, a multimeter needs to be connected across the dummy load (ie with some alligator clips) so that the voltage across it can be monitored while fine tuning the potentiometer. Here, a unit will then be adjusted correctly once the maximum voltage across the load has been obtained.
Finally, although not generally necessary, teams wanting to regularly re-adjust their electronics units may start to find the practice of accessing the electronics within the car, unplugging the motor and replacing it with the dummy load somewhat monotonous and time consuming.
As such, there is a solution to this problem and this comes in the form of a 3-pin DC socket, which is typically used to disconnect batteries from a device when adaptor power is available.
By applying its function to a model solar car, this same component can instead be used to insert a resistor bank without having to physically unplug the motor or open up the car. For this, the dummy load must simply be plugged into the socket using a matching plug and a small hole made in the car body so as to gain external access to either the button of the Easymax or potentiometer of the Engelec or Scorpio units. To assist teams, an example of such an arrangement is given below.
Here, 2 of the terminals from the DC socket are normally shorted together by spring loaded contacts and this completes the path from electronics unit to motor. The connection is then however broken when a matching plug is inserted and power diverted through the resistor bank until it is removed.
As has been touched on in a previous section on this page, a significant reduction in solar panel weight currently exists for cars running without an electronics unit (as per rule 8.13 in the 2012 regulations). Such is the extent of this reduction that the new ballasting formulas now have the potential to give a notable advantage to a well set up non-electronics car (ie not having to carry as much weight means better acceleration and less rolling resistance).
This advantage will vary depending on the intensity of the sun (where a greater advantage can be obtained in overcast and lower sunlight conditions), and mathematical modelling and simulations run by the AIMSC Committee have shown that it may be in the order of 2-3 seconds at 20% sun.
To gain a better understanding of the kind weight reductions involved, let's consider the new ballasting formulas and put some quantities to a few example panels.
First consider a 10W solar panel. With an electronics system on board this panel needs to weigh a total of 1600g. That is, if the panel weighs less than this value then some form of additional ballast will be needed to make up the remainder. Add another say 400g for a chassis/body and that makes a 2kg car.
Remove electronics from the equation, however, and the 1600g panel all of a sudden only needs to weigh 850g. This is effectively halving the required weight of the solar panel (53% to be exact). This percentage will continue to drop further and further with decreasing panel powers and, as an example, a 6W solar panel will see the required weight go from 600g down to just 250g (ie 42%).
The reason why all this has been done is to encourage teams to operate without electronics (or at least investigate going without) and thus increase their knowledge and understanding of the interactions between the solar panel, motor, wheel size and chosen gear ratios.
Given that this can however be a lot more difficult, and even impossible to master in highly variable weather conditions, it is advised that all teams (whether beginner or advanced) first set up their car to run with an electronics system. Only once sound operation has been achieved in this configuration should the no-electronics option then be considered.
In order to run without electronics, teams will almost certainly need to set up their solar panel in a way that will allow its configuration to be easily switched between at least a couple of different arrangements. The choice of several different gear ratios will also be required.
Changing either gear ratio or solar panel arrangement alone will not allow a car's performance to be fully optimised in all conditions and it is a combination of the two that will give teams the best chance in all levels of sunlight.
Also, although the optimal car configuration for a given set of conditions can be approximated theoretically to some extent, fine tuning will ultimately require on-track testing. More on this later.
The reason why the configuration of a model solar car panel needs to be made changeable when going without an electronics system is quite simple. It reduces the voltage and increases the output current of the panel at lower sunlights.
The maximum power voltage (or Vmp) on most cars using an electronics unit will be around 14V - 16V at 100% sun and this is getting too high for non-electronics cars, particularly in lower sun conditions. This means that a configuration change is needed since simply making a lower voltage panel to start with may no longer see it suitable for use with an electronics system. Even if this isn't the case, electronics units also tend to operate less efficiently with lower voltage panels.
Without changing a standard car panel, a Faulhaber 2232 6V motor would otherwise need to be geared to spin at a very high rpm for max power transfer from the solar panel. While there isn't necessarily anything wrong with this from a panel-loading perspective, and the car may still run, the frictional losses of the motor at these kinds of shaft speeds will tend to reduce its overall running efficiency at lower sunlights and ultimately result in less power going towards propelling the car forwards.
Perhaps another way of looking at it is that a motor requires a certain amount of current just to overcome its rotational running losses (no load losses).
For the Faulhaber 2232 this is around 25-30mA at 6V or 7100rpm (datasheet states 35mA although it tends to drop after the motor has been run in a little). These no load losses will increase with motor speed and at say 15 or 16V (ie 17000 - 19000rpm) this current will climb to around perhaps 70mA (determined experimentally).
So for say a 16V (Vmp) panel @ 450mA in full sun, this panel would produce just (10/100) x 450mA = 45mA at 10% sun (solar panel current is directly proportional to sun intensity). This is even less than what is needed to run the motor on its own at the max power voltage of the panel and so there are going to be losses all round.
If the panel is instead switched to series-parallel and produces 8V @ 900mA in full sun (voltage is halved and current doubled) then it would produce close to 90mA at 10% sun. Since the max power voltage is now much lower, so are the motor speeds and frictional losses (these may now be closer to 40mA or 50mA). This now leaves a spare 40mA or 50mA to drive the car.
The other good thing about switching panel configurations is that teams won't need as many gear ratios since some will be able to be used twice (once with each panel configuration). The drive wheel setup also doesn't need to be made to cater for such a large range of gear ratios.
The question now therefore turns to how does one wire up a panel to easily switch between the different configurations?
Although a solution to this problem is quite simple, teams often have problems with it or trouble working out the best way to do it. As a result, an example of how a panel might be set up is depicted below.
Here, a three position double-pole-double-throw toggle switch with a centre-off position is used to switch between all-series and series-parallel configurations. Alternatively, a 2 position version could instead be used if a standard switch for turning the car on and off is also included.
The above diagram is probably most relatable to a panel with Engelec modules although the wiring is essentially the same for Scorpio and other panels as well (boat panels too). This setup allows an electronics unit to be simply unplugged and removed and the motor then connected directly to the switch. When using this setup just be sure to carefully mark which side of the switch is all-series and which is series-parallel so the two don't become mixed up. If you switch to the wrong configuration then it will slow the car down and, if an electronics unit is being used, then the car probably won't run at all if the panel has switched to the series-parallel arrangement (due to there not being enough voltage).
Notice that the 2 Micro Deans plugs coming from the panel in the diagram above are a different colour to the one used between the switch and the electronics. These have also been arranged so that the panel is symmetrical (ie male pins on both panel plugs are +ve). Both these precautions minimise the chance of any errors being made when connecting everything together (ie mixing up the plugs). Alternatively, a single 4 wire plug could be used between the array and switch.
Finally, only solar panels with an even number of modules/sections should be used on non-electronic cars as these allow voltages to be halved (and currents doubled).
Panels made up of a prime number of modules/sections such as 3, 5, 7, 11, 13, 17, etc are not divisible by anything other than themselves and one and so mean that string voltages cannot be matched properly (this can cause problems).
Although odd numbers such as 9 or 15 can be used (ie 3x3 or 3x5/5x3), the TMSC would probably discourage this as more wiring would be required.
Wheels, guide rollers and bearings
The use of high quality wheels, guide rollers and bearings is paramount to building a good model solar car. These all affect the rolling resistance of the vehicle and so friction and vibrations must be minimised for best performance.
To achieve this, wheels and guide rollers must be well balanced and run round and true while ball bearings must have very low friction.
Given that the actual steering of a model solar car is carried out by its guide rollers running along the guide channel, and not by the front wheels, all wheels and guides other than the drive wheel should have a smooth and hard rolling surface (no tyre). There is no need for a tyre here as one will simply add to the rolling resistance of the car.
Only the drive wheel may require a tyre (usually an O-ring is used) and this is purely used to improve the traction between the wheel and track surface (in order to help limit or eliminate any wheel slip from occurring during the early stages of a model solar car race). This is however usually only an issue in high levels of sunlight and so a tyre can also often be removed in overcast conditions to further reduce the rolling resistance of the car (there will be a certain sun level at which there is a cross-over point and a tyre no longer beneficial).
Many years ago when the solar challenge first began, car teams were given the option of steering their cars around the track by either a single guide peg down the centre of the guide channel or by having a peg on either side. The first of these options was then eventually discarded in the early 2000's to allow the committee to include channel inserts around the track and improve alignment. All serious competitors up until then had run with guides on the outside of the channel anyway.
In the early days, most top teams used ball bearings as guide pegs. While this can still be seen on some modern day cars, most now fit the bearings into some form of roller or wheel. The idea behind this being that this increases the size of the guides and will help them overcome any mismatches in the solar car track more easily.
A variety of guide wheel sizes have been seen over the years but most typically range between around 20mm and 50mm in diameter. Here, a larger wheel will be less affected by a guide channel misalignment while a smaller wheel will add less weight to the car.
While bearings with seals will work, open or metal shielded ball bearings offer the least rolling resistance and so should be used. Of these, the latter of the two would be recommended by the TMSC as the metal shields will keep out most dirt.
Compared to sealed bearings, both types are also easier to clean out which is needed to improve car performance (bearing grease will just increase rolling resistance). As mentioned in John Jeffery's car help file at the top of the page, bearings can be cleaned out using White Spirits and several goes will be needed in order for all grease to be removed.
As will be discussed, most of the solar car wheel suppliers listed below will also stock matching ball bearings at a reasonable price. In fact, they will probably be less costly than what a lot of local bearing suppliers will ask for.
Cheaper alternatives may exist on the internet on sites like Ebay but be wary of the quality as well as be sure to select the correct ones.
The cheapest option for teams seeking out model solar car wheels and guide rollers is often to make their own on a lathe. Local plastic suppliers frequently have offcuts of PVC rod lying around and will hand these over at low or no cost, particularly if you let them know what it's for. Even if no offcuts are available, purchasing a short length should not put much of a dent in the budget.
The problem with this is that many schools however don't have the facilities or personnel with the skills to work a lathe and produce a set of wheels with the level of precision and quality needed for a good solar car. As a result, several suppliers of model solar car wheels and guide rollers have emerged over the years and some of these are discussed below.
These have now been around for a couple of years and are the best commercially available model solar car wheels as far as the TMSC is concerned. They have been designed specifically for the model solar car challenge and SolarMPPT has all wheels CNC machined to a high quality.
Tasmania's 2011 AIMSCC entry "The Weapon" from New Town HS used these wheels to great effect and went on to finish second at the nationals last year in Hobart. Along the way to achieving this the car also set the fastest lap of the competition and would have smashed the 5 year old Tas track record had the racing distance been consistent with previous state events.
At $10 each these are two thirds the cost of the wheels that RI sell and also around half the weight. In addition, their smaller 54mm diameter (RI wheels are around 65mm in diameter) tend to reduce the ride height of the car slightly and this will make it marginally more stable.
Although the wheels are made from a low friction Polyacetal, cars with no steering will still suffer from some steering losses around the track corners. A form of steering mechanism such as trolley-wheel steering would consequently be recommended.
3-wheel cars like "The Weapon" (or any 10 of the top 11 cars at the nationals in 2011) require just 2 of these wheels. A drive wheel will then need to be sourced from elsewhere (more on this a little later). Each wheel takes two 7mm OD, 3mm ID flanged bearings and SolarMPPT also supplies these for $4.50 each as seen at the link above.
The bearings are then typically pushed onto a threaded M3 (3mm) screw/bolt in order for the wheels to be mounted to the car or car steering mechanism.
Tightening a regular M3 nut against the bearings will more than likely see it make contact with the moving outer race and greatly increase the rolling friction of the bearings (and in some cases even cause them to lock up completely). To solve this problem, a lock nut with a domed face as seen in the example CAD images below is often used instead.
Because there is a small gap inside each wheel between the 2 bearings, the TMSC would also strongly advise teams to use a number of small 3mm ID, 5mm OD washers to fill this gap (as seen in some of the above images). This will prevent any damage from occurring to the bearings when applying the lock nut too tightly. It also makes car setup (and wheel adjustments during an event) far easier as there is no time wasted adjusting the lock nut so that it isn't too tight or loose.
Even though a number of RC model and parts manufacturers produce the particular washers needed, not every RC store will stock them. As such, buying them from online may be the easiest option.
There are several hobby stores around country the with online shopping systems and RC Hobbies which are based in NSW are one such example. These guys possess a good range of RC washers and the ones to go for here are probably the Yeah Racing Washer 3x5x0.8mm. These come in a pack of 10 for $4 and 2 of these are needed per SolarMPPT wheel.
New Town HS also used these same SolarMPPT wheels as guide rollers in 2011. This can however become rather expensive, especially if a school is looking to buy parts for several cars, and so there is another cheaper alternative for teams trying to keep costs to a minimum. This will be covered below.
RI Gear
As seen in their catalogue up the page or on their website, RI basically sell everything needed to put together a complete car. All RI components are manufactured to a high quality but this comes at a considerable cost and buying an entire kit can be a significant hit to the budget.
There are however a number of individual components from RI that are worth having a look at. In fact, RI is the only known commercial manufacturer of model solar car drive wheels and this means that teams will at the very least need to order some of these in if they do not wish to make their own.
These drive wheels come with a groove around the outer rim for an O-ring and are around 63mm in diameter with one in place (the drive wheel may need to be attached to the car in a way that takes into account the different wheel diameters if SolarMPPT wheels are used at the front). RI then has a 100 tooth black acetal spur gear that has been designed to be mounted to the wheel via two screws.
Similar to the SolarMPPT wheels, 2 flanged bearings are needed per wheel. These however need to be 10mm OD and RI supply them with an ID of 6mm for $5.40 each. Many Victorian teams then slide these onto fixed and non-steering 6mm carbon fibre/aluminium arrow shafts and use RI retaining collars to hold them in place.
For teams wishing to incorporate trolley drive wheel steering into their design or mount the drive wheel to the car a little differently, the TMSC suggests one of two options.
The first of these would be to replace the 6mm ID bearings with ones that have a 3mm ID. This allows an M3 screw/bolt to be used for mounting the wheel and bearings to the car.
The second option would be to purchase 2 of RI's guide roller sleeves which cost around $1 each. These are basically bearing inserts that reduce the inside diameter from 6mm down to 3mm so an M3 screw can again be used.
Both methods will work and teams may like to investigate either option to see which is less expensive.
As with the wheels from SolarMPPT, there is also a danger of bearing damage if too much axial force is imparted on the inside races. Consequently the TMSC would once again advise teams to use washers or some other form of spacer to keep the bearings separated. For this, a number of standard 7mm OD 3mm ID washers may suffice (these can be found in most suppliers of screws and washers and each are typically around 0.5-1mm in thickness).
Some other RI components definitely worth looking at are the guide rollers. These are 25mm in diameter and cost just $1.95 each (which is considerably less expensive than using SolarMPPT wheels as rollers). Like the drive wheel these also use two 10mm OD flanged bearings per roller and the same options listed above then again exist for reducing the inside diameter to 3mm. Either way, remember to pack up the gap between the bearings to prevent any damage as well as make things easier.
Not only are these guide wheels lighter than those from SolarMPPT, they are also designed in such a way that allow the screw head to sit up inside them. This allows the running surface of the guide rollers to be lowered closer to the track to improve car stability.
Teams with access to a lathe may also like to taper the rollers slightly so that only the very bottom runs up against the guide channel (unmodified rollers are cylindrical and 9mm high).
Scorpio Technology
As seen in their Solar Catalogue, Scorpio Technology also sell a set of model solar car wheels. While their quality is not at the same level as those from SolarMPPT and RI, they come at a much lower cost and so may like to be considered by car teams looking at cheaper options, particularly those involved in sprint car challenges.
Of these wheels, the ones to go for are those with the 7mm or 10mm bore hole sizes which allow a flanged bearing to be used on either side.
Unfortunately the wheels are rather large and at a diameter of 70mm will likely raise the height of the car somewhat (depending on its design). Unmodified, their size also makes them rather excessive for guide rollers and so teams with access to a lathe may like to consider turning them down to a more suitable size.
Scorpio also supply some ball bearings to suit these wheels. Upon closer inspection of the Solar Catalogue, one may notice that the same 10mm OD 6mm ID flanged bearings that RI stock are listed but at a lower cost ($28.35 for a pack of 10 compared to $5.40 each).
Be wary of this however as the TMSC recently ordered in a pack and found some of the bearings to be lacking in quality. Consequently, sourcing these from RI or elsewhere may be a safer option for best performance.
Technology Education Centre
Probably the last common supplier of wheels for the model solar challenge is the Technology Education Centre.
Tech Ed once sold an array of different kinds of wheels and the '05 & '06 national champions from Hobart College used a fibreglass wheel that was stocked at the time.
Tech Ed's range has however been on the decline since then and it appears that only one wheel suitable for the solar challenge remains (which the link above will take you to). This is the 60mm red nylon wheel and, like the Scorpios, may not quite be up to the same standard as the RI or SolarMPPT wheels. The website lists these as costing $44.00 for 100 wheels and so they are very low cost. This number however seems a little excessive and so teams would perhaps be advised to contact the supplier and find out whether a smaller quantity can be negotiated.
As mentioned on the website, this wheel has a bore diameter of 4mm and so allows for 2, 3 or 4mm bush bearings to be used (which may be adequate for a sprint car). Car teams intending on entering the main car challenge would however be advised to turn/bore out the wheels in a lathe to fit 7mm or 10mm ball bearings.
Getting a model solar car into motion has seen many different ideas applied over the years. These have generally involved coupling the motor with some type of drive wheel or drive axel, but even air propulsion by propeller or fan has most likely been attempted at some point.
Although fitting a drive wheel or roller directly onto the motor shaft has the potential to work with the right motor, car setup and amount of sunlight, most solar car motors (the Faulhaber 2232 6V for example) are not well suited to this and so must be "geared" down to drive the car.
This gearing down has been achieved in a number of ways since the competition first began and a few of the most common methods have included gears, belts, O-rings and chains.
Without going on to discuss these in great detail, gears are the one to go for here. Just take a look at any photos of cars from the last 5 years and you will struggle to find a car that doesn't use them.
Forget multistage gearboxes, worm and bevel gears though. What is needed is simple single-stage reduction gearing using spur gears.
As seen on basically all modern day solar cars, this is typically achieved by fixing a spur gear directly to the drive wheel and then fitting the motor shaft with a much smaller pinion gear.
(2 Faulhaber motors comparing 12 tooth plastic KHK (left) and brass RI (right) pinion gears)
For best performance, precision spur gears need to be used and a few suppliers of these are listed below.
Purgon Engineering and Ronson Gears
Both these Australian companies are suppliers of the plastic injection moulded spur gears used by many teams all over the country in years gone by. These are manufactured by KHK in Japan and made from a self-lubricating material meaning that no oil or other lubricant is ever required for sound operation.
They are particularly popular amongst Tasmanian teams and were used by the Hobart College team that won the national event in 2005 and 2006 as well as the runner up from Clarence HS in 2002 and New Town HS in 2011.
Of these gears, the ones to go for are the DS0.5 (0.5 modulus) line which encompasses an assortment of gears with teeth numbers ranging from 12 to 80.
Here, the 12 and 15 tooth gears have a 2mm bore which is a perfect press fit onto the shaft of the 2232 6V Faulhaber motor. Although either gear would suffice, most teams use the 12 tooth as it allows for the possibility of a higher gear ratio to be obtained if needed.
Once pressed onto the motor shaft the gear should then remain there as long as possible. This is because constantly removing and re-applying a press fit pinion gear may eventually lead it to loosening its grasp on the motor shaft and cause it to slip or, even worse, regularly placing a push and pull axial force on the shaft could see the shaft itself become loose and result in motor brush damage (which will ruin the motor).
If the motor pinion gear is left untouched after the initial fitting then this means that the drive ratio will need to be changed by either changing the spur gear or varying the size of the wheel. Either way will work although changing wheels can lead to variations in car height and the need to therefore adjust the height of the guide rollers accordingly. As such, gears are usually exchanged or even whole same-sized wheels with different spur gears already attached swapped around.
The spur gear or gear ratio to use to optimise car performance will depend on an array of variables including the size of the drive wheel, type of motor and solar panel output as well as the sun intensity and top speed of the car.
As discussed in the electronics section above, an electronics system can significantly simplify gear selection. In many cases an electronics unit can reduce the number of gear ratios needed to just one or two. For example, the New Town HS car that finished second at last year's nationals used only a single gear ratio at all times. This included sunlights ranging from as low as 10% all the way up to over 100% (intensities higher than 100% can exist when clouds funnel and concentrate sunlight).
The most commonly used KHK spur gear for cars running with an electronics unit on board is the 72 tooth gear (DS0.5-72). This gives a 6:1 gear ratio with the 12 tooth motor pinion gear and is well suited to cars using a 50mm to 55mm diameter drive wheel and a solar panel with a maximum power voltage of 14V or more. This gear is a good starting point for teams and is perhaps at its best in mid to high sunlights even though it will still do well in overcast conditions.
Please note that even the largest 80 tooth spur gear from KHK is not particularly well suited for use with the larger 63mm diameter drive wheels from RI.
Since drive wheels with diameters ranging from around 50mm to 55mm would be more appropriate, teams will probably need to make their own or modify a solarMPPT or Tech Ed wheel to suit.
RI may be able to reduce the size of their drive wheels upon request although this could be quite costly and have a minimum order quantity. Even if this is a possibility, the larger KHK spur gears come with a 6mm inside diameter bore and will still need to be bored or turned out on a lathe in order to be fit to the wheel.
RI Gear
As mentioned in the wheel section up the page, RI has a 100 tooth black acetal spur gear that has been designed to be fixed to the drive wheels they sell.
Unlike the KHK stock gears above, RI then have a selection of brass motor pinion gears for changing gear ratio. These are fit onto the 2mm motor shaft by a grub/hex screw and so can be easily added or removed without placing any stress on the motor. This means that the large spur gear and drive wheel can be left unchanged and the pinion gear simply swapped out.
RI offer a number of brass pinion gears ranging from 9 to 18 teeth. Which one of these to use will depend on the solar panel and sun intensity although somewhere around a 14 tooth gear would be a good starting point for a solar panel with a maximum power voltage of 14-16V. Operation in high sunlights may see a 16 tooth gear more fitting while a 12 or even 10 tooth gear more appropriate in very low sunlights.
As seen in the RI catalogue above, the PINION SET C has a good range of gears and could be a worthy investment for testing purposes. This has pinion gears with 10, 12, 14, 16 and 18 teeth.
(RI Gear Brass PINION SET C: 18, 16, 14, 12 & 10 tooth gears)
Although not as popular, RI also sell a range of spur gears for teams not wishing to use the standard 100 tooth gear. Similar to the KHK gears these come with a 6mm ID bore and will need to be turned out in order for them to be attached to an RI or any other type of wheel.
Scorpio Technology
Despite a lot of car teams using either KHK or RI gears Scorpio also offer a selection of gears at a low cost.
Because these are not as widely used the TMSC is unable to comment on their quality but it is thought that they could be well suited to the sprint car challenge.
Model solar car gear ratio selection (with electronics)
As touched on in the previous section, the gear ratio needed to optimise car performance will depend on a range of variables.
The Excel spreadsheet below should help teams approximate a gear ratio for their car if an electronics unit is being used.
model solar car gearing
This same gear ratio can then be used in most sun levels as the electronics will take care of things in the lower sunlights. In saying this, remember that while the file above should get you get in the right area, truly optimising the gear ratio for a particular car must be done by testing it out on the track and at different sun levels. No amount of calculations or simulations can be a substitute for this.
The file basically works by taking into account the solar panel output at full sun, the drive wheel diameter, car top speed and some motor characteristics.
It then uses this data to calculate the wheel speed (rotations per minute) at top speed as well as the motor rpm required for the panel to be loaded at max power. The ratio between the two rpm is then determined and this gives you the gear ratio.
For those interested, formulas can be followed by highlighting the relevant spreadsheet cell (mouse clicking in the function field at the top will colour code cells used in the formula).
Please note that teams with solar panels that have a high max panel voltage (say more than 18V) would be advised to limit their spreadsheet input to perhaps 17-18V and then use this gear ratio instead. This is because the 2232 motor speeds at voltages greater than this begin to become quite high (20 000 rpm +) and the rapidly increasing no load rotational losses of the motor start to drop its overall efficiency (particularly if using a lower power solar panel).
Model solar car steering and trolley wheel brackets
Even though many model solar car wheels are made from a low friction material, cars that don't have steering will ultimately experience more drag when negotiating the corners of a figure-8 model solar car track (as seen at the following link on solarfreaks).
model solar car corner drag
This additional rolling resistance may not be extensive but remember that every little bit counts in model solar car racing, especially at the lower sunlights.
It is true that many cars without steering will defeat cars that do and it is common to hear things like "car so and so without steering beat car so and so with steering and so it doesn't matter", but the simple fact of the matter is that steering will reduce car cornering losses.
The reason why a car without steering can win and be a very good car is because there are so many other areas that affect car performance, many of which are far more influential than steering. Add steering and the same car would however be even better.
The advantages of model solar car steering are most evident in lower sunlights where the power from the solar panel is much lower. This holds true due to the fact that steering losses remain constant across all levels of sunlight. In other words, a larger percentage of panel power goes towards overcoming these steering losses at low sun when it could otherwise be used to make the car go faster.
This being said, some improvement in vehicle performance can even be seen in mid to high sunlights.
Even though some complicated steering mechanisms have been seen over the years, trolley wheel steering is typically used. This is both simple and effective as the wheels follow the track around the corners rather than resist them (resulting in little or no steering drag).
Photos of cars from past events will give you a good idea of various trolley wheel designs. You may also notice that some cars only possess font-wheel steering while others have all-wheel steering (including the drive wheel). Of these, the all-wheel trolley designs are most efficient given that any wheel that doesn't steer will need to be dragged around the corner.
Perhaps the simplest trolley wheel design uses U-shaped aluminium extrusion.
The use of this channel means that only cutting and drilling are required throughout the manufacturing process and this makes things easier for teams.
Sheet metal bending, as is required by other designs, is completely eliminated and this can only help to improve build accuracy (as bending will not only add extra work but also make it a more difficult to keep everything nice and square).
Although most aluminium suppliers will stock, or can order in, a good range of U-shaped aluminium extrusion, places like your local Mitre 10 should supply several varieties.
Mitre 10's most common sizes are their 10mm x 10mm, 12mm x 12mm and 16mm x 16mm U-channels (the 12mm and 16mm channels are pictured below).
All these have a wall thickness of around 1.5mm and come in lengths of over 2m for less than $10 (which is a reasonable price given that enough brackets for a whole range of cars can be made from a single length).
Teams must then simply cut this channel to shape with a metal band saw or hack saw and use a file to clean up the edges and any burrs.
Note that some of the U-channel section should be left when the cutaway is made (leaving a shallow "U") as this will greatly increase the strength of the bracket (see below).
As seen above, a 3mm hole is also made (using a 3mm drill bit and drill press or cordless drill) for attaching the wheel. The following photos show the brackets fitted with a SolarMPPT wheel.
Be sure to clean up any aluminium lips or burrs that result from the drilling or otherwise the wheel may not sit square to the bracket. The exact location of the hole and length of the bracket itself can vary although these will be largely dependent on the size of the wheel being used. Just make sure that there is a few mm clearance between the wheel edge and bracket.
The final step is to then fix this bracket to the car. This is usually done by fixing the bracket onto some form of axel using a pin or screw/bolt where the wheel is then allowed to swivel around this point.
Many Tasmanian car teams use axels made from square or rectangular lengths of timber which can be sourced from local RC hobby shops, but other materials can also be used including carbon fibre.
Note that a single carbon fibre rod on its own will probably not make a fantastic trolley wheel axel (particularly on a heavy, high powered car) due to its relative weakness to torsion (twisting).
The following example shows a 7mm thick wooden axel with 1mm sections of lite ply glued to the top and bottom to thicken it to 9mm which is the inside size of the 12mm x 12mm Mitre 10 U-channel (ie 12mm minus two lots of 1.5mm). Although a 10mm x 10mm U-channel bracket could instead have been used here, this wouldn't have left any room for the lite ply which strengthens the axel against splitting and other damage.
Please note that particular care needs to be taken when drilling the hole for the swivel pin and fitting the wheel to the bracket. It is crucial that every effort is made to properly align the pin and wheel as otherwise you will be adding rolling resistance to the car. This is because a wheel that is not correctly aligned will want to continuously try and line itself up with the pin, causing it to drag as a result.
Bad alignments will see there not even being a point of having steering and can quite readily make things worse than on a car that has no steering to start off with. This is due to the fact that wheel drag is then not just seen around the corners but along the entire track, even on the straights.
(Front trolley wheel arrangment)
Whether you drill the hole for the swivel pin first and then use washers to line up the wheel or fix the wheel to the bracket and then mark where to drill the hole, it doesn't matter as long as it results in proper alignment at the end. Just make sure that you also take into account where the hole needs to go on the axel. A distance of perhaps 5mm to 6mm in from the end would be advised and this would mean that the hole on the bracket needs to be around 8mm in from the outside of the bracket (if made from the 12mm x 12mm extrusion). This leaves 2-3mm for the 1.5mm wall thickness and a small gap at the end of the axel to allow the bracket to swivel. In fact, a right-sized swivel gap will limit the outswing of the trolley wheel and take care of the "at no time may any part of the car extend sideways more than 190mm from the centre of the guide rail" rule in section 8.3 of the car regulations.
Although only non-drive wheel steering has been considered so far, a similar arrangement is also used for the drive wheel.
The following example shows the rear wheel drive setup used by the New Town HS cars in 2011.
Here, the motor has been fixed to a 16mm x 16mm Mitre 10 U-channel bracket and then a custom-made wheel fitted with a 72 tooth KHK stock gear used (as discussed in the gear section up the page). As this was the first year of competition for the students involved the gear ratio was kept constant for simplicity.
Be sure to notice that the wheel has again been lined up with the pivot point and this is the reason that the larger U-channel has been used. The 12mm x 12mm extrusion would not have had a deep enough channel for a correctly aligned hole to be drilled for the pivot pin.
Things can get a little more complicated when using an RI drive wheel and gears since these are thicker and the brass pinion gears longer. This means that even the 16mm x 16mm channel is no longer deep enough and something deeper would need to be used. This may be more difficult to source (if it in fact exists) and so some form of spacer may alternatively need to be used to push the motor back from the bracket as can be seen below (image yet to be included).
Guide roller brackets
Guide rollers are typically fixed to the front and back of a model solar car in a way that allow them to withstand a considerable amount of force. This strength is critical as the force imparted on the guides on a good solar car during a race can quite easily exceed what the regular wheels experience.
For example, let's go back to school and consider a 1 kg model solar car travelling around a 5m radius track corner at a top speed of 8m/s.
Using Newton's second law of physics of F = ma, the centripetal force experienced by the car (the force of the guide channel on the car guide rollers) can then be calculated as being equal to 1kg x (8m/s x 8m/s) / 5m = 12.8 Newtons of force (where the acceleration, a, is given by velocity squared / radius).
In comparison, the weight force experienced by the regular car wheels equals 1kg x 9.8m/s/s or 9.8 Newtons of force (also calculated using F = ma but this time with an acceleration of 9.8m/s/s, the acceleration due to gravity).
Not only is this lower but it is also spread out over 3 or 4 wheels (depending on how many wheels the car has). With the guides rollers, only 2 are engaged on each corner (these are the rollers at the front and back of the car that are on the inside of the curve).
Anyway, so as has been seen, the part of a model solar car that the guide rollers are attached to is particularly important.
To take care of this, a lot of teams fit the guides directly to the car frame. This option however doesn't necessarily translate to all car designs and so sometimes the guide rollers need to be fixed to model solar car frame via a guide bracket. This is common on 3-wheeled designs at the single wheel end (this is usually the rear drive wheel end) where many cars use a basic T-shape frame (or similar) and then have nowhere to attach the rear guides (unless some kind of bracket is fitted to the car). Several examples of such a bracket can be observed below and these are usually made from aluminium sheet metal.
(Example brackets from past cars, L-R: 2004 New Town HS, 2006 Hobart college, 2007 Rose Bay HS, 2010 Rose Bay HS, 2010 St Patrick's College)
Numerous bracket designs have been seen over the years, some easier to manufacture than others. As seen above, many of these brackets tend to include folds so as to remove the need to use long and thin screws for getting the guides close enough to the track. Perhaps a simpler design worth considering is as follows.
These particular brackets are made from 2mm thick aluminium and require no sheet metal folding, only cutting and drilling. Compared to other designs this eliminates an extra step in the manufacturing process and, more importantly, reduces the risk of the guide wheels ending up at angles to the track surface (which can arise due to inaccurate folding).
The aluminium used for these guide brackets can be acquired from an array of sources. Local sheet metal suppliers are usually one of the lowest cost options out there and offcuts of various thicknesses can often be picked up for very little or even no cost at all.
In order to lower the rollers to the correct height above the track some aluminium blocks can be used. Those pictured below have been cut from 4mm thick aluminium although the exact thickness needed will vary from car to car.
For safety's sake, a thinner set of blocks ought to be made up in the event that the guides need to be raised by more than the thicker spacer will allow.
Small adjustments for fine tuning are then typically made by adding or removing washers.
On well-aligned tracks that posses a gradual overpass slope (such as is the case with the NSW/national track), the guides can generally be set somewhat lower than on others which are not so smooth.
To give a car the best chance of staying on the track, guides should be lowered as much as possible without any chance of scraping or catching any mismatches.
A gap of around 4mm is typically a good starting point for the distance between the bottom of the rollers and the track surface.
Drag plate, aerodynamics and downforce
The new AIMSCC regulations for 2012 require cars to carry a removable 150 square cm drag plate made from at least 1.2mm aluminium sheet. This plate can be any shape and have any number of cutouts or holes as long as it meets the required area (note that holes and cutouts are not counted as being part of the plate area).
For teams unfamiliar with the term "drag plate" the clue is in the name. The purpose of this plate is to make all cars have a particular cross-sectional area (at least 150 square cm) at some point along the vehicle. This then ensures that a certain amount of air drag is experienced (hence the drag plate name).
What this all means is that the drag plate must sit vertical and across the body of the car when it is racing (transverse to the direction of travel). A plate that sits at non right angles or lengthways in the car does not fit these criteria (in fact a 1.2mm plate sitting lengthways in the car will offer barely any wind resistance at all).
As an example of what is allowed consider the following 2 rectangular drag plates. The first is 30cm wide (remember that the max car width allowed is 32cm) and the other 15cm wide. In order to then reach the 150 sq cm area, the first of these plates would need to be 5cm high and the other 10cm high. Both plates are perfectly acceptable yet completely different.
Please note that if either of these plates were to have any cutouts or holes then this would need to be accounted for. In this kind of situation the outside plate dimensions will need to be made slightly wider or higher.
Odd-shaped drag plates (where the area is not easily calculated by the scrutineers in a short period of time) may be check-weighed on a set of scales to ensure that correct area has been met.
As a reference, a 150 sq cm aluminium drag plate that is the minimum 1.2mm thick will weigh just under 50g.
Given that the drag plate will need to be carried by the car when racing, teams should aim to construct this so that it is as light as possible (ie 150 sq cm in area and 1.2mm thick). Not all suppliers however stock aluminium at this thickness and so the next size up (ie say 1.5mm) may need to be used (this will equate to roughly an extra 10g).
Anyway, so moving on, this drag plate now presents teams with an aerodynamics design challenge.
Nothing in the rules states that a car body is required and so the plate can simply be attached to the frame if so desired.
With nothing to push the air around the plate or limit flow separation and turbulence after the plate, the resulting aerodynamics of such a design would however be poor. Simply put, this will then translate into a lower top speed and slower car. In fact, air drag is the single biggest opposing force that a good model solar car will face at top speed and so streamlining is critical for optimal performance.
The difference between having good and bad aerodynamics on an otherwise soundly designed car can sometimes be up to a full second over a 1 lap race in good sunlight (this equates to around 7m - 8m out on the track).
So what can be done to improve the aerodynamics of a car? Simple. Construct a body around the drag plate.
Remember to keep the design nice and streamlined and don't just concentrate on the front of the car which is cutting through the air. The rear is just as important and square ends will disrupt air flow and lead to flow separation & turbulence.
A quick internet search on the topic of aerodynamics will uncover a vast amount of material on the matter. After going over just some of this, one thing tends to become clear and that is importance of streamlining.
Streamlined bodies such as tear drops and aerofoils possess the smallest coefficients of drag and so generate the least amount of air drag for a given cross-sectional area or width/diameter (as seen in the following diagrams).
World Solar Challenge entries are a great example of what to look out for and so several photos have been included below.
Due to the very limited supply of power, all these cars need to be highly streamlined and teams spend huge amounts of time and money designing, simulating, testing and fine tuning the aerodynamics of their vehicles.
The following selection of model solar cars shows some of the more aerodynamic designs that have been seen over the years.
(Low air drag cars seen over the last decade, L-R: "Helios" BHHS 2002, "Nigresta" BHHS 2009, "The Weapon" New Town HS 2011, 2009 Test Car, "Excelsior" BHHS 2008)
Depending on the specific material/s used for the model solar car body, 3D compound curves such as those seen on the second and fifth cars above can often be more difficult and time consuming to produce.
To this end, many teams therefore tend to concentrate on 2D shapes that either mainly push air around the sides of the vehicle, as is the case with the fourth car, or over and under the vehicle, as is the case with the first and third cars.
Also notice that similar to the World Solar Challenge entries, a number of the model solar cars above are designed in such a way that enclose some (if not all) of the wheels within the body. Wind Tunnel tests conducted by Mr. Ian Gardner on various model solar cars in Victoria have shown this to reduce air drag (by a considerable amount in some cases).
Another area that potentially needs to be looked at in terms of air flow is aerodynamic lift or down force. This hasn't been much of a problem in recent years since minimum car weights have been close to 1kg and floor areas quite low, but the regulations in 2012 now allow for lower power panels and possibly some very light weight cars. For these, getting too much air underneath the vehicle could prove disastrous and see it lose control at top speed.
Don't specifically go designing for down force and concentrate on it too much though, particularly if you're running with a high powered panel and need to carry a lot of ballast.
Rather, try to simply eliminate any unnecessary lift from the design. Excessive down force resulting from air flow over and/or around the car will just increase its overall aerodynamic drag and prevent it from reaching a higher top speed. As an example of this, consider a present day Formula 1 race car where these possess an adjustable flap on the rear wing known as a drag reduction system or DRS. This flap is driver controlled and, when closed, gives cars more down force during cornering. The flap is then opened to reduce air drag on faster sections of the track (ie the straights).
Remember, down force will place more force on the wheels and increase a model solar car's rolling resistance.
Car designs, chassis, bodies and stability
Many different solar car designs will work and evidence of this can be found by taking a look at successful entries in years gone by.
(AIMSCC winners from the past 5 national events, 2007 - 2011, L-R)
3 and 4 wheeled cars, cars with either front or rear wheel drive, cars with and without steering, etc, have all won state and national events so what then makes a good model solar car?
The answer to this question is really quite simple. See, it all revolves around minimising vehicle losses and optimising the conversion of energy from solar panel to car motion over the course of a model solar car race. To fully achieve this, every aspect of car design needs to be considered and that means that no area can be overlooked.
Having said this, even cars with plenty of room for improvement in certain areas can still perform up to a reasonable level. The reason for this can typically be put down to how well the team has addressed a couple of key areas. These areas usually account for the largest losses in performance and so should really be concentrated on before anything else.
The first of these key areas is model solar car build quality.
This is often the stand out difference between a good and not so good solar car and scrutineers regularly see this to hold true at events each year. Poor build accuracy and imprecise alignment of gears, wheels, guide rollers, etc, will not allow any car to perform up to its full potential, no matter how good the design is in other areas. In extreme cases, where combinations of bad misalignments exist, the difference can extend out to a number of seconds over a 1 lap race.
The second key area concerns the power transfer from solar panel to drive wheel during a model solar car race. That is, the correct combination of solar panel, electronics, motor, wheel size and gear ratio need to be selected. Get this wrong and even good build quality can't save you.
When both of these areas are taken care of then teams will already be well on the way to building a good solar car. Beyond this, things like car aerodynamics, weight, wheel configuration, etc, all start to come into the picture and will end up being the difference between a good and very good car.
Teams in Tasmania usually opt to go for a 3 wheel design with all-wheel trolley wheel steering (since this is most efficient in terms of rolling resistance).
Bearing this in mind, there are then still a number of different wheel configurations to select from (ie 2 wheels at the front and 1 at the back, 1 at the front and 2 at the back, front wheel drive, rear wheel drive, etc). Of these, having 2 wheels at the front at maximum width will be most stable.
Although any of the 3 wheels can be driven, the single wheel at the rear of the car is then usually made the drive wheel. This is because it offers the most traction and will remain in contact with the track at all times. Cornering at high speeds has seen cars go around the bend on just 2 wheels and this means that drive traction can be lost for short amounts of time if the drive wheel is at an end that has 2 wheels (ie at the 2 wheel end on a 3 wheel car or either end on a 4 wheel car).
So taking this all into account, a 3-wheel all-wheel trolley wheel steering design that is rear wheel driven and has 2 wheels at the front and 1 at the back is commonly used by Tasmanian schools. In fact, 8 of the top 10 cars at the 2011 national event used this type of design.
Most Victorian schools and quite a few in NSW and WA go for a 4 wheel fixed wheel design similar to the "Photon Cruncher" example car seen in Ian Gardner's design hints file. The reason for this is that these are probably the easiest to build (given that virtually all components can be purchased in a kit from RI in Victoria).
Although these cars may not be quite as efficient, they can still be made to perform to a very high level and even win events if built properly.
Many 3-wheel cars with trolley wheel steering use a basic T-frame chassis design and an example of this can be seen below (please note that the rear guides and drive wheel bracket have yet to be properly attached to the frame here).
This particular example uses SolarMPPT front wheels and RI guides rollers + rear drive wheel and, as can be seen in the photo, weighs just 280g including motor and Easymax III. Using balsa wood, foam or some other lightweight material for the body, a total car weight of somewhere between 300g and 400g is then easily achieved (not including solar panel and ballast).
Not only is a T-frame very easy to construct, but the design also allows for a whole host of different car bodies to be built around it. For example, the following two cars from New Town HS in 2011 use the same T-frame chassis yet are quite different from one another.
Notice that these cars have all-wheel trolley wheel steering and that the rear drive wheel is aligned with the centre of the car. Here, the guide rollers therefore need to be offset to prevent the drive wheel from running along the inside of the guide channel.
This type of design was first seen on Clarence HS' 2002 national runner-up and has been commonly replicated to improve the balance and stability of 3-wheeled cars ever since (given that aligning the guide channel with the centre of the car would instead require the central drive wheel to be offset slightly).
Be sure not to offset your guides by too much though. Remember, section 8.3 in the regulations states that "at no time may any part of the car extend sideways more than 190mm from the centre of the guide rail" and so this needs to be taken into consideration.
For example, if you have a car that is the full 32cm wide then half of this is 16cm. Offset this by more than 3cm and the car will not comply.
Also remember that the 190mm includes any possible outswing from trolley wheels (this includes the wheel, bracket, screw head, thread and nut) and so the offset can only really be more like 2-2.5cm (if the wheels are at full width).
On the other hand, if the offset is too small then the central drive wheel may get too close to the guide channel and cause problems.
The example T-frame above has an offset of around 22-23mm. This comes as a result of lining up the more central 25mm diameter RI guide rollers (front and back) with the centre-line of the car. In total, the 22-23mm offset comes from the roller radius (ie 25mm/2 = 12.5mm), half of the guide channel width (ie 16mm/2 = 8mm) and around a 1.5-2mm gap on either side of the guide channel.
All up, the distance between RI roller centres here is around 44-45mm (ie 2x 12.5mm roller radius + 16mm wide channel + 2x 1.5-2mm gap) and this leaves about [190mm - (160mm + 23mm + 2mm)] = 5 mm for trolley wheel outswing (the 2mm gap between the channel and roller must be included here since the car is pushed against the channel to engage the rollers at scrutineering).
Given that 5mm for steering outswing is perhaps cutting things a little fine, the front wheels on the frame above have been brought in slightly so that the pivot pins are separated by around 29.5cm (with the axel being about 30.5mm wide).
Before moving on, note that teams constructing a 3 wheel car are not limited to a T-frame chassis. A-frames and numerous other designs can be used as long as the wheels can then be attached in the basic tripod configuration at the end.
Various materials can be used for model solar car framework and structural bits, some better than others.
Close grained, lightweight timbers such as beech, spruce and radiata (used in the earlier T-frame example above) are well suited to the task. These can be sourced from most local RC hobby shops and are easy to work with and glue.
Round carbon fibre arrow shafts are also regularly used, particularly on fixed wheel designs using RI gear. Here, as mentioned previously up the page, wheels and bearings are pushed onto carbon fibre axels and then held in place with retainers via a grub screw. Having the wheel centres in line with the axel then mean that there is no real torsion present.
The story is however a little different if trolley wheel steering wants to be incorporated (given that an ordinary carbon fibre rod will then either need to be reinforced to prevent it from twisting or have the amount of twist accounted for when the car is fully ballasted).
Perhaps the other main downside to using carbon fibre arrow shaft is that it can be more difficult to work and join than timber. Gluing and attaching other components or a body to its round surface can also be tricky at times.
This being said, carbon fibre lengths with square, rectangular and other cross-sectional areas can sometimes be found at hobby shops and these may be easier to work with and offer better resistance to twisting.
Aluminium is another material that may warrant some consideration although it is typically too heavy for anything other than trolley wheel and guide roller brackets and motor mounts.
To be continued ...
To make them as effective as possible, model solar car bodies are constructed from a light weight material to maximise acceleration and minimise rolling resistance. Three of the most common of these include balsa/liteply, polystyrene/styrofoam/any other type of foam or thin folded/vacuum formed plastic. Some examples of these are given below:
(Some past car bodies consisting primarily of balsa and/or liteply)
(Some past car bodies consisting primarily of a form of foam)
(Some past car bodies consisting primarily of a thin plastic shell)
2012 Example Model Solar Car
Below are some images of an incomplete example car that has been constructed according to the information listed on this page. More on this to come ...
(Applying a balsa body to the T-frame example seen previously up the page)
(Incomplete 2012 example car fitted with components)
To be continued ...
Model Solar Boat Help
Until any more information on building model solar boats is added, here is a little something about solar boat panels and solar panel wiring that may assist teams in the meantime.
In order to construct a competitive model solar boat it is imperative that the weight of it is kept to an absolute minimum. This means using the very lightest parts and materials.
In many cases the solar panel is likely to be the single heaviest component of a model solar boat and so requires particular consideration. Using glass and resin encapsulated solar modules as is the case in the car challenge is out of the question. These are simply far too heavy to even contemplate using. What is instead needed is a special type of high energy density solar panel.
In the early days of the model solar challenge, top boat teams would make up their own custom-made high energy density solar panels from bare solar cells and attach them to a light weight backing. This practice has been previously discussed under the bare solar cell section up the page, where a couple of suppliers of pre-cut bare cells are also listed.
For example, the panel below was made back in 2006 from 27 x WD40 Plastecs solar cells on a polystyrene base. This panel weighs in at just under 50 grams and belonged to the fastest boat at the 2006 nationals. It was wired up with 3 strings of 9 cells in parallel to give an output of around 4.5V @ 1.2A.
(A custom made boat panel using bare Plastecs solar cells)
Although making up a custom-made panel does allow for a greater selection in voltage/current output, a far easier path to take these days is to simply purchase a boat panel from Scorpio Technology. These panels have been designed specifically with the model solar boat challenge in mind and take into account the 350 square cm cell area rule. Scorpio now also uses high efficiency solar cells which will put out somewhere between 0.5 and 1W more power than the Plastecs cells. To offer the cells a reasonable level of protection they are encapsulated in light weight fiberglass and Scorpio includes solder tabs to make electrical connections very straightforward.
In summary, these panels weigh around 50g and are nearly impossible to better from a weight and power output standpoint. As such, they are highly recommended to anyone thinking about competing in the boat event. The latest panels have an output of around 7V @ 0.85A, giving close to 6 watts of power, and are also well-matched to an array of motors. Perhaps their only downside is that they cannot be used in conjunction with most commercial solar challenge maximisers like the Automax, Easymax III or Engelec Max 4 (these aren't needed if a boat has been set up properly for the conditions it's running in). Even then, Scorpio sell a special low voltage maximiser kit for $10 that can be used in combination with a single boat panel.
Given their superiority, the popularity of the Scorpio boat panel has increased dramatically over the last 5 years to the stage where virtually every boat team around the country now has one. To this end, let's take a look at how teams should go about wiring them up.
The new Scorpio boat panels come in two sections of 7 cells each. Hold a panel up to the light and you will see a small gap separating the individual cells from one another. Each section of 7 cells then has 2 tabs (a positive and a negative) for soldering wires onto.
(Scorpio Technology's latest type of boat panel)
Now, no matter how big or small, the make up of a single silicon solar cell gives it a voltage of around 0.5 volts. Changing the cell area will only affect its current output, where large solar cells can put out several amps and small ones only a few milliamps.
Anyway, so each section of 7 cells is connected in series and has an output of around 7 x 0.5V = 3.5 volts (the current output of each section is around 0.85 amps in bright sunlight).
The two 3.5V, 0.85A sections on a Scorpio boat panel can then be connected together in either a series or parallel arrangement to give a total output of email@example.com or email@example.com. Since panel power is given by volts multiplied by amps, some quick arithmetic will show us that both of these configurations result in the same amount of panel power (ie 7V x 0.85A = 5.95W = 3.5V x 1.7A).
So which arrangement is therefore better? Well, this all depends on the type of motor being used, the sun level as well as the propeller.
In general, most motors used by teams in the boat challenge will perform better with the solar panel in series in bright sunlight. In lower light levels this same motor will then however operate more effectively with the two panel sections arranged in parallel. In some cases the motor won't even spin in low sunlights if the panel configuration is kept in series.
So, in other words, a Scorpio boat panel should ideally be set up to allow for both configurations to be used. If only they could be changed by simply flicking a switch ...
Well, it so happens that this is possible. In fact, with the right type of switch the panel can be wired up to allow teams to toggle between the two arrangements to their heart's content.
Although not overly complicated, it is this wiring that has often given students and teachers some problems over the years. As such, many avoid it altogether and only wire up their panels in a single configuration, despite there being all the necessary information online like in Ian Gardner's boat help file or further up this page in the car wiring section.
Fortunately for everyone, series/parallel wiring will now be discussed below using the latest Scorpio boat panels. This should then give teams the best possible chance of succeeding on race day, no matter the weather.
Firstly, consider the polarity of the solar panel tabs. As seen above, the new panels have 4 tabs between the two solar cell sections, where the two located further out should be +ve. The two inner tabs are then -ve. If you are in possession of another type of Scorpio panel and are uncertain as to the polarity of the tabs, use a digital multimeter to check the voltage across the panel sections (make sure that the red and black leads are in the correct sockets on the multimeter). If the multimeter then reads a positive voltage (ie no minus sign) then the +ve tab is the one that the red multimeter lead is in contact with. If it reads a negative voltage then the polarity is reversed and the +ve tab is the one in contact with the black multimeter lead.
Now that this has been sorted out, colour coding of wires can be put into effect. That is, all the same colour wiring can be used for each polarity to make things easier to follow. As mentioned in the car wiring section up the page, Dick Smith Electronics sell 10m rolls of red and black figure 8 twin conductor cable which is ideal for model solar car and boat wiring. Using red for positive and black for negative, this wire can then be soldered to the Scorpio panel tabs accordingly. How exactly the wires are soldered to the tabs and attached to the panel will be up to the teams however a reasonably tidy way of doing it with the 2011 boat panels is by drilling a couple of small 3mm or 4mm diameter holes between the 2 panel sections and pulling the wires through these before soldering them on. The free ends of the wires should then be tinned and prepared for soldering to either a switch or a plug.
Focus should now be turned to the switch. Here, teams will want to get hold of a six leg double pole double throw (DPDT) switch as seen a little further down the page in one of the below images. If teams can get hold of a 3 position on-off-on version like this one from Jaycar then this would be optimal as it will function as both a series/parallel and on/off switch all in the one. A 2 position on-on toggle switch version can also be used but however requires a second switch to perform the on/off function.
Soldering the free ends from the solar panel to the switch then follows and a full model solar boat wiring diagram is given below.
2 of the 6 switch legs must be connected to one another. This can be done with the same type of wiring used on the rest of the panel or alternatively with a short length of wire like that of a resistor or diode leg for example.
Once this has been carried out the 4 unused switch terminals will remain. To further assist teams with their wiring, the following diagram has been included to demonstrate connections in a more visual form.
(Scorpio boat panel wiring in combination with a on-off-on 3 position DPDT switch)
As illustrated by this wiring diagram, 2 more wires need to also be soldered to the switch in order to give an electrical connection with the model solar boat motor. For this, a plug is included so that the panel can be easily removed from the motor (and boat). Alternatively, a 4 leg plug or two 2 leg plugs could be inserted between the panel and the switch and then none included between the switch and motor. The first option would however be recommended though as this allows the switch to be fixed to the panel. The whole setup can then also be easily removed from one motor and boat hull and attached to another.
Whenever a 2 wire plug is needed, a micro deans plug would be recommended (as discussed in the car wiring section). These are very robust, compact and make it impossible to plug things in the wrong way. Because of this they are very popular among model RC enthusiasts and most local RC hobby stores will stock them.
Below are a few images of the finished product. The switch and wiring has simply been attached to the back of the panel with some tape.
(A Scorpio boat panel set up to switch between series and parallel using a on-off-on 3 position DPDT switch)
More model solar boat and sprint car design hints coming soon ...