   Why is Pacejka not related to speed?
  I'm trying to implement the Pacejka formula into my racing sim. I'm struggling to understand why the lateral force formula is not related to speed. The only inputs are the a0..a13 constants for the tyre, the slip angle and the camber. What i don't understand is how the same force can be generated at 1mph as at 70mph?I've looked through this forum, read the physics of racing series and read through the racer.nl documentation. Unfortunately, this hasn't helped.On a side note, I've plugged in the formulae given by Brian Beckman, and from the 'racer' source code (which give different results) and neither get me a force of 0 with a slip of 0. I found a paper referenced on this site which does, but again the formula is completely different to the other two. Can any one suggest a definitive (as much as can be for a formula like this) somewhere?   
  I think it takes lateral and longtitudal velocity as input, so it is related to actual tire velocity / path. ;  Hmm I must be missing something then. Looking at the formula on Beckman's pages: http://www.miata.net/sport/Physics/Part22.html which only takes the slip angle, the camber and the Pacejka constants. Any suggestions where velocity comes in?   ;Quote:The horizontal axis measures slip angle, in degrees. The vertical axis measures lateral, cornering force, in Newtons. The deep axis measures vertical load on the contact patch, in KiloNewtons.I've just skimmed through the docs, but it seems it does take into account the amount of load on the tire, the slip angle, and gives you the lateral force.Quote:As a final comment, let me say that I am somewhat dismayed that the magic formula does not account for any variation of the lateral force with speed. Intuitively, the forces generated at high speeds must be greater than the forces at low speed with the same slip angles. However, the literature-sometimes explicitly, and sometimes by sin of omission-states that the magic formula doesn't deal with it. Hmmm, something to chew on...   ;  Yup you're right, I forgot the load on the tire. Thanks oliii I hadn't spotted that comment.Has anyone had experience dealing with pacejka at low speeds. Perhaps  the forces cancel each other out so as to not make it unrealistic?   ;  I've implemented Pacejka quite succsesfully for games and demo's.Here is one example.  Plug, plug.http://www.reflexive.com/AutocrossRacing.htmlI've read through Brian Beckmans papers and coded his formula.But i did find an error in his calculations, which i e-mailed him about.Unfortunately i never got a reply back.This was maybe 3-4 years ago.I can't remember if it was the lateral or long formula either.I ended up porting across the Racer C++ Pacejka source, as it's shown to be working code and freely available.And i too got different results from the Beckman formula to the Racer code.I found in practice, that the procedure of reducing lateral velocity via the lateral force returned from pacejka gave pretty decent results suitable for a game.I'm not entirely sure if lateral velocity actually has much of an effect on the force returned in real world situations.  Different material compounds having different friction properties.And if i remember correctly, the coeficient of friction is a constant, dependant on the materials causing friction.ie, once an object is moving and causing friction, the friction returned is a constant ( coeficient of friction ) depending on the material of the object.And it's something i've never found a definitive answer too.So, my assumptions there could be in error.With the inputs Slip angle, load and camber, the Slip angle is by far the most important variable for returning force.  I suspect that any variation in force due to velocity would be negligable.And of course longtitude Slip is dependant on longtitude velocity over angular velocity.The A and B Pacejka coeficients define the properties of the tire and are constants not inputs.But as i've said, for my needs, game dev, it hasn't been a problem.KJM   ;  Thanks for that KJM!Perhaps that's what I need to look at. By "procedure of reducing lateral velocity via the lateral force returned from pacejka". Do you mean the combination methods described in part 25 of the physics of racing series and the bottom of racer.nl's pacejka page?btw, your link doesn't seem to point to your game :-P   ;  Ahh, i forgot to mention, that i didn't use a combination method of keeping the longtitude and lateral forces to a friction circle / elipse, cap of maximum forces.I just handled mine seperately.  And it all seemed to work well enough.The lateral pacejka force acts in the opposite direction to the lateral world velocity of the wheel, thus slowing the tires lateral velocity down.It pushes against the wheels direction of travel to slow it down.I found i needed to scale the force by some arbritary amount in order to find the correct value needed to slow the lateral velocity.  This was basically because the mass of my car was not based on real world data.I would advise writing a small routine similar to the racer pacejka player, to plot the pacejka curves across the screen.  If you havn't already done so.To verify that the pacejka code is returning proper forces and curves.KJM   ;  Thanks for that, I think I'm a bit clearer on the concept of pacejka and it's mostly going to be a case of getting the implementation working properly now :)   ;  I also came across a good source for Pacekja formulae. The Giancarlo Genta book Motor Vehicle Mechanics has a few sections on the magic formula and has tons of detail. I found a preview of it on google books. It's pretty hardcore as it's written for car engineers, but a good place to check the formulae.Its probably a safer bet than following physics of racing or racer source code :)   