(or sidewall dynamos) attach to the seatstay or fork and are rotated by a small wheel in contact with the tire sidewall. These are easier to retrofit than hub dynamos, but are prone to slipping, especially in wet conditions. Bottom-bracket dynamos are attached between the chainstays behind the bottom bracket and are powered by a roller against the rim. These are easy to fit and do not wear the tire sidewall. Other types of dynamos exist, but are less common. For example, there are dynamos that are attached to the fork and driven by the wheel's spokes. Output and optics Dynamos are generally limited to about 3 W of output power, although the best 12 V hub dynamos can produce 6 W at speed A bottle dynamo is likely to slip if run at twice the nominal power, a hub dynamo does not have this problem. At speed a dynamo can overpower the lamp, causing it to fail. Historically this was a nuisance, but modern lamps and dynamos often incorporate Zener diodes to prevent overvoltage, and dynamos can be designed to saturate beyond a certain voltage to protect the lamp. Good dynamos can achieve efficiencies of up to 70% (i.e., under 5 W of the rider's output is diverted to produce 3 W of electricity) and provide good light output at low speeds. To produce light when the bike is stationary, some dynamo lights have a stand light facility, usually a single blue-white (front light) or red (rear light) LED powered by a capacitor, which runs for around five minutes. Advantages of dynamo lighting Usage time not limited by battery life No recharging or change of batteries necessary More environmentally friendly and inexpensive to run than battery-powered lights Typically do not have to be removed when the bicycle is parked Disadvantages Provide no lighting when the bicycle is moving very slowly or is stopped, unless the system is equipped with capacitors or back-up batteries Maximum power output obtainable is lower than with batteries Difficult to remove by the owner if this is necessary to thwart theft or vandalism, or to use them on more than one bicycle Retrofitting a bicycle is more difficult because mounting is more difficult (due to lamp and power source being separate) Cheap or poorly aligned dynamos produce noticeable drag Old dynamos and lamps did not limit their output voltage, leading to early failure of the