Known in the US as a sidewall dynamo, the bottle dynamo is a small generator which runs off the sidewall of the tyre or occasionally the rim to power the lighting system. Named after their resemblance to bottles they work by using a roller at the top (like the lid of the bottle) which is spun by the turning wheel when it is placed against the tyrewall. Obviously, if the bicycle stops then the dynamo ceases to provide power to the lights - but more of that later.
Unlike the hub dynamos which work inside the hub of the wheel and which provide a small and constant resistance, the bottle dynamo can be withdrawn from the wheel when not in use and therefore have no resistance. They are also generally cheaper than hub dynamos and can be easily retrofitted to a bicycle. There are some downsides though;
- They can slip in wet weather which can interrupt and reduce the amount of electricity being generated - the lights fade.
- Because of the fact that they are placed against the tyre, there can be quite some resistance which can be felt by the rider.
- They must be adjusted to the optimal position to function perfectly.
- They can be quite noisy.
- The metal top can cause the tyre to wear out prematurely through the sidewall so it is worth specifying a tyre with a heavy sidewall if a bottle dynamo is going to be used.
Dynamos are usually limited to 0.5 amps output - 3 watts at 6 volts - but because they are usually constant current and not constant voltage devices, it used to be that going too fast would burn out bulbs which were not designed for the higher voltages produced at high speed. More modern designs, however, have things called zener diodes which prevent this happening. Some of the best dynamo systems are manufactured in Germany (Busch and Muller and Schmidt for example); the reason being that in Germany it is illegal to light a bicycle with anything other than a dynamo based system unless the bike is deemed a 'racing' bike and weighs under 11 kgs.
As necessity is the mother of invention, even the problem I mentioned earlier about the age-old issue of dynamos producing no light when you stop, has now been eradicated. They are now made with a ‘stand light’ facility - usually a blue-white LED powered by a capacitor which can run for about five minutes without any power being generated. In fact, modern dynamo units now use a number of electronic devices to power LED lights of varying watts for front headlights, rear and stand lights.
Take a look at the Dynamo products available from Pedal-Pedal