Bottle dynamo
bicycle dynamo mounted in such a way that its friction wheel can be driven by the sidewall of the tire
A bottle dynamo or sidewall dynamo is a small electrical generator for bicycles employed to power a bicycle's lights. The traditional bottle dynamo (pictured) is not actually a dynamo at all (which creates DC power), but a low-power magneto that generates AC. Newer models can include a rectifier to create DC output to charge batteries for electronic devices including cellphones or GPS receivers.
Named after their resemblance to bottles, these generators are also called sidewall dynamos because they operate using a roller placed on the sidewall of a bicycle tire. When the bicycle is in motion and the dynamo roller is engaged, electricity is generated as the tire spins the roller.
Two other dynamo systems used on bicycles are hub dynamos and bottom bracket dynamos.
Advantages over hub dynamos
No extra resistance when disengaged
When engaged, a dynamo requires the bicycle rider to exert more effort to maintain a given speed than would otherwise be necessary when the dynamo is not present or disengaged. Bottle dynamos can be completely disengaged when they are not in use, whereas a hub dynamo will always have added drag (though it may be so low as to be irrelevant or unnoticeable to the rider, and it is reduced significantly when lights are not being powered by the hub).
Easy retrofitting
A bottle dynamo may be more feasible than a hub dynamo to add to an existing bicycle, as it does not require a replacement or rebuilt wheel.
Price
A bottle dynamo is generally cheaper than a hub dynamo, but not always.
Disadvantages over hub dynamos
Slippage
In wet conditions, the roller on a bottle dynamo can slip against the surface of a tire, which interrupts or reduces the amount of electricity generated.
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