Bottle dynamo

A bottle dynamo or sidewall dynamo is a small electrical generator for bicycles that is usually used to power lights. However, being a simple electrical generator, it can be used for other electrical applications, such as charging batteries for small electronic devices, like cellphones or GPS units [http://www.treehugger.com/files/2005/03/pedal_power.php] . Dynamos of this type are referred to as bottle dynamos because they have the appearance and shape of a bottle. They are also referred to as 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 roller is spun by the tire. 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 disengagedFact|date=May 2007. Bottle dynamos can be completely disengaged when they are not in use, whereas a hub dynamo will always have added dragFact|date=May 2007 (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 hubFact|date=May 2007).

* 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.

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. In turn, this can cause the lights on the bicycle to go out or become inconsistently lit.Fact|date=May 2007 This is not a problem for hub dynamos, as they are sealed from the elements.
* Increased resistance: Bottle dynamos typically create more resistance than hub dynamosFact|date=May 2007 though it may be so low as to be irrelevant or unnoticeable Fact|date=May 2007, and there is no resistance when the bottle dynamo is disengaged.

* Tire wear: Because bottle dynamos rub against the sidewall of a tire to generate electricity, they cause added wear on the tire. Hub dynamos do not function this way and thus do not cause wear.

* Noise: Bottle dynamos make an easily audible mechanical humming or whirring sound when engaged. Hub dynamos are silent.

* Switching: Bottle dynamos must be physically repositioned to engage them, to turn on the lamps. Hub dynamos are switched on electronically. Hub dynamos can be engaged automatically by using electronic ambient light detection, providing zero-effort activation.