An electric generator is a device that converts mechanical energy to electrical energy through the use of electromagnetism. There are multiple styles of bicycle generators, but all bicycle generators use a dynamo and magnet to produce energy converted from the forward motion of the bicycle that in turn powers a headlight or a head and tail light system. Traditionally, these generators produced little power and weak lighting, but recent improvements in technology are beginning to advance generator power to the forefront of bicycle lighting once again.
Never Quite Right
Traditionally, generator lights mount somewhere on the bicycle and constantly contact a moving surface on one of the wheels. The old-style generators caused an enormous amount of drag and slowed the progression of the bicycle while emitting a weak front or front and rear light. The old bulbs were known to fail prematurely when the bicycle was ridden quickly downhill. The lights were barely bright enough to see and ceased operation when the bicycle stopped moving, rendering lights useless when halted at stop signs and traffic lights.
Stay Mounted Generators
The most common generator is the stay mounted or "bottle" style, according to the late Sheldon Brown, a cycling authority. These generators mounted to a seat stay or fork stay and were shaped like bottles. The cap end of the bottle contacted the tire sidewall, and as the tire spun it spun the cap and turned the dynamo past a magnet thus sending an electric signal to the light system. It is possible to "turn off" this style of generator by moving the bottle so the cap no longer contacted the tire.
Bottom Bracket Generators
A bottom bracket generator mounts under the chain stays where a kickstand is normally mounted. A spring-loaded wheel or roller is pressed against the tire, and as the tire turns, it spins the wheel and dynamo and sends an electrical signal to the light system. It is possible to turn off this style of generator by locking the spring-loaded wheel away from the tire.
Wheel Hub Generators
Hub dynamo generators are usually built into the front hub of a front bicycle wheel. As the wheel spins, it turns the internal dynamo attached to the hub shell past the magnet attached to the fixed--nonmoving---axle thus sending an electrical charge to the light system. Hub dynamo powered lights are possible to turn off, but the dynamo itself is always on and thus always exerts a drag.
The Generator Renaissance
Recent technological innovations have resulted in many bicycle parts manufacturers building efficient hub dynamos essentially the same as the older style except the new systems are able to turn off and reduce drag during the day. These dynamos are being paired with energy collecting and storing battery packs that enable bicycle generator powered lights to emit constant light, remain bright and not switch off when stopped. This recent development has managed to make generator lights a valid option next to expensive rechargeable lights and battery operated lights that can run on rechargeable batteries.
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