If you've ever been riding your bike at twilight and found that your lights don't work when you flip them on, the good news is that the situation, neither comfortable nor safe, could have been easily avoided with the right technology. Bicycle lights are available that don't require batteries, providing needed protection for night riding.
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The basic concept behind a battery-free bicycle light is to use the motion of the bicycle wheel to generate electricity. As the wheel rotates, it turns a dynamo of some kind, which acts as a generator. The dynamo produces an electric current that is then delivered to the lights. The faster the wheel moves, the more power is generated.
Direct Contact Dynamos
Typical battery-free lights consist of the metal housing containing a dynamo. An adjustable bracket attaches the housing to the bicycle frame. At the other end of the housing is a roller, attached to the spindle within the dynamo. The roller lies against either the side wall or outer surface of the tire, depending on the design. As the tire turns, it will move the roller, which turns the dynamo, generating electrical current. Wires from the dynamo deliver the electric power to lights. The light gets brighter as the wheel turns faster.
In hub dynamos, the dynamo is an integrated part of the front wheel axle and hub. As the wheel turns, the dynamo moves within the axle, generating electric power. Wires leading from the hub dynamo connect to lights. Because the dynamo is built directly into the axle, resistance is significantly lessened. Hub dynamos tend to be more expensive and require specialized installation on the front wheel.
Magnetic Induction Lights
Some battery-free lights were designed based on the principle of dynamic induction, which explains how a magnet, turning within a wire coil, generates electric current. This light uses special, powerful magnets attached to the spokes of the bicycle wheel. The light itself has its own separate magnet, mounted to the bicycle frame. As the wheel moves, the magnets on the wheel closely pass by the magnet attached to the frame, causing it to briefly turn within a wire coil. An electric current is generated that makes the light flash. As the wheel turns faster, the light flashes more frequently.
Pros and Cons
Both direct contact dynamos and hub dynamos provide near constant light when the bicycle is moving. However, because direct contact dynamos actually come in contact with the tire, they can slow the bicycle down. Direct contact dynamos can also cause increased wear on the tire and may slip in wet conditions. Hub dynamos do not suffer from the same disadvantages as they are water proof and self-contained. Induction lights do not provide a continuous beam of light, instead flashing periodically. Because there are no moving parts, these lights and the tires on your bicycle tend to last longer. Also, no direct contact means no resistance on the wheel to slow the bicycle down.