Dynamo

From ChapmanCentral

A dynamo (generator in US parlance) is a device for generating electric current. There are three types of dynamo in common use on bicycles:

• Bottle dynamos use a small roller which is brought into contact with the tyre sidewall, which in many cases has a track provided to improve traction. The drive roller may be metal, plastic or rubber. The bottle dynamo has three advantages: it is cheap, easy to fit, and when not running, it adds no drag. It has three main disadvantages: drag is generally the highest of all dynamo types, tyre sidewall and roller both wear, and rain and other contaminants can cause the drive roller to slip and thus losing power to the lights.

• Bottom bracket dynamos run off the tread of the tyre rather than the sidewall. Like the bottle dynamo, they are moved away from the wheel when not required. They are generally somewhat dearer than bottle dynamos but less prone to slip in the wet and, because they run on the tread, they are less prone to reduce tyre life.

• Hub dynamos are built into a wheel hub (usually the front wheel). They are not vulnerable to slippage, do not reduce tyre wear and modern hub dynamos are generally the most efficient types of dynamo. The disadvantages are cost, and the fact that a wheel build is necessary in order to install them.

Dynamos all score over battery lights by being able to drive the lights indefinitely. They produce less power than most modern rechargeable battery systems, but compensate in part by use of more efficient optics. Dynamos produce no power when the bike is not moving. Lights can be kept running briefly using inbuilt capacitors (standlights) but they are not usually considered suitable for offroad riding.

The largest markets for hub dynamos in the UK are probably audax riders and commuters.