Self-powered bicycle lights are nothing new. In fact, the traditional dynamo light has been around for years, and many cyclists still prefer to use dynamo-powered lights on their bikes for several reasons: these lights are always on a bike and always ready for action; as a result, there's no subsequent forgetting to put the light back on the bike, and there are no worries about the batteries unexpectedly giving out mid-ride. Dynamos, however, do have their own drawbacks. For example, friction-powered sidewall units slow a bike down and wear out a bike's tires, while dynamo hubs must be built into the wheel and add to the bike's revolving weight. In other words, dynamo lights take energy away from the momentum of the cyclist, who can feel a clear resistance when these lights are used.