Rear-wheel gear mechanism on bicycle. Bicycle gears allow you to change the number of revolutions the rear wheel will make for every revolution of the pedals. This offers control over the amount of force needed to turn the pedals. The chain transfers the turning force from the pedals and chain wheel (not seen) to the rear wheel. The derailleur (centre, black) moves the chain between the various gear-wheels. If the chain sits on a small gear-wheel, the rear wheel will perform more revolutions for every turn of the pedals; if on a larger gear-wheel, it will perform fewer.