Drivetrain

For more details on this topic, see bicycle gearing.
Shimano XT rear derailleur on a mountain bike

The drivetrain begins with pedals which rotate the crank arms, which are held in axis by the bottom bracket. Attached to one crank arm may be one or more chainrings or sprockets which drive the chain, which in turn rotates the rear wheel via the rear sprockets (cassette or freewheel). A gearing systems is used to vary the number of rear wheel revolutions produced by each turn of the pedals.

Since cyclists' legs are most efficient over a narrow range of cadences, a variable gear ratio is helpful to maintain an optimum pedalling speed while covering varied terrain.

When the bicycle chain shifts to a larger rear sprocket, or to a smaller front sprocket (a lower gear) every cycle on the pedal leads to fewer rotations in the freewheel (and hence the rear wheel). This allows the force required to move same distance to be distributed over more pedal cycles, reducing fatigue when riding uphill, with a heavy load, or against strong winds. The reverse process allows the cyclist to spend fewer pedal cycles to maintain a higher speed, but with more effort per cycle.

Road bicycles have close set multi-step gearing, which allows very fine control of cadence, while utility bicycles offer fewer, more widely spaced speeds. Mountain bikes and many entry-level racing bicycles offer an extremely low gear to facilitate climbing slowly on steep hills. Fixed-gear bicycles have only one gear combination.