Cassettes: The bicycle cassette is a cluster of sprockets attached to the rear wheel. This is the main mechanism of torque in propelling the bicycle forward. The cassette is geared in order to let the rear wheel continue to spin when no torque is being applied to the casette. Casettes are designed with a specific number of different sized sprockets to provide gears. The larger sprockets are your lower gears for climbing hills The smaller sprockets are your high gears for speed on level or downhill surfaces.