Chain drives use a chain connected between a sprocket at the transmission output and a sprocket attached to the rear wheel. Drive chains consist of a number of links that contain pins, rollers, bushings, and plates. A chain drive is a system of transferring power from one shaft to another by means of sprockets and an endless chain. Chain drives are used on engines as timing chains or cam chains to control the timing of opening valves. Chain drives such as a roller chain drive are composed of one chain that wraps around two or more sprockets. The roller chain is made of alternating inner and outer links assembled in pivots by bearing pins and bushes. A unity chain drives chain drives with sprockets attached to each roller. When a unity chain is used, the sprockets of the roller have only several teeth exposed to the chain. Chain drives can also have a rigid sprocket mounted to a motor, which engages a chain comprised of numerous links. Engaging the chains also engages a sprocket attached to the drive shaft. A reverse transmission chain drive provides a reverse driven sprocket connected to an output shaft, in which the torque is transmitted between the sprockets by the chain drive.