The inner thread of the flywheel is fixed on the right end of the rear axle by screwing, keeping the same plane with the sprocket, and connecting with the sprocket through the chain to form the driving system of the bicycle. It can be divided into single-stage flywheel and multi-stage flywheel in structure.

The chain, also known as vehicle chain and roller chain, is installed on the coupling wheel and flywheel. Its function is to transfer the pedal force from the crank and sprocket to the flywheel and rear wheel to drive the bicycle forward.