Single-stage flywheel, also known as single-chain wheel flywheel, is mainly composed of outer sleeve, flat block and core, jack, jack spring, washer, wire block, steel ball and other parts.

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.