A freewheel is a set of sprockets of different sizes mounted on the rear hub (rear axle) of a bicycle. Together with the front chainring set, the chain, and the rear derailleur, it forms the bicycle’s drivetrain system. Simply put, the freewheel is the “rear gears.” The rider changes gears by using the derailleur to move the chain across different sprockets on the freewheel, thereby adjusting the gear ratio.
The freewheel provides a wide range of gearing options: sprockets of different sizes—from small to large—offer different transmission ratios. Smaller sprockets are used for higher speeds, flat roads, or downhill riding, while larger sprockets are ideal for climbing, starting from a stop, or riding into headwinds, making pedaling easier.