Freewheels

A freewheel is a mechanical device that permits rotation of a shaft in one direction while allowing it to rotate freely in the opposite direction. It typically employs a one-way clutch, such as a ratchet and pawl arrangement or a sprag roller, to engage when driven forward and disengage when rotation would come from the driven side.

In bicycles, a freewheel assembly combines the sprockets with an internal ratcheting mechanism that engages when the rider applies power and disengages when pedaling stops, allowing coasting. Modern systems often use a freehub body with a separate cassette rather than a single freewheel. Freewheels wear over time and can require maintenance or replacement to prevent slipping or skipping under load.

In automotive and industrial contexts, one-way or overrunning clutches function as freewheels to transmit torque in one direction while allowing free rotation in the opposite direction. In some older automobiles and motorcycles, freewheel devices were used to decouple the engine from the drivetrain during deceleration to reduce engine braking, a practice largely replaced by modern electronic or hydraulic systems.

Typical designs include ratchet-and-pawl assemblies, sprag clutches with asymmetrical surfaces, and roller ramp mechanisms. The choice of design depends on required torque, operating speed, misalignment tolerance, and wear characteristics. Freewheel concepts have a long history in machinery and bicycles, with bicycle versions becoming common in the early 20th century. Related terms include one-way clutch and overrunning clutch.