freewheel

A freewheel is a mechanical device that allows rotation in one direction while permitting free rotation in the opposite direction. It is used to transmit torque selectively and to decouple moving parts in a range of machines, from bicycles to industrial equipment. A related concept in electronics is the freewheeling or flyback path, which provides a current path for inductive loads when a switch opens, protecting components.

In bicycles, a freewheel typically refers to a cluster of sprockets with an internal ratcheting mechanism. When the rider pedals forward, pawls engage the sprocket teeth and drive the wheel. If pedaling stops or the wheel turns faster than the cranks, the pawls disengage and the wheel can coast. Traditional bicycles used screw-in freewheels that thread into the rear hub; many modern systems use a separate cassette that mounts on a freehub body, which is a related but distinct arrangement. Freewheels require regular maintenance, lubrication, and occasional replacement of worn pawls or sprockets.

Outside bicycles, freewheels or one-way clutches appear in various machinery to transmit torque in a single direction while allowing free movement in the opposite direction. In power electronics, a freewheeling diode serves a similar purpose by giving inductive current a safe path when a switch changes state, reducing voltage spikes.

See also: one-way bearing, ratchet mechanism, freehub, and bicycle cassette.