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The freewheel is a ratcheting mechanism, that coupled with a set of gears, is that threaded onto the rear hub of a bike. The freewheel allows the rear wheel to engage and move when the pedals are spun forward, but coast freely when the pedals are stationary or spun backwards.

The together with its associated is referred to as a . The serves the same purpose as a although they are mechanically different and cannot be readily interchanged.

A discussion of the difference between a and can be found on Sheldon Brown's website: Traditional Thread-on Freewheels

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