Freewheel

 

A sprocket that screws onto the hub of the rear wheel with teeth to engage the chain. A freewheel drives the wheel forwards but not backwards. A single speed freewheel consists of just one sprocket; a multi-speed freewheel consists of a cluster of sprockets, each of a different size.

 

villiers freewheel576p

7-speed-freewheel-576p