FREEWHEEL

In mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. An overdrive is sometimes mistakenly called a freewheel, but is otherwise unrelated.

Applications: 

1. Fixed Gear Bike
2. Bike Tryall
3. single speed flywheel
4. removable