1. Usually refers to the action of a vehicle on a downgrade when the overdrive over-running clutch is slipping with a resultant loss of engine braking. This condition will occur only after the overdrive unit is engaged but before the balk ring has activated the planetary gearset .
2. The removable cluster of gears at the rear wheel of a bicycle . Usually there are five or more gears. It also contains a ratcheting mechanism inside that allows the wheel to rotate forward while the pedals, chain , and gear sprockets remain still or move in reverse. The rear derailleur moves the chain from one gear to the other to change the gear ratio . The other end of the chain wraps around the chainwheel .