A slipper clutch is a device used in high-performance motorcycles. The clutch is manufactured to allow the clutch to slip if the rear tire attempts to push the cycle's engine faster than it is operating. This design eliminates the dangerous rear tire hop, or chatter, that cycles are famous for in the event of a blown engine or from simply slowing rapidly. Used in four-cycle motorcycle engines on the street, the slipper clutch made slowing a motorcycle by engine braking or down-shifting much more manageable than a non-slipper clutch equipped version.
The slipper clutch was first used in the 1970s by motorcycle drag racing teams who understood the safety benefits of the system. The first slipper clutch units were created by combining earth-mover components with an automobile transmission. The results were a clutch that would apply tremendous amounts of power when being driven in the proper direction. The slipper clutch would also allow the rear tire to slip on the clutch when the engine slowed prematurely. The slipper clutch, albeit refined, has been standard equipment on most high-powered, four-cycle race bikes sold since 1980 worldwide.
Many of the high-powered performance motorcycles used on the street are now equipped with a slipper clutch. There are retro-fit clutch kits available for cycles not originally equipped with the slipper-type clutch from the factory. Motorcycles are not the only vehicles to use a slipper type clutch. Many experimental aircraft have used this clutch behind the propeller to avoid damage from a propeller striking an object. Automobile use of the clutch has been limited to experimental type vehicles that use a motorcycle-type drive train.
The difference between a slipper unit and a one-way sprag clutch lies in the slipper unit's ability to be adjusted. The slipper clutch can be adjusted to slip at a predetermined force to slip the clutch. This force can be calculated taking in the weight of the rider, the cycle and the speed at which the cycle is expected to operate. The surface of the track the cycle is racing on is also factored into the equation when adjusting the clutch.
In a one-way sprag clutch, the sprag is connected to just half of the friction plates in the clutch and a spring allows the clutch to slip past half of the plates in an engine braking situation. This system, while somewhat better than no slipping, still offers some control issues for the rider; the cycle's rear tire is still prone to hopping upon sudden deceleration.