No, not a group of slippers but a ‘back-torque limiting’ clutch mechanism. You may have heard the term used regularly when discussing the technical details of a modern four-stroke racing motorcycle. It seems that almost every race machine in the two-wheeled racing world has a version of these specialty clutches. Even many modern street-going race-replica motorcycles are adopting them due to their rather successful use at the highest – and toughest – levels of racing. So, what is it?
Simply put, it is a clutch assembly which allows the clutch to ‘slip’ in order to eliminate dangerous engine braking when decelerating into corners. If the rear tire is driving the engine – instead of the other way around – it can be forced to ‘chatter’ or lock-up completely as the compression in the engine fights against the force pushed into it by the rear wheel. If that were to happen while racing a powerful four-stroke motorcycle, a nasty crash often results. To deal with the problem, the slipper clutch will allow the drive and driven clutch plates to disengage momentarily to stop any rear-wheel instability as the rider slows down quickly. It allows the rear wheel to spin, preventing it from having to work against engine compression. In practice, this ‘slip’ happens at very high speed and is not easy to see in action, but it is easy to feel if you are piloting the bike yourself. It is also possible to ‘tune’ a slipper clutch by adjusting how much force it takes to disengage the clutch plates.
In addition to better rear-wheel characteristics when slowing down dramatically, the slipper clutch will also assist the motorcycle’s suspension. In a situation where the rear wheel is not performing smoothly, the rear suspension is also being jarred up and down. That causes the motorcycle to become unstable – another good way to crash. A slipper clutch removes this from the equation and the racer will have a much smoother corner entry, thus a faster lap and a better chance to win!
So, the next time you hear someone talking about a slipper clutch, you will know exactly what it is and how it works – and you won’t imagine a pile of house shoes.