The Slipper-Clutch on your Motorcycle

Slipper clutches are now in the limelight thanks to manufacturers fitting them as stock parts on their sportiest motorcycles. But what do they do, how do they work and perhaps more importantly, do you actually need one on your next motorcycle?

There are several different designs of slipper clutch mechanism, but they all achieve the same thing – to convert some of the back torque into linear movement that can be used to reduce pressure on the plates. The simplest design uses a ramp to do this with a modified clutch center. Turned in one direction the torque acts on the square edge of the ramp and everything works as normal to drive the motorcycle forwards. Turned the other way, torque acts upon the ramp and part of the clutch center rides up the ramp relieving pressure on the plates.

Other designs work in the same way, but rather than using ramps, they use ball bearings inside a helix to achieve the same result – a bit like a nut spinning on a bolt. There are benefits to each method of course – although arguably, as long as it works it doesn't matter how it's done.

Getting a slipper clutch to work is for the most part quite easy. The engineers designing it have the hardest job getting it to work in the first place. Once they've done this all we can really do is play with two variables: pack thickness and the back-torque threshold.
Pack thickness is, as the name suggests, the thickness of all the clutch plates together and is vitally important. As the pack wears it becomes thinner and so the pressure holding it together aslo drops. In turn this affects how and when it slips. Generally, the thinner the clutch-pack the easier it will slip. Of course the thinner it is, the easier it slips and the quicker it wears, which causes it to slip even more and so on...

The other setting normally involves some sort of spring (a diaphragm or spider spring). These help define the disengage, and re-engage characteristics of the clutch. For instance, if the slipper clutch disengages too easily, you may fit a stronger spring, meaning the wheel has to try and turn the engine over harder before the slipper activates. Likewise, if it doesn't slip enough fit a weaker spring.

The most skilful rider in the world would not require a slipper-clutch. So it speaks volumes that the most skilled racers in the world have been the ones pushing for their development. And the reason? They take away workload from the racer in a way that lets them concentrate on riding hard – and those are the keywords; racer and riding hard.

When racers ride hard they take everything to extremes, especially braking. The problem is of course the physics gets involved and sets limits on how hard a certain motorcycle can stop. Bend the rules and you'll do a stoppie, which in most cases is quite entertaining. But if you don't want to do a stoppie, and you're concentrating on keeping the wheels in line as you change down to gears, while leaning into a 200km/h bend, the lack of weight can be a problem.

You see as long as the rear wheel is gripping it keeps the rear wheel behind the front. As soon as you lift it off the floor, or reduce its grip, you reduce its desire to stay in line. It gets jumpy too. Slipper clutches reduce this. They can't stop the wheel coming off the floor, but they can stop it locking when you down-shift. And that keeps things inline and easier to control.

What is a slipper clutch?

There two types of technology – those that physically help you, and those that don't. Gear position indicators would be a good example of a useful, but non-physical technology. Slipper clutches are very much in the pro-actively helpful camp. So what do we know about them?

A conventional clutch consist of two main parts, a clutch housing that's meshed to the gear on the crankshaft and a clutch hub that's splined onto the gearbox input shaft. Both parts turn independently of each other. Steel and friction plates are then fitted alternately to make a clutch sandwich.

The 'steels' are forced to turn with the clutch hub, while the friction plates turn with the housing. By pressing the plates firmly together, stopping the two parts from turning independently, the whole assembly is forced to turn as one and toque passes through the clutch.

Pulling the clutch lever removes the pressure from the plates so they no longer turn as one and you eventually come to a stop.

Slipper clutches work in exactly the same way as a normal clutch, but with one major difference. In a normal clutch, as long as the plates are being pressed together the unit always turns as one, so torque can pass through it in both directions – from engine to wheel and vice-versa. In a slipper clutch, the plates are only held firmly together as long as the engine is trying to turn the wheel.

As soon as the wheel tries to turn the engine, that back-torque actually reduces the clamping pressure on the plates and allows them the slip slightly.

The harder the wheel tries to turn the engine, the more the pressure is reduced on the plates. As the name suggests, they limit the toque the wheel can apply on the engine.
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