How a slipper stops a slide...

WE’VE been hearing a lot about slipper clutches recently – they’re fitted to exotica like Aprilia’s RSV and anyone watching the TV coverage of the Phillip Island WSB will have seen what can happen if you don’t have one.

Turning into a low-speed corner in the wet, Pier-Francesco Chili shut the throttle on his Suzuki GSX-R750 and was pitched off the bike. Why did this happen? Simple… there was too much engine braking for the slippery conditions and the back wheel locked up. Things could have been so much different if his GSX-R had been fitted with a back torque limiter – that’s a slipper clutch to you and me.

The physics behind the problem is simple. With the throttle open the engine is burning fuel to turn the crankshaft. This drives the transmission and, finally, the rear wheel. Shut the throttle and the engine stops producing power. It still spins as it’s still connected to the rear wheel. But the process is now reversed, with the wheel turning the engine.

An engine with a closed throttle takes quite a lot of effort to turn over, as there’s compression to cope with (even higher compression on a race bike) plus friction from all the internal components. So if there isn’t enough grip, the rear tyre will slide.

In normal road riding, there is more than enough grip at the rear tyre to spin the engine when the throttle’s closed, but there are circumstances which alter this. For example, when the brakes are being used hard, a great deal of the bike’s weight is transferred to the front wheel. That means less weight over the back, which equals less grip for the rear tyre.

Revs are important, too. Turning an engine over at high revs takes more effort than spinning it slowly. So if you’ve been riding hard and revving to the red line – like Chili on the track – more grip is needed at the back tyre to keep it spinning as you shut the throttle.

Another factor is the gear you’re in. In a high gear, the engine only turns over a few times for each revolution of the rear wheel, maybe as little as once or less. But in a low gear it will turn over many more times, possibly six to 10 per rear wheel revolution. When the throttle’s closed, in a low gear the wheel might have to spin the engine 10 times per revolution, which takes 10 times the effort of spinning the engine in a high gear.

Now add all these factors together, as you’re likely to be doing on a race track or while riding very hard on the road. You scream the engine up to the red line just as you reach your braking point for a corner (and you might only be in third gear if it’s a tight circuit). Then you slam the throttle shut and brake as hard as you can. It’s quite possible the back tyre could be close to lifting completely clear of the road, a situation where it has almost zero grip. Yet you’re asking it to spin the motor at high revs in a low gear…

Inevitably, the tyre will lose its grip and slide, while the engine’s revs will drop below the level they should be at for your road speed, neither of which does your ability to control the bike any good. There’s a good chance if you’re also turning into a corner you will lose the rear end and fall off.

That’s where you need a back torque limiter. It’s a device within the clutch which causes it to slip when the power being transmitted in one direction only exceeds a pre-set amount. Under acceleration with the throttle open there’s no slip. But with the throttle shut and the power passing back up the transmission, the clutch will release if too much is being asked of the rear tyre.

The principle is straightforward. The clutch hub faces up against a series of engagement dogs with sloping faces (see diagram, bottom right). When the power through it is reversed, the hub slides up these dogs, pushing the outer plate (which it’s directly attached to) and decreasing the pressure on the rest of the clutch plates so they slip more easily.

As a bonus, this design can also be used to prevent slip as the clutch is engaged. When power is fed in from the engine, the clutch hub slides down the sloping faces of the dogs, pulling the outer plate inwards and increasing the pressure on the other clutch plates (see diagram, top right). This design, as used by Suzuki among others on its TL1000 and Hayabusa, allows the clutch to be smaller and lighter than it would otherwise be, although if not set up properly it can sometimes result in a grabby action.

A more sophisticated variation is used on the RSV Mille. A sealed air line runs between the engine’s intake tracts and a vacuum-operated servo at the base of the clutch. When the throttle is closed the drop in pressure causes the servo to exert a load on the clutch against the springs, decreasing the pressure on the plates so they will slip when there is enough back torque.

As with the Suzuki design, the Aprilia’s also allows the plates to be pushed together more tightly when the throttle is opened (when the inlet tract pressure increases) so the clutch lever feels lighter and the whole assembly can be made smaller.