Kawasaki Ninja 300: Slipper clutch explained

The new Kawasaki Ninja 300 adds in its list of upgrades a slipper clutch. And what you are about to read on all the online forums, discussion groups and all is that the slipper clutch is the bees knees. But is it? Let's find out.

When you downshift at any time on a motorcycle or indeed, in a car, the chain of mechanical events goes like this. What your shifting the lever does is effectively alter the gearing the engine runs (think of it as changing the sequence of all the various cogs that connect the crankshaft to the rear wheel, the specifics are not important). When you go down the gearbox, the change you make is that small throttle changes produce bigger changes in the useful torque at the wheel as you go down the gearbox. And all the various cogs have to readjust their speeds to match the new setup.

Now think about a motorcycle cruising at a steady 50kmph. You can, let us say, do this in third, fourth and fifth gears. What would the revs be like in each gear? Let us say that the motorcycle is at 7000rpm in third, 5500rpm in fourth and 3000rpm in fifth. This engine speed to road speed relationship is for all practical purposes a constant. This motorcycle will always do 7000rpm if you're at 50kmph in third gear.

What that also means is that should you downshift from fifth to fourth at 50kmph without changing the speed, the engine needs to change its revs from 3000 to 5500rpm in an instant.

And this where the slipper clutch has a role. Now normally, when you downshift on most motorcycle at low or mid revs, the engine can, in fact, smoothly change these revs without stress. If the rider knows how to match revs (a quick blip on the throttle while downshifting so that, for instance, the revs are momentarily high so that when the shift goes through, the bike is already at the right revs for the new gear) the downshift can be very smooth. Which is great for jerk free riding, but also critical when traction is at a premium or you are near the limit of the motorcycle.

Before we go into the slipper clutch itself, let us consider one more situation. Let us say you have a high performance motorcycle and you are riding quickly. Going into a corner, you downshift aggressively which raises the revs neatly to the peak power point. If the engine cannot accept the sudden change of revs (easy to imagine if the engine is a high compression motor, for example) then the difference is made up by the rear wheel's excess inertia being absorbed by the powertrain in reverse. The rear wheel actually tries to drive the engine into raising it revs quicker. Which not only means the piston has to go up and down faster, but the cams have to open the valves more often and so forth. This is not easy to do and that is usually the reason why a badly executed, aggressive downshift will usually lead to a chirp from the rear wheel as it will momentarily lock as it applies all the excess torque to the powertrain and slows to a stop as a result.

As you can imagine this is a stressful situation. It compromises both traction and stability and this is not a distraction you need when you are going fast and need every iota of available traction for the purpose.

What the slipper clutch does is allow the clutch to freewheel in one direction more or less like a bicycle sprocket. If you stop pedalling a cycle, you hear that whirring sound as the bicycle freewheels, right? A slipper clutch is far more complex but what it does is when the rear wheel tries to apply its excess torque backwards up the powertrain, it allows the clutch to partially disengage, bleeding away the excess forces without seriously affecting the gearbox and engine. This also allows the rear wheel to continue to rotate rather than lock up which is why its alternate name is Back Torque Limiter.

In theory, it is possible to still lock the rear wheel with epic stupidity and impeccable timing on a downshift but a well designed slipper clutch will usually prevent wheel lock up in almost all cases in practice. Finally, one of the advantages of a slipper clutch - that we hope no one has to actually ever see - is that in the event of a transmission failure or an engine seizure, it should be able to prevent the rear wheel from locking up allowing a safe stop.