What is a slipper clutch?

What is a slipper clutch?

Racing is about far more than power down the straights. Well-controlled high-speed corner entry is just as important to a quick lap as sheer grunt. The trouble is that big four strokes have a lot of engine braking, and that can really destabilise their rear suspension.

Large-scale engine-braking is the result of closing the throttle on a high-compression four-stroke. Even though there’s not much air going in past the closed throttle, the piston still has to compress what is there. As the rider shuts off, the engine slows the rear wheel – connected via clutch, gearbox and drive chain.

It becomes a problem when it affects the suspension and the back tyre.

The rear-wheel chain drive is a critical part in the way a motorcycle works on track. Motorcycle designers use the positioning of the gearbox output sprocket and the swing-arm pivot to create a pull on the swing-arm that pushes the rear tyre into the ground under power.

Conversely, if the rear wheel is driving the engine, the reverse happens. The chain tries to hop the rear tyre off the ground, the more so when it is fighting a lot of compression.

As far as the rider was concerned, as soon as he shuts the throttle, the rear end of the bike starts to feel different … unstable. The problem becomes much worse while changing down through the gears. Each shift puts a spike in the load on the chain. If the rider is being very aggressive (as you would expect on a racetrack) the revs can increase to the point that they are pushed up through the rev limit, making the engine braking more severe and also potentially damaging the engine. The forward pitch of the bike under heavy braking also takes weight off the rear wheel, further exacerbating the problem.

Riding fast on a circuit, or for that matter on the road, requires smoothness: it is about loading up the tyres to their limit of grip (or controlled slide) and holding them there at that fine margin – just before they really let go. All the suspension and chassis set-up effort goes into helping this situation to be predictable and accurate. What you do not want is the big pistons at the other end of the chain jerking the swing-arm about, either while floating into a high-speed corner or coming into a slow, hard-braking corner. You need smooth progressive deceleration as you roll off the throttle, and no jerks that would result in the chain moving the swing-arm.

There are two ways to try to eliminate these effects.

One attacks the root cause: slightly open the throttles to a fast idle, so that there is little or no engine-braking in the first place. On the earlier ride by wire race bikes the problem with this was that different degrees of throttle opening were required to get the right effect on each corner, depending on the speed, engine revs and gear position. In later years the use of programmable ride-by-wire technology has made a big difference. This solution however does not give any protection against over revving on down shifts.

The other solution is to fit a slipper clutch, which automatically disengages whenever too much resistance is felt. As the ride-by-wire programs have become more sophisticated the clutch has had less to do. In MotoGP, even with the latest Dorna control electronics, most bikes rely on the electronics to control their entry into the corner, and use the clutches to allow smooth gearshifts and to guard against major over-revving should the bike be downshifted over-enthusiastically.

The combination of the two solutions lets the bike’s suspension work much better coming into (and going through) corners, and also helps the engine to take the abuses of racetrack use.

So how does a slipper clutch work?

There are several types. Various designs have been tried over the years, but all those on the MotoGP grid are ramp types. In these, most of the clutches have a separate centre section that has a ramp system built into its base.

The clutch is a purely mechanical device. When the throttle is closed and the rear wheel starts to turn over the engine (let’s call it reverse torque), a simple system of angled ramps (with or without ball bearings, depending on manufacturer) inside the central drum forces the drum up against the outer pressure plate, so pushing the clutch plates apart. As soon as the clutch starts to slip, the forces are controlled, and the clutch is held in perfect slipping mode – just enough power is transmitted to maintain the equilibrium. In a normal riding situation the clutch operates just like any other. The point at which the clutch releases, and the way it does it, makes a real difference to the way the engine-braking affects handling, and thus also to the suspension set-up.

When the engine power is driving the bike forwards in the normal direction, the little ramps lock solid, and the clutch acts completely normally. The diagram shows what occurs when the power is shut off and the engine is being turned over by the rear wheel. As gearshaft B is turned by the rear wheel, the clutch (attached to gearshaft A) is forced to take the load in the opposite direction. This forces the centre of the clutch up the ramps; thus starting to move the clutch pack apart.

As soon as the clutch pack stops gripping (i.e. when the centre has risen slightly on its ramps) most of the force to hold the pack apart is lost and, theoretically, clutch grip is restored as the pack tries to come back together. In practice, the clutch establishes an equilibrium position where there is just enough force being transmitted to hold the clutch apart (i.e. to hold the centre partially up the ramps), yet just enough to stop the engine being revved up.

Slipper clutches are essentially a game of ramp angles and springs. The ramp is what forces the clutch apart, while the springs are trying to keep the clutch together. The method of adjustment with the biggest effect is to use a different ramp angle but in private ownership this can get expensive so combinations of different main springs or preload spacers can be used to resist the “rising centre” more strongly. Maintaining the correct thickness of the clutch pack becomes much more important with a slipper clutch as the preload on the springs affects the operation of the clutch.  Long term reliability depends on a clutch which has springs that are strong enough to take the bikes power under acceleration but which isn’t held together too strongly to prevent it slipping under deceleration.

The stiffer the springs are the more effort is required to push the clutch apart, so creating a feeling of more engine braking. The stiffer spring settings mean that the clutch is working harder to stay apart, so generating more heat and causing more wear, care must therefore be taken to Raising and lowering the closed throttle engine tick-over speed can also make a big difference but care has to be taken to ensure the engine still shuts down effectively once the throttle is closed. Test any changes to make sure you get the effect you want before you start going fast.. 

Most slipper clutch designs require the clutch to rise about 1.0 mm before the clutch starts to disengage, we call this the ‘clearance’ and this measurement is critical to the correct operation of the clutch. Some Sigma clutches are designed for different ‘clearances’ and it is important that the minimum recommendations are respected. Using the Sigma approved method of measurement helps to get an accurate initial setting, but it is worth checking the clearance after each initial track outing so each rider gets used to the amount of wear his choice of set up and preferred use creates.

One potential drawback is that the slip of the clutch makes it difficult to bump-start an engine. Most bikes in pit lane now retain their electric starters, so they can set their clutches up with lower-angle ramps for easier slip.

Some bikes have to be bump started however, the NSF250R being an example, so that is the only bike in the Sigma range with a locking mechanism to help the bike start. Older race bikes that have had their electric starts removed can be fitted with slipper clutches and started but this requires slightly different settings and the use of second gear for starting. For every other instance we recommend that the electric or kick start be retained as this allows the slipper clutch to be set up purely for the track, not bodged to allow a bump start too; this improves its effectiveness.

Finally, a slipper clutch is on the bike as an extra, it is there to help the rider get around the track more quickly, correctly set up and used, grip levels will improve, the suspension will be much more settled and the engine will take less of a beating. But the clutch is only there to help, it isn’t there to replace the riders clutch use. The rider should still use the clutch as normal on downshifts and they will feel the slipper clutch operating through the lever, just a series of little clicks.