How Slipper Clutches Work
Slipper clutches, the most underrated track accessory??
The Slipper Clutch
Racing motorcycles quickly on a circuit 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 four strokes have a lot of engine braking, and that can both reduce the amount of grip from their rear tyre and really mess up the stability of the bike.
As a bike comes into a corner the rider starts to brake really aggressively and hopefully starts to lean and steer towards the apex of the corner. The act of using the front brake hard means that a lot of the weight of the bike is pitched forward onto the front tyre. In some ways that’s good as the tyre contact patch flattens out and allows it grip harder but as the weight moves forward there’s less weight left on the rear. And that rear tyre is still trying to turn over an engine where the throttle has been shut and where the engine breaking is starting to really mess up the rear suspension stability.
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 a 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.
Unfortunately, if the rear wheel is trying to drive 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, once the braking has started, and as soon as the throttle is shut, the rear end (of the bike!) starts to feel different … unstable. The problem becomes much worse while changing down through the gears. Each shift jerks the drive chain. If the rider is being very aggressive (as people tend to be on a racetrack) the engine revs can also increase to the point that they are pushed up through the rev limit, making the engine braking more severe and potentially damaging the engine.
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 for the really, really fast riders, into a controlled slide) and holding them there–before they 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 engine at the other end of the drive 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, with no jerks that would result in the chain moving the swing-arm.
If we assume that any tyre has let’s say a maximum of 100% grip capacity. As you roll a bike onto its side into a corner up to 50% of that tyres available grip can end up being used simply to turn over the engine. That then starts to become a grip issue. If we could reduce the engine braking all of the grip that’s currently being lost into turning over the engine is available to stop the bike sliding out sideways.
And that’s not all, a bike is designed so that under hard acceleration the chain drive and swingarm is forced down for more grip. Unfortunately under braking the reverse is also true. With a four stroke engined bike the rear chain can try to lift the rear wheel off the ground, and as the braking forces have also pitched the bike forwards compressing the forks, the back of the bike can easily feel very loose and unstable.
So.. The target is to smooth out the bikes corner entry, cancel out any rear suspension instability while also losing as little side grip as possible. In a perfect world you would also try not to raise the engine revs too much but still leave just enough engine braking to keep the rear wheel behind the front….
There are several ways to try and address this. The first is to reduce the engine braking itself. On modern bikes equipped with the very latest ride by wire throttles there will often be an engine braking control system. This will slightly open the throttles as the bike slows down killing off a lot engine braking. Older bikes, indeed everything with an old fashioned cable throttle, and most bikes that weren’t intended to be sports bikes in the first place, will not have this feature. There is a way to get some of the effect though and that is to slightly increase the engine idle RPM. Raising the idle speed very slightly, just 200 rpm or so, reduces engine braking but you have to be incredibly careful to make sure that the resulting engine revs don’t make the bike try and go too quickly into slower corners. This solution however does not give any protection against over revving on down shifts.
You can clearly see the six ramps and ball bearings. Spot the balls.
The simplest way, and for years the complete solution, is to fit a clutch that deliberately slips when engine braking becomes a problem. These clutches are called slipper clutches and they have been around for normal customer use for at least 20 years now and were used on works level race bikes for about 10 to 15 years before that. Slipper clutches automatically disengage whenever too much resistance is felt, this allows the bike’s suspension and tyres work far more effectively coming into (and going through) corners, and also helps the engine survive some of the abuse of racetrack use.
A test of a Sigma clutch on a Honda CBR600RR in Performance Bikes magazine included the following analysis of their riders data after his first ride.. ‘Alex took over a second off his lap time using the Sigma slipper clutch. But far more significant are his traces through the corners. This one at the end of the long straight is typical, the Sigma slipper clutch allows much later harder breaking (.863 G stock versus .950 G with the Sigma) and Alex shed speed faster thanks in no small part to an increase in confidence. Because the suspension is more settled the benefits continue through the corner. He turns harder and holds a tighter line, his mid corner speed rises by 4.64 miles an hour from 54.81 mile stock to 59.43 with the slipper. He’s on the power earlier thanks to the suspension being more settled.’
Yamaha R1 - Assembled
So how does a Sigma slipper clutch work?
Various designs have been tried over the years, but most, including all those on the MotoGP grid, are ramp types. These 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 re-engages, and the way it does it, also makes a real difference to the way the engine-braking affects handling, and therefore also to the suspension reaction. The re-engagement has to be smooth and gentle. Nothing must put any additional stress on the tyres.
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 re-established 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, this equilibrium point can be adjusted either by varying the spring rate or the effective preload on the springs holding the clutch together or by varying the height of the assembled clutch pack. This is why you will often see theoretically rich well-funded race teams sorting through endless piles of seemingly knackered clutch plates trying to get precise pack thicknesses they want to give their rider the feel he is used to.
When the engine power is driving the bike forwards in the normal direction, the little ramps are locked solid, and as shown in Diagram A the clutch acts completely normally.
Diagram B 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 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 operation of the clutch depends on a clutch which is strong enough to take the bikes power under acceleration but which isn’t held together too strongly so it slips as required 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 not to overdo it. Raising and lowering the closed throttle engine tick-over speed, by say 200rpm, is a good way of adjusting the ‘rider feel’ but care has to be taken to ensure the engine still shuts down effectively once the throttle is closed.
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 for each clutch are respected. Using the Sigma approved method of measurement helps to get an accurate initial setting, but it is worth checking the clearance after the initial track outings so each rider gets used to the amount of wear his choice of set up and method of 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 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 operated 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.
Sigma has a range of clutches covering over 80 different bikes, some are for bikes that do not have a slipper fitted as standard and some are ‘track focused upgrades’ for bikes that already have a slipper clutch …
All Sigma clutches come with detailed fitting instructions. Normal tools only are required. Have a look at the FAQ on www.Sigmaperformance.com or ring us for more info.
Copyright Neil Spalding 2003-22