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SIGMA's Slipper Clutch FAQ

Will the clutch bolt straight in; will I have to modify anything?
Can I use my bikes standard clutch plate packs?
Will the clutch plates wear more quickly?
Do I need to change down normally or can I ignore the clutch lever?
What other benefits will I get from my slipper clutch?
I can feel the clutch lever moving under my hand, is this normal?
Will my Sigma slipper clutch require a lot of maintenance?
Will my bike feel different?
Can the ball bearings in the centre of the clutch escape in to the motor?
Can I adjust my Sigma slipper clutch?
I have a motorcycle engine in my racing car. Will a slipper clutch help me?
Why do Sigma Clutches use coil springs?
What is Rotational Inertia?


Will the clutch bolt straight in; will I have to modify anything?

All Sigma Slipper clutches fit straight on with no modification, each clutch comes with proper instructions, the only special tool you will need is your bike manufacturers standard clutch holding tool. Any competent mechanic familiar with the type of bike will have no trouble fitting the clutch, just make sure they read the instructions as each clutch has its own idiosyncrasies.

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Can I use my bikes standard clutch plate packs?

Yes, in most cases they are the best option, but on rare occasions, and for specific uses, there are better solutions, just have a look in our race setup page for more info. Our top of the range Ducati Clutch comes with a special 48 tooth friction plates for less noise and long-term durability.

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Will the clutch plates wear more quickly? 

Our experience is that for normal track day use and up to national level racing the extra wear is negligible The plates can wear more, its just a question of use and how much work you ask the clutch to do.  In normal circumstances the only work the plates are doing is holding the clutch centre up the ramps; that doesn't require that much power in the great scheme of things so the clutch doesn't get too hot and therefore there is little additional wear.

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Do I need to change down normally or can I ignore the clutch lever?

The clutch is designed to stop rear wheel hopping under rapid deceleration, if you decide to ignore the clutch lever the clutch will do the work for you and will use the clutch friction plates to do the work, wearing the plates quicker if you do it all the time. If you ride normally, using the clutch lever for downshifts, the work level for the clutch is reduced and less wear occurs. Most of the time it's a case of just go out and ride.

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What other benefits will I get from my slipper clutch?

Your slipper clutch works anytime the rear wheel tries to overcome the engine braking, you can adjust it to work sooner or later. On its standard settings, if you bang it down one gear too many, it should declutch automatically, helping you stop the crank being revved up past its rev limiter, helping keep your engine in one piece!

In close racing one of the most invaluable reserves is the rider/drivers mental capacity. If you are concentrating on getting into a corner and fighting off close opposition then a clutch that makes any mistakes you may make on the down shifts less of a problem is a distinct advantage.

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I can feel the clutch lever moving under my hand, is this normal? 

If you are playing or feeling the lever as the clutch is working for itself then you are going to have occasions when you are resting your fingers on the lever when the automatic mechanism decides to let out the clutch; in these circumstances you will feel it.  The usual feeling is a small kick through the lever. It can feel a little funny at first but you soon get used to it. If it stops doing it then it is worth having a look at the clutch as it may need servicing.

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Will my Sigma slipper clutch require a lot of maintenance?

No, but if you abuse it then it will require some. For normal use the plate wear is negligible.  In extreme situations, as an example if you never use the clutch lever for downshifts and you are racing on the GP circuit at Donington with three consecutive hairpins then it doesn't seem unreasonable to assume clutch and plate wear will be accelerated.  If however you are working the clutch lever as normal, just allowing the Clutch to help maintain grip, then plate wear should be near normal.  The mechanism itself and the condition of the plates should be checked more often if it subjected to regular extreme use.

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Will my bike feel different?

There will be small differences, engine braking was an extra brake, an uncontrolled one. So you may need to use more front brake. Because the extra brake was purely at the rear the extra braking you are putting through the front may require a slightly stiffer front setting. Equally, although you didn't think about it, your rear suspension damping was set to eliminate some of the fluctuations coming down the chain; most probably by adding compression damping. You might find an improvement by experimenting with less control over the rear shocks movements. These are small differences, ones that the average rider will not feel, but they are worth considering if you are really going for it.

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Can the ball bearings in the centre of the clutch escape in to the motor?

The clutch is designed to lift by a maximum of 2.5mm. When assembled on to the gearbox input shaft correctly they will be limited to this by the washer under the main fixing nut acting as a travel stop. The ball bearings in the clutch ramp mechanism are 6mm in diameter; with everything correctly assembled a 6mm ball cannot get out of a gap, which at max will be 2 .5mm. All you have to do is correctly assemble the centre nut and its washer according to the instructions that came with the clutch and to the recommended torque settings and it can't go wrong.

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Can I adjust my Sigma slipper clutch?

The standard setting for one of our clutches, when used with a new set of plates, is for a clearance between the clutch centre and the pressure plate of 1mm. (See the instructions that came with your clutch for more information on how to measure this). This setting provides a reliable combination of low pack wear, reduced rear wheel hop and overrev insurance and works well for 99% of our users. For extreme situations however you can use alternative clutch pack thicknesses and spring rates to get slightly different effects.

When the rear wheel tries to turn over the engine the clutch centre is pushed upwards by the ramps inside the clutch hub. The further it has to travel, the longer it will be before the pressure plate is pushed off. Thinning the clutch pack using alternative thickness metal plates means that the centre to pressure plate distance can be reduced right down to 0.4mm.  This means the clutch disengages very easily and often.

If you want to change the feel you can adjust the spring rates; to feel more engine braking heavier main springs will make the clutch work harder to stay slipping.  This will result in higher levels of clutch pack wear however.  Conversely lighter main springs will require less effort and will help the plates to last longer.

For really big differences in clutch feel different ramp angles are required; on supersport engines, 600cc ones we use ramp angle of between 37 and 33 degrees depending on bike.  For some bikes, such as the Honda CBR600RR, we have clutches available in 37 and 33 degree angles depending on power out put.  This is because lower angles are better at dealing the stronger main springs required to transmit high power outputs under acceleration.  All these lower angles mean that the clutch will slip when bump starting is attempted so electric starters are required.

Historically we have used 45 degree angle ramps on the larger bikes.  This is because most were bump started in a racing environment.  Now however race teams are leaving the electric starter mechanisms on so we are reducing ramp angles in the pusuit of a significantly smoother action than any of the OEM slipper clutches.

Have a look at our race set up pages for more info.

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I have a motorcycle engine in my racing car. Will a slipper clutch help me?

Sigma slipper clutches are used in a lot of different vehicles, ranging from racing bikes to open wheel and closed wheel race-cars. In bikes the clutches ability to increase available rear wheel traction going into the corner is the most important aspect, its ability to help reduce over revving on down shifts is a nice extra.  For cars (or sidecars!!) it is the other way round.

Motorcycle engines in cars are very sensitive to being over-revved on down shifts; slipper clutches automatically declutch under aggressive down shifting and help reduce over revving and therefore damage to the motor. The additional rear wheel grip going into corners is not as critical as it is on bikes but there is definitely an advantage to be had by allowing the tyres available grip to be used solely on side grip.. Fitting a slipper clutch will however let the rear wheels use all their grip for holding the line and will allow the car to run into the apex far better.

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Why do Sigma Clutches use coil springs?

Our clutches are deliberately as simple as possible by design. For most uses the clutch springs we recommend are the standard ones. We only use diaphragm springs where the original clutch used them; thankfully now Yamaha has reverted to coil springs for the '2004 and on R-1' clutch we only have to deal with a diaphragm spring on the first generation R-1 motors.

In normal use, with coil springs, the clutch feels the same as the original. Coil springs are easy to preload and are insensitive towards small changes in preload. In other words as the clutch plates wear the coil springs still provide enough 'clamping force' to the clutch pack to prevent slippage under power, unlike the other options. In a coil spring clutch the coil springs are all located near the centre of the clutch, concentrating weight near the centre and minimising rotational inertia.

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What is Rotational Inertia? 

Inertia is a "resistance to a change of motion". The more inertia an object has, the less it responds to being accelerated or pushed. You are no doubt aware that, all other things being equal, that a heavier bike will accelerate down a drag strip slower than a lighter bike. The more weight or mass in an object, the less it responds to being accelerated or pushed. 

Rotational inertia is a resistance to change of rotation.  The more rotational inertia designed into an object, the less it responds to being spun up. An engines flywheel is designed to use this principle to slow or accelerate an engines throttle response depending both on its weight and how far out from the centre the weight is placed. The further out from the centre the weight of a spinning object is placed the greater inertia it creates. This is particularly important on Supersport bikes as the FIM Supersport rules prevent modifications to the crank. You cannot then modify the crank for quicker throttle response to make up for a clutch that adds rotational inertia. 

It is quite possible to have two different, in this case, clutches, weighing the same but with one giving significantly more rotational inertia than the other—for a swift demonstration fill a bucket with some water; attach a 2 metre rope, grab hold of it and spin it round. Do it first with both 2 metres, and then try it with 1 metre. The shorter length requires less effort and less speed, it has less rotational inertia, that's why we prefer to keep the heavier spring materials near the centre of our clutch.

Copyright Neil Spalding 2002- 2005

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