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A BASIC CLUTCH
A standard motorcycle clutch does one main thing – it connects and disconnects the engine’s power from the rear wheel, facilitating gear changes in the transmission. It is the middleman between the crankshaft and gearbox. A good analogy is that it serves as a lawyer arbitrating conflict between an arguing couple - the rear wheel, drivetrain, and gearbox on one end, with the motor and crankshaft on the other. When all is well the clutch serves as the enabler, facilitating glorious horsepower from motor to rear wheel in seamless fashion. During aggressive downshifting at speed, things quickly become turbulent between the normally harmonious couple. Physically and metaphorically, the clutch is sandwiched between two hot ends of a motorcycle.

WHY A SLIPPER CLUTCH
When a motorcycle’s throttle is opened, its pistons and crankshaft are powering the rear wheel through the drivetrain via the clutch. Conversely when the throttle is closed, the motor in a four-stroke bike is overrun, the engine acts as a braking mechanism, and the power is reversed. In this state, the rear wheel is turning the pistons and crankshaft via the clutch resulting in back torque. The lower the gear and higher the RPMs, the more pronounced the back torque will be. Large displacement and high compression four-stroke motors create the most back torque. A Ducati V-twin for example, has an over square set of pistons that put a great deal of pressure back on the clutch. If a motor puts out 100 Newton meters of torque to drive itself forward, then just imagine reversing some of that power back through the clutch.

Clicking down from 4th to 1st gear at once would put a lot of stress on this system. In this scenario, the rear wheel speed will exceed the engine speed of the motorcycle. The clutch will do its best to keep up but at an expense, unless a rider uses a technique to match engine speed to each downshifting gear. By blipping the throttle during downshifts, the motor has a better chance of keeping up with the rear wheel if executed correctly. If no blipping is done, the chain, swingarm and rear sprocket take a heavy dose of stress, creating traction instability. This can create skidding, wheel hop, and even catastrophic rear tire lock up. When instability is the last thing a rider wants at corner entry or during an emergency stop, this is a problem. For a racer or track day rider, smooth and settled braking at corner entries is not only safer but also shorten lap times.

A slipper clutch allows a rider to brake and downshift at corner entry without the worry of losing stability. Out-braking the competition isn’t as labor intensive when a rider can downshift to the correct gear with less worry of upsetting the bike’s chassis. Clutch slip can also eliminate the need for throttle blipping, decreasing the mental bandwidth needed to arrive and dive into a corner. The rear shock is able to work as it should, no longer trying to keep up with a hopping wheel and allowing the shock to keep the rubber where it should be – on the ground. Of course, a slipper isn’t a magic fix. Proper installation, testing and tuning of the suspension, brankes, in addition to the clutch are required for optimum performance.

HOW IT WORKS
A standard clutch package is a series of spring-loaded steel plates coated with friction material akin to brake pads. As discussed, when engaged, the function of the clutch is to connect the crank to the transmission and transmit torque. This is the same in a slipper, but unlike a conventional clutch, it will disengage once it meets enough back torque and absorbs this load in the opposite direction. It allows a set percentage of back torque to transmit away from the gear box, just enough to a create neutrality between the opposing forces of the motor and rear wheel. With this may come a loss of engine braking, however most slipper clutch designs will allow adjustment for level of engine brake and slip sensitivity.



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The common slipper design uses a mechanical hub, that upon pressure of back torque unlocks, slides and rotates up a set of angled ramps that are mounted on the clutch back plate. The unlocked hub then pushes the pressure plate out enough to pull the clutch pack apart at a calculated distance to allow slip. If a rider does not abuse a slipper clutch, the clutch plates and basket will operate reliably without major wear or failure. Different manufactures have their own proprietary designs that include ramps of varied angles with or without ball bearings, an interchangeable diaphragm, and disk or coil springs to enable tuning the amount of slip and its sensitivity to it.

Depending on the intended use, type of machine, and available budget, there are numerous options when shopping for a slipper clutch. A low-end clutch can start at five hundred dollars, or at the high end can cost over two thousand dollars. Many newer sport bike clutches come standard with some slipper type function, so it is up to the rider to know what they need for the desired application and result.

Should you be shopping for a slipper clutch or upgrade to a new bike with one? They’re are not for everyone, but then again no mod is. Riders have been going fast without them since the dawn of motorcycling and many will continue without them. A slipper clutch isn’t easy to engineer or manufacture, and therefore aren’t cheap. Like many modern rider-assist technologies it comes down to your riding style, performance goals and of course the thickness of your wallet. Are you a roadracing, trying to shave down your lap times? Riding a lot of supermoto lately, using back torque to help break the rear tire loose into corners? Whether a slipper clutch is for you or whether it will improve your riding can only be answered by you, but at least you’re now armed with a little more knowledge.

Thanks to our friends at Yoyodyne for contributing technical expertise