These two pieces make up the inner hub of the slipper clutch used on the Honda CBR1000RR. The intricate angles on the interlocking ramps determine how much the parts separate on deceleration and how much they are forced together on acceleration.
How does a slipper clutch work, and why do I need one?
The main benefit of a slipper or back-torque-limiting clutch is that it reduces the effect of engine braking on the rear wheel under deceleration. When you close the throttle on your bike, the rear wheel tries to turn the engine rather than the other way around, resulting in a sometimes considerable braking force. Four-stroke engines typically have more engine braking than two-strokes, larger engines more than smaller, and big twins are usually the worst. Excessive engine braking can cause the rear wheel to skid and can even effect the rear suspension action, causing wheel hop or instability. Some riders also tend to over-rev the engine on downshifts (not mentioning any names here, Bradley…), and a slipper clutch can help prevent this from happening.
Honda introduced slipper clutches on its four-stroke racebikes during the ’80s, including the NR500 and FWS1000, and production slipper clutches first appeared on Honda’s Sabre, Magna, and Interceptor V-4 models. This setup used a one-way bearing to allow half the clutch plates to freewheel under deceleration. It was quickly replaced by the ramp-type unit as more manufacturers adopted slipper clutches, and this is the style used in most sportbikes today. Here, the inner clutch hub is made in two pieces with a set of ramps between them. On deceleration, the rear wheel tries to turn one piece of the hub faster than the other, which is being slowed by the engine, and the ramps force the hub apart, separating the clutch pack and allowing a certain amount of slip. Once the clutch starts to slip, the force on the ramps is reduced and the plates close up, introducing a certain amount of equilibrium into the system.
From a simple concept with few parts, the ramp-style slipper clutch has evolved into a tunable performance part. This STM unit can be adjusted using springs and washers to determine when and how much slip is allowed in specific conditions, allowing just the right amount of engine braking to be dialed in.
In its simplest form, the ramp-style unit has no additional parts, and the two hub pieces are in direct contact. This is the arrangement Suzuki used on early TL V-twins and Hayabusa models; the Hayabusa especially was prone to clutch chatter on a hard launch, one of the drawbacks of this simple design. Over time the ramp-style clutches have been significantly refined, however. Ball bearings are used between the two pieces of the hub for smoother and more consistent action, and springs—separate from the main clutch springs—are used to adjust the amount of slip. Slipper clutches began receiving a lot of attention when MotoGP was introduced with four-stroke machinery, as the bikes had a lot of engine braking; relatively crude electronics at the time meant a slipper clutch was the main way to keep that engine braking in check.
Just as ramps are used to spread the two pieces of the hub apart on deceleration, many units use ramps to force them together under acceleration. This reduces the chances of clutch slip (when you don’t want slip) and allows a smaller, lighter clutch pack for a given engine; additionally, lighter main springs can be used for less effort at the clutch lever. Ducati uses its APTC (Adler Power Torque Clutch) unit to reduce lever effort by as much as 40 percent on some Monster models, an important consideration for new riders.
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