Slipper clutches are commonly used to reduce rear-wheel hop under rapid deceleration on powerful large-displacement sportbikes like the factory-backed and privately supported machines raced in MotoAmerica Superbike competition. Tim White
Why are slipper/assist clutches showing up on smaller-displacement and non-sportbike models? Aren’t they just made to smooth out corner entry for fire-breathing big bikes? That used to be true, but a new reality has taken its place. Slipper/assist may even become the new “standard” in clutches. Here’s why.
When powerful four-stroke roadracers reappeared in the late 1970s—1-liter Superbikes and Honda’s oval-piston NR500 Grand Prix bike—engine-braking overnight became a big problem. When the rider closed the throttle on a high-power engine, the rear wheel suddenly had to send forward through the clutch enough power to overcome engine friction, which in race engines at peak revs can be as much as 25 percent of its horsepower. During braking for corners, weight transfers to the front wheel, often leaving too little grip at the rear to handle engine-braking torque. The result was sliding or hopping of the back tire, denying riders the stable platform needed for corner entry. As MV Agusta and Giacomo Agostini struggled to compete with fast-improving two-stroke opposition in the early 1970s, engine-braking was their biggest problem.
Riders were left to deal with this on their own. AMA Supersport champion Tom Kipp routinely pulled his clutch during braking, held it through the corner, then re-engaged it to accelerate. Five-time 500cc World Champion Mick Doohan, competing in the Suzuka 8 Hours endurance race, improvised the same technique. Because this divided the rider’s attention, new technology was clearly needed.
Kawasaki has been fitting a growing number of its street motorcycles—from the entry-level Z400 to the Versys 1000 SE LT+ adventure-tourer—with “assisted” slipper clutches, making it easier for riders to control the clutch in practically any situation. Courtesy of Kawasaki
Honda, first in the NR500 and then in the 1-liter FWS V-4 Daytona racer of 1982, attacked this problem with the first slipper clutches. In a slipper, when reverse torque is sent through the clutch from the rear wheel, a set of rotary ramps is activated, which reduces spring pressure on the clutch plates, allowing them to slip enough that rear wheel slide out and hopping are, if not completely eliminated, at least greatly reduced. When torque goes the other way, from engine to rear wheel, normal clutch spring pressure operates.
Here was a function every rider could appreciate because it allowed clutch spring pressure to be reduced, making it easier for the rider to pull and smoothly control the clutch.
More recently engineers realized that a slipper could work both ways, reducing clutch spring pressure to solve the engine-braking problem yet also increasing spring pressure when under power (now called “assist”) in proportion to engine torque. Here was a function every rider could appreciate because it allowed clutch spring pressure to be reduced, making it easier for the rider to pull and smoothly control the clutch. The more throttle the rider used, the more pressure the ramps applied to the clutch plates, preventing slippage.
Vincent, which in the 1950s produced legendary 1-liter V-twins, designed its postwar clutch with a centrifugal assist feature, with the goal of achieving high clutch torque capacity without a heavy lever pull. It didn’t always work as planned, but modern slipper/assist clutches work well.
Because every rider appreciates a non-grip-exerciser clutch pull, the slipper/assist feature is now appearing on a wider range of motorcycles.
