In shopping around for a new bike, do I need one with a “slipper clutch”? Seems like many of the bikes I’m interested have one, but a few don’t. Will I feel something less than complete without one? What does a slipper clutch do exactly anyway?
Clutching at Straws
The slipper clutch is designed to do one thing really – eliminate the dreaded “wheel hop” when braking hard into a corner, mostly on the racetrack, but it can’t hurt if you ride hard on the street either. The typical motorcycle clutch, of course, is a stack of about 10 thin plates that when clamped together, transmit power from the engine to the transmission (then to the back tire). The more power you’re transmitting, the stouter the clutch pack needs to be.
When big powerful four-strokes began taking over racing again, engine braking began to be a problem – that’s when the throttle is closed and the bike is braking for a corner. In that condition, the rear wheel is trying to spin the engine (instead of the engine driving the back tire), and since the rear tire is already light (sometimes airborne even), and since modern four-strokes have high compression ratios and aren’t so easy to spin, it could get the rear wheel bouncing, locking up, skidding, and generally giving the rider a whole handful of problems at a time when all he wants to do is bend the bike smoothly into the apex of the corner.
By far the most common type is the ramp-type slipper clutch; the high-end Sigma Performance one in our photo uses simple ramps and ball bearings that, when the rear wheel is trying to spin the engine, causes those 10 plates to separate a smidge just as if the rider were pulling in the clutch lever a bit – but without the rider having to do so. The less expensive versions on most production bikes are just like it, but minus the ball bearings. Both types work fine for us on the track. The coolest ones are adjustable, letting the rider adjust how much engine braking he wants.
This illo is from a BlogHonda site that nearly gave me RC213V-S overload.
A lot of new sportbikes now also let you adjust engine braking electronically by propping the throttle butterflies open a smidge, which might negate the need for a slipper clutch, really, just like in the bad old days when a work-around for too much engine braking was to just turn the idle up to 2500 rpm or so.
Anyway, a good slipper clutch makes it possible for the rider to sail into a heavy braking zone with silk scarf billowing, whip two or three quick downshifts as needed, and have the bike brake smoothly with no fear of rear wheel non-cooperation – also with no fear of over-revving the engine and bending a valve or something terrible. Electronic rev limiters don’t work when the rear tire is what’s spinning the engine, and all the race schools teach you should bang those downshifts as soon as possible while braking so you can concentrate on turning.
When it’s time to reverse drive direction and get back on the gas, the slipper works in that direction just like any other clutch. The clutch hub slides back down the ramps and clamps the plates tightly together again. Away we go.
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