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GP Week : Issue 78
SLIP-SLIDING IT was former MotoGP rider Toni Elias who articulated the problem. The standard slipper clutch supplied to all Moto2 competitors was not, he revealed, anything like the sophisticated instrument he had become used to using riding MotoGP Yamahas, Hondas and Ducatis. That was why the bikes were getting all sideways into the corners. Hopping back wheels and slithering rear tyres demanded a new adaptation of riding technique -- whether you had come down a class from above, like Elias, or moved up from 250s and 125s. And the crude operation of the Suter- produced clutch wasn't helping. Once upon a time, motorcycle racers had to use the clutch just the same as a road rider does. Especially on down-shifts -- an upshift could be snicked through clutchless with a quick dip of the throttle, but going back down the box required at least a partial declutch, a throttle blip to equalise the revs, and smooth re-engagement. That change with the two-strokes. Without the engine braking of a four-stroke, riders no longer had to be quite so pedantic with down-shifts, and it was common to see people just stamping on the pedal, sometimes fully declutched and without any blipping. It was, as we shall see, always part of Mick Doohan's technique, and he was much imitated by other riders hoping they might also imitate his lap times. It meant the rider could concentrate totally on front and rear brakes, needing only to get to the right gear to accelerate out again. A really good rider might still be braking when he cracked the throttle open, just to get the fires burning cleanly. Then it was power on to spin the rear wheel, to initiate the exit phase. The point being that the clutch played little part in the finely balanced equation of how to get a two- stroke round a corner fast. And then it changed again, with the arrival of MotoGP. Changed big-time. As well as a major weight gain, engine braking was back. And corner entry became a whole lot more complicated. The engineers' name for it is "reverse torque", and it had never been away, of course, in World Superbikes and other forms of four-stroke racing. And it had re-emerged briefly in GPs back in the 1970s with Honda's ill-fated attempt at beating the two-strokes ... the oval-piston NR500. Massive engine braking was one of many huge problems with this powerful but difficult motorbike. Honda pioneered and patented a solution ... a sprag clutch, that gripped in one direction while a load-sensitive ratchet system limited the amount of grip in the other. This in turn created another problem ... finding enough grip for the dead-engine push-start that began all races at that time. A better solution came from drag-racing ... a mechanism that physically disengages the clutch, to allow a little (or even quite a lot) of slip. A slipper clutch is like a free-wheel device that isn't very free. It gives full grip when the throttle is open, but allows a certain amount of slip in the other direction. So when riders crunch down a gear or two, the back wheel keeps right on turning and the engine revs are equalised only gradually. Like a two- stroke, drag is minimised. Sounds easy. But the devil is in the detail, and good crisp performance is difficult to guarantee -- as the Moto2 riders are finding out. The amount and character of the slip is crucial. The drag must not overload the rear tyre, but the clutch needs enough grip to equalise revs with speed quickly, so the rider can take up drive smoothly when he opens the throttle again. The best riders learned to bypass the system. Mick Doohan freewheeled into corners on a big four-stroke Suzuka Eight-Hour bike as well as on his 500 two- stroke. He once explained to me: "I used to declutch completely to backshift, coast into the corner, then feed the clutch in again as I got to the apex." Valentino Rossi, as the on-board cameras reveal in fascinating detail, doesn't trust the slipper clutch either. He still eases the clutch lever into the turns. But not every rider has that degree of confidence, or competence. With GP racers still on two-strokes, slipper-clutch development came in World Superbikes and in Supermono racing, and by the time MotoGP four-strokes showed up in 2002, there were a couple of different systems on the SBK grid. Ducati and others used a cam-operated system that slipped the clutch in proportion to the amount of back-torque -- the more drag, the more the cams (actually ramps) disengage the clutch. Suzuki still used the older and simpler sprag clutch. It worked well enough in this context. The demands of the lighter, more powerful and more specialised grand prix bikes, however, were rather harder to meet. Suzuki dispensed with its sprag clutch before the first season was out; at the other end of the scale Yamaha was using an electronically operated slipper clutch ... since abandoned. In the early years, it was an education to watch different riders conducting their bikes into the corners. Not only for the clutch action, but because of the blipping. Software was being pioneered that meant the electronics were meant to take care of this for the rider, but it was rare to see anybody trust them fully, and not infrequent to see some dramatic sideways corner entry techniques when the auto-blip let them down. Mind you, the bikes were pretty sideways anyway, both into and out of the turns. Ducati even tried an extraordinary free- wheel system that forced riders to freewheel into turns, and then automatically re- engaged the clutch as the braking phase ended. Test rider Guareschi came to terms with it; full-timer Capirossi tried it, and crashed. It was never used in anger. By now, however, careful development has given MotoGP riders slipper clutches that perform well enough that they are no longer a cause for much concern. Moto2 riders find themselves a long way from this comfortable position. One overlooked technical change is contributing to the Moto2 spectacle. Michael Scott investigates AWAY Moto GP FEATURE >> 41