Photographs showing what looks like a big black wing nut mounted on top of the left side of the Ducati GP19's upper fork crown appeared during MotoGP preseason testing at the Sepang International Circuit in Malaysia. Without actually saying so, some people are implying this might be a kind of ratchet device that can lock down front and/or rear suspension in a compressed state to improve the launch at the start of the race.
Such devices have been used in both drag racing and motocross to increase the acceleration possible at launch. Normally, maximum acceleration is limited by the wheelie point, the rate of acceleration that lifts the front wheel. In drag racing, the obvious answer (given sufficient rear-tire traction) is to lower the center of gravity, allowing more acceleration to be used without lifting the front.
Back in 1972, one of our Arlington Motor Sports mechanics had been taking his brand-new 750cc Kawasaki H2 to the dragstrip and having a terrible time. His bike was wheelying wildly at the start and “getting left bad.” This had been part of the background noise in the dealership until one Saturday evening I decided I’d heard enough. Time for action.
“Why don’t you lower that bike?”
“Uh, why would I?”
“Dragsters are long and low so they don’t wheelie. Your bike is tall so it wheelies like crazy. So lower it.”
No, we didn’t have shorter shocks in the parts department. No, we didn’t have metal stock suitable for making struts.
“So make ’em out of wood,” I said. “There’s crate wood in that pile over there.”
“Yeah, but wood is weak.”
“Make it real thick then. Remember, this is just a test.”
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We cut down the rolled tin spacers from the fork legs to lower the front maybe 3 inches and short struts did the same for the rear. The following day at the strip he launched like a champ and came back grinning with respectable time slips.
MotoGP riders say it’s essential to start from the front row because that way you might get clear of the six or eight other bikes trying to cram onto the racing line, pushing and shoving. Getting a strong launch is basic no matter where you’re gridded.
How would you compress the suspension? The engineering guys make up suspension ratchets that can be released. To compress the front, activate the ratchet and brake hard for your grid position. Click, it locks down.
You want to compress the rear too? The time of max rear compression is not during acceleration because your bike’s anti-squat has been carefully designed to prevent squat as you accelerate off turns.
Ever hear of the dreaded “squat and push”? Honda had a terrible case of it in early 1988. The rider would gas it off a turn, the back would squat, pulling weight off the front, and the bike would head for the outside of the track. Once everybody could adjust swingarm-pivot height, they could tune anti-squat to riders’ tastes.
But back to compressing the rear suspension: One way is to send a large mechanic to the grid to just push it down. Another way is to lock it down during the last turn before reaching the grid; the cornering force compresses both front and rear suspension.
If both ends have ratchets, the whole bike could be lowered 2 or 3 inches, substantially raising the wheelie point, all set to make possible greater launch acceleration. Once you’re launched, the down-lock ratchets are disengaged, either by the rider or by a line of code.
And there’s more: If the front suspension is locked down, its springs can’t give any help to lifting the front as the bike launches. That makes it even more difficult to wheelie, further increasing the wheelie point and initial acceleration.
More about the Ducati rear brake linkage...
In the past, the two obvious choices for arranging a rear brake caliper were:
The usual—to bolt the caliper to the swingarm itself. This, by transmitting 100 percent of rear brake torque to the swingarm, compresses the rear suspension. Back in 1980, when Kenny Roberts was languishing in bed after his preseason testing crash, Skip Aksland was having trouble being outbraked in practice at Daytona by Dale Singleton. Roberts said to Aksland, "Did you ever think about using the rear brake just before the front?" That initial application of rear brake pulled down rear ride height, allowing the rider to use the front brake a bit harder without lifting the rear wheel off the pavement.
An alternative—to make a full-floating rear caliper with a strut running forward to the chassis (and with the strut being parallel with the central plane of the swingarm). By feeding rear brake torque forward into the chassis, this makes it possible to use the rear brake over rough pavement without causing upsetting brake hop.
Now consider the whole range of possibilities. A full-floating rear caliper with its torque arm attached to the chassis at the height of the swingarm pivot is the same as bolting the caliper to the swingarm; applying the rear brake compresses the rear suspension. If we now lower the point of attachment to the frame, we begin to feed less rear brake torque into the swingarm and more into the chassis. If we lower the point of attachment until the brake torque arm becomes parallel with the central plane of the swingarm, all rear brake torque goes into the chassis and application of the rear brake feeds no torque into the swingarm.
But if we now raise the point of attachment of the rear caliper’s torque arm above the swingarm pivot (assuming the caliper is below the swingarm), we can multiply the torque it applies to the swingarm, causing the rear suspension to be compressed even more than if the caliper were bolted to the swingarm.
Make yourself models of all this, using thumbtacks and cardboard.
