Ducati MotoGP Mystery Strut Explained | Cycle World
Courtesy of Ducati

Ducati MotoGP Mystery Strut Explained

What is that thing on the right side of the GP18’s rear suspension?

Writers dealing with MotoGP have recently speculated on Ducati’s latest test hardware, a slender aluminum strut extending forward from the rear brake caliper. Is it a rebirth of Kawasaki’s 1974 four-bar, twin-swingarm rear suspension known as “Fu-Bar”? Is it a warp drive that derives free energy from ions in the air?

Nothing so esoteric. It is just a means of varying the degree of torque coupling between the rear brake and the swingarm. The rear caliper bracket is pivoted on the axle centerline and a strut extends forward from it to the frame. When adjusted as a parallelogram—consisting of the swingarm, the caliper on its pivoted bracket, plus a pivoted strut of length equal to that of the swingarm extending forward to a pivoting attachment point on the frame—no brake torque is reacted to the swingarm. It all goes into the chassis at the forward end of the strut. But as the attachment point of the front end of the strut is raised by degrees, more and more of the rear brake’s torque is applied to the swingarm. The resulting torque on the swingarm tends to lower the rear of the bike when the brake is applied.

In recent years, it has been normal to attach the rear caliper directly to the swingarm (100 percent of brake torque applied to compressing the rear suspension) because with this setup, applying the rear brake an instant before the front lowers the rear of the machine. This allows the rider to brake a bit harder before the bike lifts its back wheel in a stoppie. When Skip Aksland went to Kenny Roberts in 1980 to ask what he could do to stop being outbraked by Dale Singleton, Roberts reportedly said to him, “Did you ever think about hitting the rear brake just before the front?” And in his little book, The Art of Motorcycle Racing, Mike Hailwood suggested applying the rear just before the front because “it steadies you up.”

Jorge Lorenzo

Longer-wheelbase bikes need some rear brake to get stopped (remember Jorge Lorenzo talking about having to learn the technique?) but the harder you brake on a bike whose rear brake torque is fed into the swingarm, the easier it is to develop brake hop.

Courtesy of Ducati

Through the Bridgestone tire era of MotoGP, front braking was powerful enough that most slowing was done with the front, so the rear brake was much less a part of the action. In the present era, two things are different. One is that tires now come from Michelin, whose rears have a lot of grip. The other is that Ducati riders find they need to supplement front-wheel braking (not all that hot, as Michelin fronts can’t do what the Bridgestones did) with fair-sized amounts of rear braking.

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Why is this a problem? With the rear caliper bolted to the swingarm, if the rear tire is being braked fairly hard and hits a bump hard enough to leave the ground, the spike of brake torque when it returns to earth is large enough to kick the rear suspension upward (because brake torque lifts the rear axle). The result can be violent rear brake hop, enough to keep the rider from using as much rear brake as he needs (possibly also enough to give him double vision). The very same used to happen at the front in the 1980 era of mechanical linkage anti-dive brake systems; hitting bumps or ripples at maximum braking with one of those “Gyro Gearloose” contraptions could set the bike hopping and the fork tubes and steering head flapping.

Danilo Petrucci

Ducati has made an adjustable linkage to explore variations of the geometry to find a sweet spot in which braking is hop-free but the value of squatting the rear of the bike is not completely lost. The lower the bike, the harder the rider can ultimately brake.

Courtesy of Ducati

Therefore, it is only natural to wonder if trying some degree of rear brake torque coupling less than 100 percent might kill the hop while retaining a useful amount of rear suspension compression during braking. This is what Ducati is testing now.

This is not a new idea. The great Manx Norton tuner Francis Beart built such a rear-brake torque-adjustment system in 1952 and I built one for my homemade Kawasaki H2R in 1972. In both cases, riders commented that with less rear brake torque going into the swingarm and more into the chassis through a strut, it became possible to use more rear brake without provoking rear-wheel hop.

Whatever happened to front anti-dive braking? It was quickly abandoned once Yamaha proved that dive is actually beneficial. As previously noted, dive lowers center-of-gravity height, thereby making slightly harder braking possible. But how could all that high-tech-looking linkage possibly have been wrong? And why are custom builders reviving it? First, remember Occam’s razor, that the simplest solution is most often the best solution. And second, never ask artists why they do things the way they do.