Tdc
TDC
What's Ducati up to?
Kevin Cameron
THE NEW CARBON-FIBER CHASSIS ON Ducati’s D16-09 MotoGP bike may represent more than simply adopting a material of higher stiffness-to-weight ratio. The last big change in chassis was exactly that: the 1980s’ abandonment of traditional welded tubular steel loop frames in favor of twin aluminum spars. The classic 1950-78-style twin-loop steel-tube chassis could no longer cope with the larger forces being generated by slick tires, so a new chassis technology was required, one able to provide the necessary increased stiffness at an acceptable weight. Twin aluminum spars provided that.
By the 1990s, it was clear that twinspar chassis could actually be made too stiff, so that when leaned over in comers, such chassis were unable to flex enough to keep tires hooked up. That led to chatter, hop and skating. The result has been the development of chassis with directional stiffness-soft enough laterally to act as “sideways suspension” in comers but stiff in all other directions.
This informal sideways suspension was definitely a step forward, but it provided only springiness, with no damping. Undamped motions (those not subject to friction losses) can easily develop into oscillations. Metal has very little internal damping, which is why bells are made of metals. With little energy lost in each cycle of flexure, a metal bell rings and rings.
Ringing-continued vibration-is not what you want from suspension. We want a suspension to move only when deflected by a bump or depression, and then stop moving.
The pretty picture of a chassis that is laterally flexible but stiff in other directions is complicated by two serious problems: chatter and weave. If lateral chassis flexibility can act as a suspension in corners, it can also permit chassis oscillations to develop in step with the front tire’s bounce frequency. This is chatter. Both Yamaha and Honda had serious chatter in MotoGP at the beginning of 2006. Honda found a solution, but Yamaha struggled long enough with the problem to fall behind in points.
In 2008, Ducati’s MotoGP bike could be seen to “pump” during the lift-up as it accelerated off corners. Pump is rider talk for what chassis engineers know as weave, a side-to-side oscillation of the rear of the machine at 2-3 cycles per second. Back in 1967, I saw Mike Hailwood’s Honda RC-181 500cc Four weave in a big way as it accelerated off lowergear corners at the Canadian GP. I saw the Yamaha team’s TZ750s weave on the Talladega straightaway in 1974, throwing their riders’ feet off the pegs. This is not a problem team managers can dismiss with a shrug. It goes straight to Engineering on documents marked “Urgent!”
In the old days, increased chassis stiffness was reliable medicine against weave. It worked for Honda in the early 1960s when it switched the 250cc GP bike from a spine frame to a stiffer twin-loop design. It worked through the 1980s as the then-new twin-spar-chassis 500cc two-stroke GP bikes fought their battles (peaking in 1989) with high-sides as rear tires slipped and gripped.
Is increased stiffness all Ducati is seeking with the D16-09’s carbon chassis? I wonder. Oscillations develop only when a driving force of some kind injects energy into a system of flexible parts faster than that energy can leak out of the system through its internal friction. Ringing bells quiet down when we touch a soft finger to them. Metal chassis-whether twin-spar aluminum or welded steel-tube trellis-provide very little friction, making them vulnerable to oscillations. This makes me wonder whether Ducati has adopted the carbon chassis because controlled frictiondamping-can be engineered into it.
Around the world, thousands of industrial robots are in use, spot-welding car bodies, positioning components on surface-mount circuit boards or assembling parts. The faster they can move, the fewer of them are required for a given level of production. But moving fast means sudden starts and stops. Those motions bend the robot’s parts, setting them into oscillations that do not promptly die away. In some applications, this means inaccurate positioning unless the machine waits for the oscillations to die away. But waiting defeats the purpose of trying to make robot work faster. As a result, engineers are now building damping into carbon robot arms, in the form of thermoplastic layers between the plies of carbon-fiber. With damping provided by the soft thermoplastic, any oscillation of robot arms dies away quickly, making more rapid operation practical.
These are complicated matters, and the analysis I have proposed-that Ducati may have adopted a carbon chassis because it can be made self-dampingcould be quite wrong. But the problem of flexible chassis that act as undamped suspension is real. I used to look out the back window of the school bus as a fellow student drove his ’52 Ford coupe close behind. After every bump he’d hit, his car’s front end would wallow up and down seven or eight times because its shock absorbers had worn out long ago. If he’d been accelerating out of a turn near the limit of tire grip, his car would have slipped sideways every time its front suspension reached the top of its oscillation. Oscillations cause tire loading and, therefore, grip to vary.
The continued existence of problems lets us say with cheerful assurance that the design of the motorcycle is far from finished. Naturally, a few of the most devout Ducati enthusiasts will be appalled that the company could even consider abandoning its steel trellis chassis, but no engineering solution is permanent. Today’s solution always evolves into tomorrow’s problem. We do the great engineers of the past no favors by enshrining their solutions as if they were eternal. When problems crowd around us, an open mind is the best medicine.
