What About Different Motorcycle Chassis Materials?

Reader Jim Adamson recently asked for more discussion of chassis materials other than today’s usual choice: cast or formed-sheet aluminum twin-beam designs in the style of the late Antonio Cobas.

In my view, what twin-beam aluminum chassis have going for them is experience; many have been built, and those builders (for example, Kalex, whose chassis dominate Moto2) now have a lot of knowledge relating to them. I'm sure that tubular steel chassis can be built to give equally functional lateral flexibility (as a means of keeping tires on the ground when bikes are at high lean angle on unsmooth pavement) but because there is less accumulated experience with this material in MotoGP, reaching that point may require building and evaluating a considerable number of chassis.

Ducati entered MotoGP in 2003 with a tubular steel "trellis" chassis conceived years before by Massimo Tamburini. Its flexibility had previously been important to Ducati's success in World Superbike, causing Colin Edwards to say, "Yeah, they wallow, but they dig in and go around the corner." When progress with Honda's RC51 Superbike was slow, a Ducati was bought for measurement; its chassis stiffness was found to be half that of the Honda. A kit of parts was created to gradually reduce the RC's stiffness, and at each step, Edwards said, he could push the bike a bit harder before problems began. He was world champion on the RC51 in 2000 and '02.

But in 2008, Ducati’s steel trellis MotoGP bike could be seen to lack stability in corners, wallowing and weaving. Had the grip of Bridgestone tires become too much for its moderate stiffness? That chassis had served well enough during Casey Stoner’s first championship in 2007. Yet a year later he was saying, “On that thing, you can’t hit the same point two laps running.”

In 2009, Ducati leaped to the other extreme, building its ultra-rigid “black-pyramid” carbon-fiber airbox-cum-steering-head-mount MotoGP frame, attaching to the V-4 engine’s two cylinder heads. It was surely very light, but its rigidity seems to have led to tire skipping and hopping in rough corners. Stoner won fewer races on it every year, commenting that front-end loss came often and without warning. In 2011, he switched to Honda and, as he had done on the Ducati in 2007, he won 10 races and the championship.

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These contrasting experiences do not disqualify the two materials—tubular steel and carbon fiber—from possible future MotoGP success. They need to be used to provide the degree of lateral flexibility that has worked with aluminum twin-beam chassis, combined with adequate bending stiffness to provide braking stability. (Brake hop with weak tubular chassis of the 1960s and ’70s was comical to behold, as braking force flexed the steering head and fork tubes until the force rose high enough to break the front tire loose, letting everything snap back, initiating a ride-’em-cowboy hop at about two cycles per second.)

Success may be easier to achieve with tubular steel than with carbon, as metal frames are quite easy to modify, while carbon fiber requires support tooling and autoclave curing to manufacture, discouraging modification.

I keep remembering the problem of chatter and the absence of damping in motorcycle chassis materials. When Head pioneered aluminum skis, chatter was a serious problem. When wood (which goes thunk instead of bong) was added to the laminate, the chatter was brought under control. This makes me wish that a magical billionaire would decide to build successful MotoGP or Moto2 chassis out of wood. As with carbon fiber, one problem would be that of feeding high loads into the material, usually accomplished by molding in place metal thimbles or other fittings capable of handling the forces from bolts or studs attaching the fork, engine, and swingarm. An important precedent for such an advanced use of wood is the British de Havilland Mosquito fast twin-engined light bomber of World War II. Its flight surfaces and fuselage were wood, fabricated as shaped and glued laminates by Britain’s furniture industry. Alas, as happened to the wood-and-fabric biplanes of WWI when operated in hot-and-humid places, they literally came unglued. This is why few Mosquitoes survive.

Street riders seldom push their machines to the extremes present in racing, so MotoGP’s technology of engineered lateral chassis flex might be irrelevant save in the right kind of emergency situation. That being the case, production machines can afford to make use of a wider variety of chassis constructions and materials.