Drawing The Line - Composite Concerns

Up To Speed

At the beginning of the 2009 MotoGP season, Ducati introduced a new carbon-fiber frame on its GP9 racebike, and after a few races its aluminum swingarm began alternating with a carbon-fiber one, each working better in certain situations. Ducati's carbon-fiber parts get a lot of attention, but will we see more structural parts like these on racebikes? Or future streetbikes?

Composite materials are usually combinations of fibers and resins (plastics), and since wood fibers and natural resins can be used to make composites, who's to say when composite materials were first used? The British Mosquito high-performance fighter-bomber of 1941 was a composite design using wood fiber (in plywood), resins and glues for its structure. The Lotus Elite sports car of 1958 had a composite body/chassis using fiberglass for structural duty. Racing car constructors started using the stronger carbon-fiber reinforcement in the '70s, and by the '80s carbon-fiber chassis were common in purpose-built race cars.

The first carbon-fiber motorcycle chassis I saw was on the Armstrong, a beautiful little British 250cc racebike built in 1983. So even in motorcycles, carbon-fiber structural parts have been around for decades. Why hasn't composite construction dominated in motorcycle racing as it has in car racing?

Two reasons: First, motorcycle racing is predominantly production-based, and therefore an innovative new chassis can't be raced in most classes. In Superbike and Supersport/Superstock series, frames are raced as they come from the factory. Some Superbike series allow aftermarket or development swingarms, but more often than not carbon-fiber structural parts are not allowed due to cost.

Second, as a general rule, composites make excellent boxes while metal makes excellent beams. A car chassis is a complex box which is difficult to fabricate but somewhat easier to mold. A motorcycle chassis, on the other hand, must usually bracket the engine with two or more beams, just as a conventional swingarm brackets the rear wheel. These construct-ions are often easier to fabricate than to mold. There are certainly exceptions to these rules, like the cast frames of the Buells or the Fischer MRX, but even these frames would not be well suited to composites.

The Ducati GP9 frame, on the other hand, is not a beam-type chassis. The steel-tube frame used on its predecessor, the GP8, bolts to the top of the engine at the front and rear cylinder heads and doesn't extend downward to the swingarm pivot. Instead, the swingarm is carried in the rear of the engine/transmission case. The case effectively replaces the beams of a more conventional frame, and the Ducati frame thus becomes a box of tubes carrying the steering head and enclosing the airbox in the "V" of the engine. This frame is perfect for composites because the carbon-fiber box duplicates the function of the steel-tube box and at the same time doubles as the airbox, reducing parts count and complexity, and providing what is almost certainly increased airbox volume.

The Ducati swingarm, though it of course has "arm" sections on either side of the rear wheel, is dominated by a large box section at the forward end, enclosing the shock, and its complexity makes it a good application for composites.

Though it's strong and light, carbon-composite construction is usually labor-intensive and depends to a great extent on care and skill in the assembly process. A high level of quality control and numerous careful inspections are necessary to get reliable results.

Recent problems in the use of composites in the aircraft industry make this clear. There have been flaws (perhaps leading to failure) in the composite tail sections of Airbus aircraft, and recently Boeing had to temporarily shut down the facility making fuselages for the new, predominantly composite 787 Dreamliner. Boeing had found "wrinkles" in fuselage sections that are apparently areas of delamination between fabric layers. Delaminations are especially problematic as they can be effectively invisible to inspectors. Sometimes the process of finding these imperfections can be as primitive as tapping the surface with a small mallet to listen for a characteristic dull "clunk" rather than a ringing tone.

So in addition to being strong and light, composite construction for structural parts is expensive and its quality more difficult to control than that of more conventional materials. With every racing series trying to save money, there will continue to be some restrictions on the use of high-tech composites. Additionally, minimum-weight rules have been written to prevent extreme, and extremely expensive, uses of super-light materials. These rules clearly reduce the incentive to use carbon fiber in structural parts.

For production motorcycle use, we see that even the high standards of the aircraft industry may not be able to guarantee problem-free assembly, and manufacturers are not going to welcome the complication and expense. But we can certainly admire the new Ducati parts, and hope that we'll see more of this kind of innovative development-at least in the relatively open environment of MotoGP.

The GP9's engine is its central chassis component. Carbon-fiber main and subframe components bolt directly to the V-4, along with the rear suspension bits.