ASK THE MXPERTS: “IT SEEMS OBVIOUS TO ME”

 

mxpertcarbonframe444
Dear MXA,
    Bicycles once came with steel frames, then aluminum and now carbon fiber. And Formula 1 used to have steel frames, then aluminum monocoques and now carbon fiber tubes. It seems obvious to me that motocross bikes will soon be switching to carbon fiber also. Am I right?

It depends on how much you hanker for a $20,000 production bike. As exciting as exotic materials may look on the sales brochure, motocross chassis are about feel, not materials. One of the reasons that KTM has made so much progress against the Japanese brands has to be attributed to the chromoly steel frame. Chromoly steel offers both tensile strength and the ability to flex in the right direction. It’s the flex of a steel frame that makes a KTM a very good all-around handling machine. Steel may seem old fashioned, but for motocross frames it is really a very high-tech solution. And, if you had the wherewithal to build a one-off race bike,  steel would be the place to start. The Japanese factory teams cannot race with steel frames without making their production bikes look suspect. Don’t forget, Jeremy McGrath left Team Honda over this exact issue back in 1997.

Don’t get too misty about aluminum frames. They have endured  a very rocky history to get where they are today, largely because aluminum does not flex and, if it does, it tends to crack its large-grain composition. Starting with the original 1997 Honda Delta Box frame, aluminum produced frames so stiff that that they lost feel and placed 100 percent of the suspension burden on the forks and shock. Over the last 18 years, the Japanese manufacturers have been tapering the aluminum extrusions, thinning the wall thicknesses and relying on the motor mounts and head stays to soften up the feel.

Aluminum, much like air forks, became the cause celebre in the factory accounting departments because it was cheaper to manufacture and didn’t require any cutting, bending, mitering or gussetting. An aluminum frame is made up of eight cast, extruded or forged parts that plug together.

There are companies experimenting with molded carbon fiber frames. These builders tout carbon fiber’s light weight and controllable stiffness. But, in a production frame, no manufacturer would take full advantage of carbon fiber’s light weight for fear of a frame failure. Thus, they would lay the carbon fiber up well beyond the fail-safe level, and that would result in not only a heavier frame, but a stiffer one. Yes, it is possible to lay the carbon sheets up to get more flex, but with a risk level that the corporate lawyers would never sign off on. Think about carbon fiber handlebars. They first surfaced on mountain bikes back in 1986, but when racer Aaron Cox’s carbon bars broke at a Big Bear race the same year, carbon fiber bars suffered a setback that they have never recovered from. The same risk applies to motorcycle frames, which do break, regardless of what they are made from, but a broken carbon fiber frame would be a death knell to carbon fiber’s future in motocross.

Plus, motocross is not held on a pool-table-smooth F1 track or on a mildly bumpy Tour de France road. It is a gritty sport complete with shotgun blasts of roost, hard crashes and mechanically inept owners. A scratch could lead to carbon fiber failure. Spraying the wrong chemical on the carbon fiber could delaminate the cloth. The abrasion of a boot can eat through the welds on an aluminum frame; imagine what it would do to carbon fiber.

Carbon fiber may well be the brave new world of chassis design in boats, planes, bicycles and race cars, but the high cost of production and high premiums for liability insurance pretty well nix it for anything more exotic than skid plates, subframes and mufflers in motocross.

 

SUBSCRIBEINTERNAL

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