Ask The Geek: Carbon Fiber Wheels And Pneumatic Valves

20 Questions In One
My friends and I were curious about why race teams use magnesium wheels and not carbon fiber. Carbon fiber is lighter and stronger, right? When molded into that certain application, is magnesium superior? Does it have anything to do with the limited availability of rim designs in carbon fiber or is it more cost related because of the R&D time it would take to test and produce?

We also want to know about pneumatic valves. We see the pneumatic valves being operated by a pressurized tank. Is it possible to use, or has anyone ever tested with, an electronic-actuated pneumatic valve system, one that would control the valves completely by the bike's electronics? The electronically controlled valves could be opened and closed by the actuators without the use of valve springs or a physical cam. If everything could be controlled by the electronics, would it be possible to program a cam lobe into the bike's electronics and test or change it with a laptop? This seems like it would take a lot of the guesswork and downtime out of tuning an engine on the race level. The pressurized pneumatic valve system being used in MotoGP right now could never be transferred to street application because when you run out of pressure your valves stop working, right? If it was electronic and there were no valve springs or cam resistance in the engine, wouldn't it free up a lot of horsepower that the additional drag of these two things are holding back? In my eyes this seems like a better design that could save weight and at some point be transferred into a superior street application. I know I am not the first person to think of this so I'm sure people have tested it before. Why isn't a system like this being used in racing or on street bikes?
Les Craig
Beaumont, TX

Carbon fiber is lighter and stronger than magnesium, but most race teams use forged magnesium wheels for a number of reasons. Primarily, many sanctioning bodies-including World Superbike and the AMA-do not allow the use of carbon fiber wheels, although they are legal for MotoGP use. This goes back to the early days of the material's use and Freddie Spencer's famous crash on a carbon fiber wheel in 1984. Although the material is stronger than magnesium, its stiffness makes it less pliable under impact-rather than deform and absorb some impact, a carbon fiber wheel may delaminate or even break. Additionally, where a mag wheel could potentially be repaired after an impact, a carbon fiber wheel may appear fine and yet be unsafe to use. That said, the technology involved has advanced significantly in recent years and safety is less of an issue.

A magnesium wheel can be forged in one piece, whereas a carbon fiber wheel must have aluminum or magnesium hubs bolted or glued on, adding complexity and cost to an already complex and expensive process. Carbon fiber wheels do not offer a significant weight savings compared with purpose-built magnesium wheels, and race teams may not consider the minimal savings to offset the additional expense and safety issues-real or perceived-of using the material.

You are correct that the pneumatic valve technology used in MotoGP is unlikely to be transferred to a street application anytime soon. Leakage in the system means a supply of air or nitrogen is necessary to replenish that loss. In a racing application, a small, highly pressurized tank can supply enough air or nitrogen for a practice or race, but for a production motorcycle a pump would be required, adding weight and drawing power. At some point, the benefits of pneumatic valve springs-higher rpm limits and reduced friction-may offset the added weight and complexity, but for now, plain valve springs provide lightweight, cost-effective valve control for streetbikes while pneumatic valve springs allow the MotoGP bikes to rev higher without adding undue complexity or weight.

Technology exists for a valvetrain with no camshaft and the valves operated by hydraulics and/or electronics. In an electro-hydraulic system, solenoids and hydraulic pressure are used to open and close the valves. An electromagnetic setup uses solenoids only to directly actuate the valves. These arrangements would allow for practically infinite control of valve lift and timing, allowing both to be changed for any rpm or load through the electronics. Eliminating the camshaft and its drive system would save a lot of weight and friction inside the engine, but the downside is that both setups add external weight and complexity. The hydraulic system requires a storage tank and a power-robbing pump, while electromagnetic solenoids draw considerable power that must be generated by the engine. Another stumbling block is finding components fast enough to deal with the high rpm of a motorcycle engine. Furthermore, both systems would require new levels of computing power, adding even more weight and drawing even more power. Currently, the tradeoffs leave camshafts with pneumatic springs the best option for high-revving four-stroke race engines.

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