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A MotoGP Clutch Design Can Make All The Difference
By Alex Edge

It is no secret that Nicky Hayden has been having clutch issues with his Honda MotoGP machine. Reporters following the MotoGP circuit generally acknowledge it, and Hayden's recent starts (particularly, his rapid fade from pole position to 17th in the opening lap at Phillip Island a few weeks ago) point to it. Hayden was testing new clutch parts following last weekend's Motegi round. While you might think it is simple enough to design an effective clutch, and that HRC could not possibly design an ineffective one; it is a bit more complicated than that.

First, let's talk about the huge demands on a MotoGP clutch. My recent experience with motorcycle drag racing, which included a lesson in the basics by famous racer Ricky Gadson, taught me that gaining maximum acceleration away from a dead stop requires heavy use of the clutch, which is modulated to apply the maximum forward drive, without lifting the front wheel or spinning the rear tire. Letting off the throttle causes the engine to lose its momentum, so to speak, so the throttle is held at a steady, fairly heavy application, and power delivery is modulated by slipping the clutch.

Slipping the clutch means that the clutch plates are not entirely locked together, but neither are they completely separated - they are lightly dragging against each other, transmitting some of the power while the rest is turned into friction as the clutch plates slide across each other. Of course, friction generates heat, and slipping the clutch generates massive amounts of heat in the discs as well as in any peripheral components. The more powerful the bike, the more heat is generated, and it doesn't get much more powerful than a MotoGP bike! Overheating the clutch can cause the plates to 'glaze', at which point they will no longer grip each other with much force - meaning that, when force is next applied to the clutch (as it is even during in-gear acceleration), it may slip.

Typically, a "slipper clutch", which is intended to let the clutch slip slightly under hard deceleration (preventing rear wheel lockup on corner entry), will actually make it more difficult to smoothly slip the clutch when launching hard from a dead stop. This problem is exacerbated by the setup of a MotoGP bike; Hayden's RC211V almost certainly runs an extremely tall first gear (to allow him to change down to first for slow corners), which will of course require a huge amount of clutch slip for a hard launch. Not only that, but a MotoGP bike's relatively short wheelbase, tall center of gravity and light weight, compared to a purpose-built drag bike make it incredibly wheelie-prone when accelerating hard at low speeds - again, a problem that is usually controlled by slipping the clutch. All this creates massive heat build-up, which can damage the clutch plates to such an extent that they can no longer hold full power without slipping.

One solution would be to switch to a larger diameter clutch, which would have more area to effectively dissipate heat. The problem with this solution is that a larger clutch carries more inertia, slowing down the engine's rate of RPM gain and thus its acceleration. Another solution would be to add more spring pressure, but this would (usually) make the clutch harder to pull, increasing rider fatigue. It might also change the way in which the slipper clutch operates, possibly requiring that system to be redesigned (if the RC211V does in fact have a slipper clutch - with so much of MotoGP developement shrouded in secrecy, Honda might currently be using a completely different system to do the job usually done by a slipper clutch). Of course, we're sure that HRC engineers have been through all these solutions, and probably a few more I haven't thought of!

Now that you have a basic idea of the technical challenges of designing a clutch to handle both the rigors of a standing start, and the entirely different demands of the rest of the race, let's talk about what happens when it doesn't. As I mentioned earlier, once the clutch plates have been glazed due to extreme heat, or otherwise damaged, they may not be able to hold the engine's full power, and can start slipping under hard acceleration. This leads to that horrible feeling when you twist the throttle all the way to the stop, only to see the needle spin quickly across the tach while the bike continues to accelerate as if you're only giving it 60% throttle.

For most riders, this is nothing more than a minor irritant - oh, gee, I'm only able to use 60% of my liter bike's horsepower during the morning commute to work! But for a rider like Nicky Hayden, battling against the world's most talented motorcycle racers in contention for a championship that Honda has spent untold millions of dollars trying to win, it's a much more serious issue. If Hayden exits a corner just behind Valentino Rossi, only to apply his throttle and find less than 100% of his Honda's horsepower being transmitted to the ground, how is he supposed to beat 'The Doctor'? That's a hard enough task aboard an equally powered machine.

Whatever happens between Rossi and Hayden in the final two rounds of the series, I'm sure most fans are hoping that both of these riders have any mechanical issues behind them.

Source: Motorcycle Daily
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