Motorcycle Frame Stiffness: The 85 Percent Solution | Up To Speed

Ducati Desmosedici RR | The 85 Percent Solution

As a lad intrigued with racing machines, I naively thought that one day all would be revealed-that even if secrecy was necessary when a racer was new, we would eventually know everything about it.

I was wrong. Mysteries always remain. Amid the oohs and aahs surrounding the new Ducati Desmosedici RR MotoGP replica, we learned that the bike's steel-trellis frame is 85 percent stiffer than that of the 1098 Superbike. Now, a double-digit number like that gets your attention, especially in an era when factories are routinely touting the reduced stiffness of their frames.

Take Rob Taylor, talking up the newest Kawasaki ZX-10R: "We've worked on the balance between how rigid and how forgiving in motion the chassis is...to allow the frame to bend here, not there...stress areas eat transmission of feedback...they hog up any kind of motion as feedback...we've made the frame more friendly to the transmission of these motions." This sounds like a lot of information, but it gives the impression that designers are groping in the dark.

This is not to single out Kawasaki. All the factories are making a mystery of the nexus of rigidity/flexibility. Take Ducati's Claudio Domenicali talking about the 2008 MotoGP bike: "The new frame is lighter with optimized torsional and flexural rigidity." Flexural rigidity? Talk about your oxymorons

Neil Hodgson, who rode for Ducati in the 2006 AMA Superbike Championship and recently signed with American Honda, revealed that his 999 tied itself in knots due to insufficient chassis rigidity. The new 1098 hasa steel-trellis frame that is very similar to the 999's, yet the Desmosedici's frame is purportedly nearly twice as stiff. While it's true that the MotoGP bike is more powerful than the Superbike, the gap is not that wide, at maybe 230 bhp for the MotoGP bike and 210 bhp for the superbike-a difference of less than 10 percent.

To get a handle on what's going on here, look at how the car guys deal with rigidity. For them, it's simple: The chassis is essentially An platform to which the four suspension systems attach. So the goal is to make the platform as rigid as possible and let the suspension provide all wheel movement. The chassis is dead, the suspension live.

Why would cars and bikes be so different? Why can't we make a bike's chassis rigid and let the suspension provide all of the wheel motion? The major culprit here is the telescopic fork, which is performing two jobs simultaneously -suspension and steering-and is allowing undamped and, therefore, uncontrolled motion in both modes. Admittedly, the modern fork allows very little uncontrolled motion, unlike the old days when you could look down at your front axle and see it moving back and forth. But any such motion is significant.

The flex designed into current frames is actually very similar to that of a fork. Though the factories don't tell us what the scale is, I figure fork/chassis flex allows 5-10mm of movement at the tire contact patch. The interesting connection here is that frame flex may be developed to counteract, or compensate for, fork flex.

An intriguing clue can be found in Ducati's specs sheets. The hlins fork on the $40,000 1098R is the Road and Track version, which retails for about $2800. The $72,500 Desmosedici RR carries the hlins FG353: a high-end racing fork of 2006 vintage, which sells for about $10K. That 200+ percent price difference definitely buys you superior suspension.

The better the suspension is on a given bike, the stiffer its frame can be. Suspension components that allow less flex and uncontrolled wheel movement need fewer Band-Aids in the form of compensating motion in the chassis. As companies develop suspension components that further limit unwanted motion and refine true suspension movement, frames and swingarms will become stiffer, and our search will parallel the automotive model: The frame becomes a rigid platform and the only wheel motion is in steering and suspension, not tuned flex.

Top-drawer suspension let Ducati engineers pen a new trellis frame for the Desmosedici RR that's dramatically stiffer than its predecessors.

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