Illustration by Ralph Hermens

Alternative Paths To Motorcycle Chassis Flexibility

“The suspension can’t do the work at 60 or more degrees from the vertical...”

Commenting on my story about chassis materials, a reader asked, “Could Ducati have built in flex in how it attached the very rigid carbon frame to the heads, say, using elastomer in the mix to tune the flex to the track? Is there some way that actual lateral suspension could be built into the hub so there would be a degree of slide for the wheel 90 degrees to the motion of the suspension? Right now, the chassis is just twisting. It’s like a torsion spring damped by the junk hung on it (gas tank, rider, etc.). Hardly ideal, but better than nothing.”

He is right to ask these questions. To keep a motorcycle hooked up over unsmooth pavement while at high lean angle, some form of flexibility other than the bike’s suspension is needed. The suspension can’t do the work because, with the bike at 60 or more degrees from the vertical, suspension movement is mostly in the horizontal plane. To allow the tires to move up and down over imperfect pavement, we need some form of lateral flexibility. Allowing the chassis to twist in order to provide this has not worked because, by tilting the wheels out of plane, this produces upsetting precession forces that generate uncommanded steering.

Messing with tire flex can be a trap that leads to chatter, which can excite the tread band into cyclical lateral motion.

The most basic form of this lateral flexibility is provided by tires, whose tread band can move from side to side via the flexibility of its sidewalls. This has been important to front-end grip on Kawasaki’s World Superbike-spec ZX-10RR. But messing with tire flex can be a trap that leads to chatter, which can excite the tread band into cyclical lateral motion.

All crew chiefs know there is a list of things to try when the rider starts losing the front without warning:

  1. Changing from stiff roller steering-head bearings to more flexible ball bearings
  2. Reducing the diameter of fork tubes (this is a fourth-power effect, so decreasing tube size by 1 millimeter increases flexibility 8 percent)
  3. Reducing front-axle stiffness
  4. Moving front-wheel bearings (which are ball) closer together

Not often discussed is the flexibility of the wheels themselves. Every top rider has a preference for a particular wheel “feel,” and this is provided. Stiffer is not always better.

The proposal to provide flex through elastomer chassis inserts is a good one, if the necessary high bending and torsional stiffness can be retained. As in, “Oh, we’re at Assen. Remove the red chassis inserts and fit the green ones.” At present, the best way to adjust lateral flex is to make, test, and produce new chassis. That limits the number of experiments per year!

It's clear that we are talking about very limited movement here. In the bad old days of dinky fork tubes (36mm!) and "right-side-up" construction, we had to provide a half inch of clearance between the front tire and radiator (or cylinder head, in the case of Ducati) at full front suspension compression. Big tubes (now carbon in MotoGP) and upside-down construction have stiffened front ends a great deal so that 1970s 12mm of flex is much less today. So let's estimate lateral flexure at something like low single-digit millimeters.

Small wonder, then, that when the new chassis appear in preseason tests or later, they may have no detectable useful effect.

Think of how complicated this could get. It’s like a pushrod valve train. As the front tire hits a pavement transition, the tire deflects first. Then the wheel flexes, together with the front wheel bearings, the fork tubes, the steering-head bearings, and finally, the “engineered flex” that we have designed into the chassis. Just as in that pushrod valve train, each element in the stack has its own natural frequency. Given the shape of the cam lobe, predict the motion of the valve.

Good luck.

Small wonder, then, that when the new chassis appear in preseason tests or later, they may have no detectable useful effect. One top rider, reminded that the latest solution had been optimized by computer analysis, told the engineer, “Okay, then maybe you’d better get the computer to ride this thing.”

Our reader is also right to point out that having the entire front end jiggle from side to side is “hardly ideal.” Yes, the mass of wheel, tire, brakes, and the entire fork assembly is large, limiting the frequency response of the system as it patters over bumps. Yet we can understand the engineers’ resistance to letting the wheel slide sideways against springs on the front axle or letting the rim move sideways on parallelogram flat spokes because those movements offset the tire footprint from the steering axis, creating who-knows-what effects. Yes, but doesn’t tire flex do that very same thing?

Thank you for stirring the pot.

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