Saturday, July 12, 2008

Shimmy Shake Rattle and Roll

There has been a lot of talk lately about front wheel shimmy. Again. Online discussions, Calfee's owners' forum and his own theories regarding fork alignment, a recent accident among my clientele, shop discussions, personal experiences and hearsay. A woman died recently after losing control at high speed-witnesses said that her front wheel was shaking violently. Everybody's weighing in, regarding forks, headsets, wheels, tires, air pressure, frames, hand position, wind, etc. It's the same discussion we had last year, and the year before that. One summer it seems we've settled on HEADSETS as the culprit. Next year, TIRES. This year, it seems we're going toward FRAMES or FORKS.

But it's serious business, as any one who has experienced it will attest. What causes it? What enables it?

I submit that front wheel shimmy can be best explained (and perhaps solved) by acoustics. At a particular speed, your front wheel is rotating at a certain RPM. That number is a Frequency, which corresponds to a particular pitch, or musical note. When that frequency matches the resonant frequency of a component (fork, frame, handlebars), that component will resonate sympathetically.  If the component is not damped (by your arms, for example), the resonance will propagate.

A fork on a bicycle is like a tuning fork. It has a fundamental frequency and a series of overtones that are related mathematically to the length of its legs. If you have a fork separated from its frame, hold it by the steerer and give the fork ends a strum with your hand, then hold the end up to your ear. Hear that low pitch? Feel it in your hand? That's the fundamental. Now, (carefully) whack the fork on the edge of your workbench. Hear the higher pitches, the klang? Those are the overtones, which might be out of tune with each other (which is why it's a klang and not a beautiful note-it's a bicycle fork, not a musical instrument). The fundamental pitch is easiest to initiate, and WHEN, not IF, the frequency of your front wheel revolutions matches the frequency of that fundamental, your bike is going to start shaking.  The pitch will resonate, and will grow (propagate) if not damped somehow.  It might be slight or unnoticeable, it might be dramatic, but it will happen. If you have a stiff fork, the frequency will be higher and your shimmy will occur at a higher speed. If you have a flexible fork, or a longer fork (ie: cyclocross), the fundamental frequency will be lower and your shimmy will occur at a lower speed. If you go slower or faster, the shimmy will disappear, or the wheel RPM will induce an overtone frequency and will be felt as vibration, or perhaps a rattle of some sort.

Your wheels are not perfectly balanced. Lift up the front of your bike by the handlebars, and you will notice that the front wheel will turn and settle to a particular spot. The heaviest point of the rim/tire/tube combination settles to the bottom, and will be located at the rim joint (most commonly), or possibly the valve, or where the heaviest part of your tube is, or where your tire liner overlaps itself, or where your sealant has pooled. Remove the wheel from the bike, hold it in front of you by the skewer, and give it a spin with your fingers. Even if you have an ultra-light, high-zoot wheel, it will want to move up and down in space.  It might wobble or cavitate if the heavy spot is on the side (say, a tire boot or computer magnet), or if the wheel is out of dish.  That's the heavy spot you're feeling, and it's also the frequency of rotation. In mechanics, it is known as RPM, but in music it is known as pitch. It's actually making a sound, something like 1 or 2 Hertz, but that's way lower than the threshold of human hearing. Which is why we call it 110 RPM rather than C-sharp.

Automotive mechanics has parallels. If your front wheels are out of balance, at higher speeds your steering wheel will shake. If you have a fouled or broken sparkplug or wire, one cylinder will misfire and the motor will jiggle on its mounts. Bad CV-joint? Big noise, big trouble. It will shake itself to pieces and put you into the ditch. The next time you're under your car, look at the driveshaft-you will probably see little squares of steel welded on here and there, which balance it. When I replaced the u-joints on my old Z, the manual stated that you must mark the position of the knuckles in relation to the shaft, and replace them in exactly the same way. They were balanced.

When you turn your cel phone to "silent," or "buzz," you engage a little out-of-balance motor rather than your ring tone. When you get a call, the motor spins and it shakes so badly that you can feel it in your pocket, and you know that a call is coming in.

There are other resonant frequencies on a bicycle. The distance from the fork end to the handlebar ends, for example. Or the entire distance from the front axle through the frame to the rear axle.  On my old noodle Tommaso, I don't think I get a fork shimmy but I do get a wobble from the frame.  It happens around 15-17mph, so its frequency would have a longer acoustic length, perhaps the distance from my seatpost to the front end somewhere, or it might be the length of the entire fork.  When I sit up I can feel it immediately, and it will damp out if I put my hands on the ends of the handlebars, but it will not damp as quickly if I grab the handlebars near the stem.  Could it be the length of the fork plus handlebars?  The front wheel seems quiet, at higher speeds as well, but the frame is whipping back and forth.  It's quite dramatic and a little wild, actually. I showed a riding buddy once, and he thought I was going down. (I didn't-it looks worse than it actually is). If I go a bit faster, or slower, it doesn't happen.  If I input an interfering frequency by pedaling, it becomes intermittent.  It will wobble, then still, then wobble, then still (this might be a parallel to the beats you can hear when a perfect interval is not tuned well).  If I damp it between my legs, it stops. I recently took pains to balance the front wheel on that bike: we'll see if it helps.

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