Things that Go Bump on the Road - Part 1

Safety Corner logofrom the January, 2006 issue of WheelPeople

This is Part 1 of a two-part series discussing some of the things that can go wrong when you’re just riding along, minding your own business—and what to do about them. Part 1 is dedicated to bike shimmy.

It’s a god-awful experience. You’re bombing down a steep, hairy hill. That’s scary enough, but suddenly things get way worse. Your bike starts to wobble violently, handlebars rapidly swerving back and forth. You’re hanging on for all you’re worth. You just know you’re going to crash, and you know it’s going to be ugly at this speed. Somehow, you manage to stop without crashing. After you stop shaking and regain your composure, you check your bike from stem to stern. Nothing’s broken, bent, or loose. Your bike’s in perfect mechanical condition. Now, what the heck was that all about? You’ve just experienced bike shimmy—also known as speed wobble. There’s no mistaking it for anything else, and you’ll never forget the experience.

What is bike shimmy? Every object has a natural frequency (actually, multiple natural frequencies, but usually only one of them is of practical significance) at which it tends to vibrate. If a stimulus causes an object to vibrate at its natural frequency, the results can be dramatic. One example is a violin string producing a beautiful note. Another example is the tragic failure of the Tacoma Narrows suspension bridge in November 1940. After being open only a few months, vibrations induced by the wind matched the natural frequency of the bridge. The bridge began to oscillate wildly and self-destructed within minutes. You and your bike also form a system that can vibrate if excited at its natural frequency. Shimmy is a vibration in which the head tube oscillates laterally several times per second, causing the handlebars to swerve back and forth at the same rate. Engineers now know to design a bridge so that wind is unlikely to produce vibration at the bridge's natural frequency. However, bike designers can’t always prevent shimmy solely through design because each rider is different and impacts the natural frequency of the bike/rider system differently.

Fortunately, most riders rarely or never experience shimmy. Conditions have to be just so. Forward motion provides the energy to drive a shimmy, so high speed contributes. The gyroscopic forces of a fast-spinning front wheel may also contribute. However, shimmy can occur at lower speeds if riding hands-free. Other factors include descending steep inclines that shift weight to the front tire, and heavy rear panniers or saddlebags (especially if loosely mounted). Large bike frames, especially if not sufficiently stiff, have a greater tendency to shimmy. Having too short a reach to the handlebars can also contribute. Perhaps most important is the rider him or herself, as excess muscular tension reduces the body’s ability to dampen shimmy. These factors by themselves will not cause shimmy, but simply make the bike more prone to shimmy. Something still needs to trigger the shimmy, such as a bump or rough pavement, a stone, or cross winds. Shimmy can be worsened if the rider attempts to correct the shimmy by actively returning the handlebars to center. The rider cannot react fast enough and tends to drive the handlebars too far, thereby amplifying the shimmy. Shivering (from cold or from fear) can also feed shimmy, as the frequency of human shivering often matches the natural frequency of the bike/rider system.

To reduce the chances of shimmy (and for better downhill control in general), maintain proper posture when descending:
• Grasp the handlebars in the drops, keeping the brake levers within easy reach. Grip the bars lightly
• Slide your hips back on the saddle to take some of the weight off the front tire
• Position the crankarms horizontally
• Divide your weight between the pedals and the saddle, and gently squeeze the nose of the saddle between your thighs
• Relax your knees and elbows, so that your legs and arms can absorb shocks.

Limit your speed to your comfort zone to help you stay relaxed. Brake and steer gently—a little goes a long way at high speeds. If it’s cold, add layers before descending so that you won’t shiver.

If your bike starts to shimmy, keep your cool. Despite how it feels, the chances are excellent that you’ll be able to ride it out. Look ahead so that you stay on the road. To help dampen the shimmy, experts suggest raising yourself slightly off the saddle, placing one knee against the top tube, and/or squeezing the top tube between both knees. Hard braking shifts weight to the front tire, which tends to increase shimmy. However, increasing speed (i.e., not braking on a steep descent) also increases shimmy. Brake lightly to trim speed gradually. If you can, relax your grip and avoid wrestling the handlebars. Executing any or all of these measures will require great discipline once a shimmy has started. It’s best to execute them before a shimmy develops—when you first feel unstable or notice yourself getting tense.

Disclaimer: Bike shimmy is a complex and poorly documented phenomenon, and the experts sometimes disagree on its causes and cures. In the author’s opinion, David Gordon Wilson [1] is the most credible among the sources used.

Sources:
1) Wilson, David Gordon; Bicycling Science; Third Edition; The MIT Press; 2004.
2) www.spectrum-cycles.com/615.htm
3) www.sheldonbrown.com/brandt/shimmy.html
4) http://64.78.31.35/tips&info/downhil.html

Remember—safety is about choices. What choices will you make?


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