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Steel Bike Shimmy(5 posts)

Steel Bike ShimmyRlieto
May 14, 2002 5:56 PM
Has anyone experienced a shimmy while on a steel bike. Just bought a 853 Renyolds bike and experienced a bad shimmy at 30 miles and hour, just one time. The wheels and head set are fine. The LBS tells me it is a characterist of steel. There advice was to put my thigh against the top tube and it would stop. Ever experienced this and heard of the fix?
re: Steel Bike Shimmyguido
May 14, 2002 8:17 PM
Here's a post from the archives that explains the phenomenon. As steel has spring-like qualities that aluminum or carbon fiber don't, your LBS may very well be right:

Posted by: Dog Oct-03-01, 03:47 PM

Subject: 8h.5 Shimmy or Speed Wobble
From: Jobst Brandt

Shimmy is not related to frame alignment or loose bearings as is often suggested. Shimmy arises from the dynamics of forward motion and the elasticity of the frame, fork, and wheels, and the saddle position. Both perfectly aligned bicycles and ones with wheels out of plane to one another shimmy nearly equally well. The same is true for bearing adjustment. In fact shimmy is more likely with properly adjusted bearings than loose ones. The bearing or alignment concept is usually offered as a cause of shimmy and each airing perpetuates the idea.

Shimmy, the lateral oscillation at the head tube, depends primarily on the frame and its geometry. The inflation of the tire and the gyroscopic effects of the front wheel make it largely speed dependent. It cannot be fixed by adjustments because it is inherent to the geometry and elasticity of the components. The longer the frame and the higher the saddle, the greater the tendency to shimmy, other things being equal. Weight distribution also has no effect on shimmy although where that weight contacts the frame does.

In contrast to common knowledge, a well aligned frame shimmies more easily than a crooked one because it rides straight and without bias. The bias force of a crooked frame impedes shimmy slightly. Because many riders never ride no-hands downhill, or at least not in the critical speed range, they seldom encounter shimmy. When it occurs with the hands on the bars it is unusual and especially disconcerting.

There is a preferred speed at which shimmy initiates when coasting no-hands on a smooth road and it should occur every time when in that critical speed range. Although it usually does not initiate at higher speed, it can.

Pedaling or rough road interferes with shimmy on a bicycle that isn't highly susceptible. When coasting, laying one leg against the top tube is the most common way to inhibit it. Interestingly, compliant tread of knobby tires give such high lateral damping that most bicycles equipped with knobbies do not shimmy.

Shimmy is caused by the gyroscopic force of the front wheel that acts at 90 degrees to the axis of the steering motion. The wheel steers to the left about a vertical axis when it is leaned to the left about a horizontal axis. When the wheel leans to the one side, gyroscopic force steers it toward that side, however, the steering action immediately reverses the lean of the wheel as the tire contact point acts on the trail of the fork caster to reverse the steering motion.

The shimmy oscillates at a rate that the rider's mass on the saddle cannot follow, causing the top and down tubes to act as springs that store the energy that initiates the return swing. The shimmy will stop if the rider unloads the saddle, because the mass of the rider is the anchor about which the oscillation operates. Without this anchor no energy is stored. The fork and wheels may store some energy, although it appears the frame acts as the principal spring.

Shimmy can also be initiated with the hands firmly on the bars by shivering, typically in cold weather. The frequency of human shivering is about the same as that of a typical bicycle frame.

http://draco.acs.uci.edu/rbfaq/FAQ/140.html
re: Steel Bike ShimmyRlieto
May 14, 2002 10:00 PM
guido,

Thanks for the info. Bikes thay are wonderful things.

PS. Was going to name the first Born male Guido Lieto, but the ex. had a fit.
re: guidoguido
May 16, 2002 7:42 PM
That's my screen name, inspired from the Italian movie, "La Dolce Vita" by Frederico Fellini, starring Anita Ekberg and Marcello Mastrioanni who played a journalist named Guido. Did you know Fellini never got a drivers' license or owned a car? He used to get inspiration for his movies by riding a bicycle through the streets of Rome during the Fifties and observing up close the countless dramas along the way.

Yes, Bikes are wonderful things!
re: guidoRlieto
May 18, 2002 11:40 PM
"The Sweet Life" saw the movie many years ago. Did you know that Albert Einstien formulated the theory of relativity while riding his bike. He said riding cleared his mind and helped him think clearly. Wow.