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High Speed Wobble Confusion(3 posts)

High Speed Wobble ConfusionBenoit
Apr 30, 2003 8:34 PM
I ride a Rocky Mountain Glide and experience a front-end wobble at speeds above 65 km/h. The wobble seems to come and go. I can descend the same section of road one day and hit 75 km/h the next day 65 km/h and the front end is all over the place and I'm waiting to do a superman onto the pavement. I have had the bike into my dealer and they can't find anything wrong with it. Any ideas?
re: High Speed Wobble ConfusionSpiderman
May 1, 2003 6:11 AM
There are a lot of things that go into a wobble. What exactly did your shop do? Try switching forks, wheelSETS (not just the front or rear one, but both), headsets. Try all of those at different times so you can eliminate any other variables (ie - switch the fork, if there is still a wobble, put original fork back on and switch something else, etc)

Also, wobbles have a lot to do with weight distribution on the bike. You can make most bikes wobble, any material, at high speeds (35+ mph) by taking your hands off the bars, sitting up and leaning back on the back end of your saddle. I wouldn't recommend that cause it can be scary and dangerous! I used to work at a manufacturer and had to do this with some larger frames to see if they in fact did wobble. Usually it was the fork or when we built up the frame (the bike shop had packed it partially assembled) it wouldn't wobble. The wobble disappeared when we built it up because everything was re-seated and aligned properly. Not that the shop did a bad job of origianally building it, but we essentially had gone over every part when rebuilding it.

I have read, and only had to do once or twice, that to stop a wobble you can press your knee against the top tube which will absorb/stop (i don't know the physical explanation of it) some of the reverberations in the frame.

Hope this helps, just some thoughts, they aren't written in stone
re: High Speed Wobble ConfusionRlieto
May 1, 2003 11:22 AM
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.

Hope this helps. Not sure if the bike you reference is steel.