Mark Wolfe wrote:
> Lots of things, check the following:
>
> headset wheels for true/loose spokes
Read the FAQ
<http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html>:
Subject: 8h.5 Shimmy or Speed Wobble From: Jobst Brandt <
[email protected]>
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.
Todd Kuzma Heron Bicycles Tullio's Big Dog Cyclery LaSalle, IL 815-223-1776
http://www.heronbicycles.com http://www.tullios.com
