Re: [CR]Violent Shimmy, Was Raleigh

> From:
> Answers to Rec.Bicycles' Frequently Asked Questions and Interesting Information
> http://draco.acs.uci.edu/rbfaq/FAQ
>
> ========================================================================
>
> Subject: 8h.5 Shimmy or Speed Wobble
> From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
>
> 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.
>
> ========================================================================
>
> Chuck Schmidt
> South Pasadena, Southern California