Bicycle Mechanics - Shimmy shake

I don't know how to fix this. My shifters are on my down tube. When I let go of the bar to shift into high gear while going down a hill, the slightest bump causes my front wheel to oscillate. It stops as soon as I grab the bar again. The wheel is true to the best of my knowledge. No lateral runout that I can see when I spin it. No loose or broken spokes anywhere. No play in steering shaft bearings etc..
What causes this? And how can I fix it?
BTW, it's an old English Viscount.

All bikes can do the shimmy-shake. There is a good explination somewhere on the web, maybe someone else can find a link for you.

It might be that slightly changing your handlebar height or weight distribution on the bike. Putting a knee on the top tube or keep pedalling to counteract the shimmy...

Changing on a fast descent is not recomended. Get into a suitable gear before you get your speed up, and stick with it.

Thanks guys, I was thinking there was something wrong with my bike. I had an old Honda 750 that did that between 90 and 100 mph. To stop the shimmy I had to pull back hard on the bar while maintaining throttle. Easing off the throttle during a shimmy only made it worse. Sounds like the same principle works on bikes too.
MichaelW, thanks. That did happen only once (so far), I think because of your exact point. I was late shifting to high gear. Since then I have made the point of putting in high just as I start my descent.

Check your REAR (and, of course, the front) spokes and bearings, as looseness there can cause shimmy. Some bikes and bike-plus-rider combinations are far more sensitive than others to shimmy; one of the worst offenders was my 1960 Capo, particularly when I overloaded the rear rack with my UCLA textbooks.

Here is the article I found:


Subject: 8.60 Shimmy or Speed Wobble
From: Jobst Brandt <jbrandt@hpl.hp.com>

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.
------------------------------

Great article RacerX! That explains it very well. In a nutshell, just keep my hands on the bar, especially during power descents, and there's nothing wrong with my bike.
I just did the same hill last night with no shimmy.

this also may be because of your technique. make sure you hold your bars on the tops, close to center. you have MUCH more control one handed there. good for shifting downtubes or for grabbing your water bottle. when you're trying to steer one-handed on the hoods or drops, it's easy to over-steer back and forth, especially when you hit a bump that pulls the wheel one way or another.

in any case, seems you already found the best solution: keep both hands on the bar when in any dangerous situation. seems to be a pretty good rule.