Shimmy appears to be one of many properties of bicycles that is not
well understood. The math involved to describe what bikes do is very
complicated- even just turning a corner is hard to describe
mathematically because the bike describes arc in two different
planes- and fortunately we don't need to do the math in order to be
able to *ride* bikes! Shimmy is something that plagues bicycles,
motorcycles and shopping carts. Shimmy in cars and trucks is usually
due to an out of balance wheel bouncing up and down and is a
different beast, as I understand it.
From what I've read, shimmy is caused by the bike frame acting as a
spring. The head tube whips back and forth, causing some rotation of
the steerer in the headset. There is probably a contribution by the
steering geometry and from the forces of the spinning wheel,
especially the front wheel. As the frame whips to the left, it "runs
into" the gyroscopic force of the wheel and then whips to the right;
rinse and repeat. The mass of the handlebars, brake levers, etc.
will also contribute. Most people find that shimmy is speed-
dependent. A stiffer frame is probably likely to shimmy less. My
guess is that a bike with a more lightly loaded front end is more
likely to shimmy (short chain stays, long top tube). A lighter, more
responsive bike may be more prone to shimmy (but I don't know that
this is really the case). Larger frames may be more likely to shimmy
than smaller frames.
A shimmy has several fulcrum points: the contact patch of the front
wheel, the rider on the saddle and the contact patch of the rear
wheel; possibly also the rider's feet on the pedals. The rider's
hands on the bars tend to stop a light shimmy, but a rider who is
cold and shivering can induce shimmy through his or her hands. There
is a natural tremor in all people which can induce shimmy
(photographers are very aware of this tremor which causes blurring at
shutter speeds below 1/60 or 1/30)
IME the fastest way to stop a shimmy is to lift my weight off the
saddle. This has always stopped a shimmy cold. I don't have to
stand all the way up, just ease the weight up until the shimmy stops.
Preventing shimmy seems to be a black art. I have a friend who's Al
De Rosa shimmied horribly on descents; it went away when re put on a
new front tire. No idea why, it was the same make and model tire.
Wider tires at lower pressure may offer more damping and reduce
shimmy. A steering damper may do the same. Jan has reported that
needle bearing headsets reduce shimmy; I haven't tried switching
headsets. Changing weight distribution (shorter or longer stem, any
bags and such on the bike, moving the saddle slightly forward, etc.)
may reduce or worsen shimmy.
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