On Mon, 07 Mar 2005 23:19:39 -0800, Mark Schecter
may have said:I have experienced shimmy on my mountain bike, and could easily control it. It
happened riding no-hands at moderate speeds, but light hands-on was all that
was needed.
Depending on the bike, rider and circumstances, it can occur at any
speed, and often seems more common when the rider has a firm grip on
the bars. The key is that in every case, this is a resonant action,
and doing something to change the resonant frequency will stop the
cycle; putting the hands on the bars when it was taking place
hands-off is an example. When it begins during a fast descent,
particularly when the rider is tense and has a near-death-grip on the
bars, it can be difficult for the rider to convince himself that what
may be needed is to relax the grip, change posture, and remove all
stiffness from the arms or take one hand off the bars altogether. One
common reaction is to go for the brakes, which may work...but also may
make the motion worse. The more instinctive reaction is to tense up
and fight the motion, but this is often exactly the wrong choice.
They key is that something that has an effect on the resonant
frequency must be changed; modifying the amount, distribution or
resiliency of some part of the reacting mass (which primarily means
the rider's arm and hand position and tenseness) is usually what's
possible.
I've only had a shimmy happen on a bike a few times. In each case,
changing grip position or taking one hand off the bars was what
stopped the motion. I've never had it happen on a fast descent, but
as I have always lived in a fairly flat area, my opportunities for
those have been few.
Quoted message said:Your confidence is reassuring - my imagination can conjure a
death-wobble, as they used to call it on early Kawasaki K1000 motorcycles.
The ones that had no damper? I remember them.
Quoted message said:Perhaps the imagination is working overtime, since a bicycle doesn't have
enough mass to overpower its rider the way a motorcycle does. Is that a fair
reading?
No, unfortunately, the situation can develop where the shimmy is quite
forceful. As stated above, the key is that to eliminate it, the
system producing it must be detuned by some action of the rider to
change the resonant frquency. If the rider has a grip on the bars at
the time, the system includes the rider, and it may call for a choice
that is not in the range of normal reactions. While changing speed
might work in theory, using the brakes reportedly often increases the
severity of the motion. My personal experience, and that of some
others, has been that the brakes are not usually the best first
choice...but it's hard to predict what will be.
All that said, many riders go for years or decades without ever
experiencing such an event. Few have it happen with any degree of
frequency, and even fewer have a really bad episode. It's not
something that I consider a sufficient hazard to be worth worrying
about, although having some idea of what to do if it occurs is simply
prudent.
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