There are many different things that can cause a vibration that some
will call "shimmy" (and that in some cases, others will call "speed
wobble"). Some of them are various kinds of rider behavior, that can't
properly be blamed on the bike at all. Cases in point: shivering.
Holding on to the bars too tight. I even recall one case (on a mailing
list) of a guy who would smack the top tube to initiate a vibration and
then complain if the bike shimmied.
On 08/23/2016 02:51 PM, Bill Lindsay wrote:
Unfortunately, nobody knows what causes shimmy. What I mean by that,
is that there is not a human being on earth that look at your bike and
say "your bike shimmies and here's why". If we took 9 bikes that
don't shimmy and 1 that does and put them in a lineup, there is no
human that can look them over and pick out the shimmying bike. If we
took a non-shimmying bike to the workshop there is no mechanic that
can do work on it to cause it to shimmy.
What we do know is that shimmy is a resonant oscillation, and
everybody who knows anything about resonsance knows that if it is
resonating, then everything that is important is just right in that
oscillating mechanical system. To stop it, you change something that
is important. How do you know what's important? Take a guess and
start changing things. If it helped, it was important. If it
doesn't, either it wasn't important or you didn't change it enough.
A needlebearing headset has proven to be effective because it adds
damping to the oscillating mechanical system. It's still resonating,
but the amplitude gets scrubbed off and the self-amplifying part of
the resonance fades away. (for electronics geeks, damping reduces the
Q of the system). The damping from a needlebearing headset is from
friction. A needlebearing headset does not turn as freely.
Weight distribution on the bike can be quite important, and is the
only thing you can easily make major changes to. Some people guess
that tire width and tire pressure can effect shimmy. Some speculate
that frame stiffness and steering geometry influence whether or not a
bike will shimmy, but you are stuck with those things.
Well said, Bill.
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