With all this talk of violent shimmy, forks, frames and some bikes do and some dont, I have to ask a question. Is it really the frame/fork/geometry that is causing it?
What is really making me ask this is that the same model was good for some, bad for others.
Here are my thoughts. The shimmy is observed as the headset oscillating back and fourth by an inch or so. I highly doubt that most of this is flex in the frame or fork, but rather it is mostly the headset bearing that is turning (as it should) due to some forces.
Since it is a resonance, we need some energy at that frequency to excite it, and though I haven't checked in a while, I suspect that the revolving wheel is revolving at about the same frequency as the shimmy, and is the source of the energy. I'll let you all think about it on your ride tomorrow, but there is too much snow salt and cold here for me to try.
But if this is the case, then the degree of shimmy that a bike experiences could have more to do the the wheelset on it (stifness of the axle, roundness of the tire) than how many pints the Raleigh fork bender had at lunch.
There are many other things that could affect the resonance as well, rider position (saddle hight, and front back placement) comes to mind the most.
So, first off, thanks for thinking about this with me. And maybe someone could switch the wheels between a bike that does and a bike that doesn't and let me know what they learn.
Dan "all my bikes shimmy from a Cannondale to an orbit track bike" Kasha in Frozen Salt Lake City
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