Originally Posted by
unterhausen
One experiment with $20000 worth of test equipment is all that is required to prove your intuition wrong in this case. Below the first resonant frequency of a structure, you will not get any vibration in the structure itself. It acts as a rigid body -- you're just shaking the frame back and forth with the fork/wheel. So changing the tubing would have no effect until you made it into a wet noodle. The wet noodle wouldn't ride well enough to even consider as a bike.
It's clear to me that a bicycle is a very complex system of interacting variables (the whole thing - wheels, racks, bags, rider, speed etc) which has some kind of resonant frequency that can result in shimmy. Anybody who has experienced shimmy knows this - it's a self-reinforcing resonance which can quickly grow out of control if you're not careful. And it's also clear to me that the stiffness of the frame seems to affect the way this system resonates - people talk about stopping shimmy sometimes simply by pressing one knee agains the top tube, which dampens down the side-to-side wobble. Just treating the bare frame like a giant tuning fork and making conclusions based on that about the more complex system, seems overly reductionist to me (no matter how much your equipment cost).
Neil