Deanster04 said:Doesn't happen on all bikes. It only happened once to me on a early bike with a British type touring fork popular in the 60's. Never since. I ride a lot of miles and my test of a bikes stability and balance is to take my hands off the bars going down hill and slap the head set. If the oscillations dampen immediately I will buy the bike.
I don't pretend to understand the problem because I have had friends who have seen the problem. My 3 classic steel bikes are Italian geometry with Cinelli type fork crowns and Italian forks. My moderne bikes are Ti and Carbon compact frames. Never an inkling of a problem. Weigh 220lbs and love to descend fast on my own built 32 H conventionally spoked wheels. Your method of stopping the problem is exactly what I did on my bike 45 years ago to stop the problem...and it is scarey when it happens.
I have a background in gyroscopes and am therefore attuned to the properties of rigid spinning bodies. (This by way of explanation. Apologies for rather lengthy discussion🙂
One of these properties of spinning bodies is called nutation; you can easily see it in any wheel by taking say the front wheel out of the fork, hold it lightly by the axle with one hand so it dangles with axle vertical; now spin the wheel up with the other hand. When it is spinning, give the tyre a sharp bump parallel to the axle; the wobble you see is the nutation. It is an underdamped harmonic oscillation whose frequency depends on the wheel's angular inertia and angular speed. Nutation is only one component in wheel shimmy in a bicycle, and is a property of all wheels.
The second component is less clear; some hold it is a function of frame stiffness and rider weight; I suspect frame stiffness while contributing is not the major part; rather it is the compliance of the rider to the saddle. But whatever the exact factors, what you have is a second underdamped system comprised by frame and rider and steering mechanism. You can imagine that if the bars are suddelny turned one way, the inertia of the rider and frame will shift momentarily and then re-center; but if poorly damped, may overshoot to the other side, and so on until fully damped. A loaded rack on the back is almost guaranteed to produce shimmy, because it can quite easily sway from side to side, reducing the damping in the system.
Now on to the third part: At some speed, the wheel nutation frequency matches the frame+rider frequency. If there is insufficient damping in these 2 systems, they will reinforce each other and become a noticable shimmy. In severe cases the oscliation amplitude will be so bad that the rider loses control.
If you ride no-hands and give the bars a light bump, the front wheel will immediately exhibit the nutation, but usually it damps out because there is no positive feedback from the frame+rider. So you will be under the impression that this bike is fine from a shimmy point of view.
Only thing is, this thing is entirely speed dependent, so there is likely another speed where the oscillation would not damp out as quick, or not at all until you put your knee agains the top tube, or perhaps lift yourself slightly off the saddle, or take the bars in hand. Any of these actions should normally be sufficient to damp out the shimmy.
Again, apologies for this long-winded explanation, but it is complex and I suspect even gurus like Brandt poorly understands the phenomenon, because in all his writings I have read he nor anybody else ever mentions nutation as a necessary part.
