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If your bike has a scary high-speed shimmy, what can you do?
Wheel stiffness has a lot to do with it, as does frame torsional stiffness and amount of vibration absorption in the fork. In general, it is only an issue for tall bikes and tall riders, with its severity increasing with rider weight.
One thing you can do right off is try some stiffer wheels.
You can also check that the hub bearings and headset bearings are not out of adjustment. If the headset is too tight and thus can’t adjust quickly enough to inputs, or if the hubs or headset are loose and rattling about, shimmy can also result. Obviously, make sure your hub quick release skewers are tight, too.
Until a few years ago, I had thought that bicycle shimmy is a resonance phenomenon, but now I believe that it is actually a Hopf bifurcation. You may know of the Tacoma Narrows Bridge Collapse (https://www.youtube.com/watch?v=xox9BVSu7Ok), which I remember as a physics student being explained as a function of the wind somehow exciting the resonant frequency of the bridge. I believe that now there is relatively universal agreement that it was a Hopf bifurcation that brought it down.
That said, relating to both things as resonance phenomena can still fix the problems. My efforts to increase the resonant frequency of the bicycle/rider combination always reduced shimmy. The taller the bike and the heavier the rider, the longer it takes to twist back and forth through one cycle. So to shorten the period of oscillation, you can make the frame stiffer, tighter, and smaller, and you can reduce the weight of the rider and of any other weight on the bike.
Shimmy is a function not just of speed and rider weight and weight positioning, but also of environmental factors like wind and road surface. My guess first of all is that on a different day without wind, you could ride the same section of road at the same speed and not have it happen.
My frame design for big bikes and riders has a number of features designed to increase the frame torsional stiffness as well as the amount of vibration absorption in the fork. I was the first to use 1-1/8” (rather than 1”) fork steering tubes on road bikes (in the early 1990s) as a way to stiffen the front end of the bike as well as the steering tube itself; now we use even bigger head tubes and a tapered fork steerer that’s even bigger at its base. It is to reduce shimmy that I lower and slope the top tube and increase the diameters of the tubes. These things all increase the torsional stiffness of the frame, as does shortening the seat tube from the bottom, too, by raising the bottom bracket to adjust for the long cranks (which maintain a standard proportionality to rider leg length). And there is a consequent shortening of the down tube, top tube, seat stays and chain stays simply by changing the angles of those tubes. And using a shallower head angle makes the fork angle out ahead more, which both increases the stability of the bike as well as allows the fork to absorb more road shock along the fork legs.
That said, frames can lose stiffness over time, and it is always possible to make a stiffer frame, if a given one isn’t stiff enough for someone.
If shimmy happens, it can be very scary. I know this firsthand, and it is why I became a framebuilder.
While I was able to make shimmy-free bikes for myself pretty quickly (at 170 pounds, I’m not as big of a challenge as the vast majority of my customers), it took me years of steady improvement and experimentation to figure out how to make a bike that doesn’t shimmy under riders much heavier and taller than me. And, the thing I also did not know at the time was a better way to come out of it safely when shimmy occurs.
The normal tendency of a rider whose bike goes into a high-speed shimmy is to hang on tighter to the handlebar and hit the brakes. The former certainly increases shimmy, and I believe that the latter does too, initially, by throwing more of your weight onto the front…
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