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Front-End Shimmy, a.k.a. Speed Wobble or Death Wobble
By Lennard Zinn
Has your bike ever started shaking when you got going fast on a descent or took your hands off the bars? Front-end shimmy has many nasty high-speed bike crashes and even more terrifying moments for riders that didn’t end up crashing. It’s the biggest reason I became a professional framebuilder—to cure the problem I had with my own bikes. It’s common on tall bikes, and I was 6’6” and racing on lightweight, thin-tubed steel frames in 1981 when it first happened to me.
Speed wobble or shimmy is the increasing-amplitude oscillation of the bicycle’s front end described mathematically by a Hopf Bifurcation. Front-end shimmy can be treated more simply by thinking of it as a resonance phenomenon. If the resonant frequency of the bike and rider is close to the frequency of things encountered on the road, like bumps, cracks, chip seal, and wind, then the oscillation of the frame whipping back and forth can be continually reinforced and magnified in amplitude by those factors.
Things reducing the torsional stiffness of the frame are:
With low torsional stiffness, it will deflect further under a twisting force and won’t snap back as quickly as a stiffer frame would. The length of time it takes for one back and forth twist of the frame (the “period” of one oscillation) will be longer and hence the resonant frequency of oscillation will be lower. Add the rider to it and the resonant frequency drops further (the period will increase)—the heavier the rider, the greater the drop in resonant frequency.
Stiffer wheels will help somewhat. Simply tightening spokes has fixed shimmy problems for some riders. Increased tire pressure may, too. So will ensuring that there is no play in your hubs or headset and that your tires are properly seated so there are no hops in them as they spin.
In addition to not being stiff enough, tall frames are often set up for shimmy by having overly steep head-tube angles as a means to reduce the length of the wheelbase. The steeper the head angle of the frame, though, the less the fork can absorb bumps and mitigate jolts coming into the frame.
A cracked or misaligned fork can be the culprit. In the days of steel forks, a misaligned fork could induce shimmy, but today’s carbon forks come straight out of a precision mold, so that’s rarely an issue. Back in the day (1980s), I cured the shimmy in some bikes by increasing the offset (rake) of the fork, which required bending the fork. That’s not possible with a carbon fork.
We build a lot of titanium bikes for tall, heavy guys, and I make sure the bikes don’t shimmy. To prevent shimmy on tall bikes, I design the frame with large-diameter, straight-gauge titanium or steel tubing. I slope the top tube to shorten the actual lengths of the seat tube, top tube and seat stays while maintaining the same effective lengths and thus the same positioning of hands, butt and feet.
I also make the cranks proportional to the rider’s leg length and design the bike with a correspondingly higher bottom bracket. This results in a shorter seat tube (since more of the seat height is taken up in the longer crank length) and down tube from the bottom up. And then I use a shallow (often 72-degree, sometimes less) head angle to let the fork take up more of the shock. And I make the wheels very stiff.
Some of my customers coming to me for a shimmy-free bike do so because they have essential tremor. Their hands continually shake the handlebar as if they were shivering, which can induce high-speed shimmy in bikes that otherwise would not shimmy under a rider of that height and weight. In such a case, in addition to taking the above measures in the design and building of their bike, I also install a headset steering damper (I used to employ Hopey Steering Dampers for this, but that company is no longer in business). For mountain bikes, Pademelon’s CS.1 Steering Damper takes the place of the Hopey damper.Simply installing a Cane Creek Hellbender 70 Visco steering stabilizer headset may cure a bike that shakes at speed.
Wheel balance can make a difference. Put the bike in a bike stand and turn the crank rapidly while in high gear until the rear wheel is spinning very fast. Let go. The bike will likely bounce up and down wildly because of the out of balance rear wheel. You can reduce this by balancing your wheel(s) like this:
Let each of your wheels spin down to a stop (the brake must not be rubbing, and on a rear wheel, the chain must be off). If the hub bearings are smooth and properly adjusted, the wheel will rotate back and forth a bit each direction before coming to a complete stop. If it’s got an aluminum rim, it will likely always stop with the valve at the top. Screw multiple valve collars onto the valve stem until the wheel spins down to a stop with the valve in random positions rather than always at the top (if it instead stops with the valve at the bottom, the fix will be to use stick-on wheel weights at the opposite side). Once balanced, when you crank the wheel up to high speed again, the bike’s bouncing will be much less. That could make the difference on a bike that shimmies.
If these fixes don’t do it, try some Rolf Prima paired-spoke wheels. I explained why paired spoking can reduce the tendency for shimmy in this column: https://lennardzinn.substack.com/p/investigating-chain-skipping-and.
― Lennard
Lennard Zinn has been designing and building custom bicycles for over 45 years; he founded Zinn Cycles in 1982 and co-founded Clydesdale Bicycles in 2017. His Tech Q&A column on Substack follows his 35-year stint as a technical writer for VeloNews (from 1987 through 2022). He is a former U.S. National Cycling Team member and author of many bicycle books including Zinn and the Art of Mountain Bike Maintenance, Zinn and the Art of Road Bike Maintenance, and The Haywire Heart. He holds a bachelor’s degree in physics from Colorado College.
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