Jim’s Tech Talk
By Jim Langley
Speed wobble, which is also known as high-speed shimmy (and “death wobble” by motorcyclists) – and likely by other names outside the USA, is a dangerous condition that every rider should know about. It can happen on any bicycle, even electric ones. Due to their suspension plus wider and usually knobby tires, as well as the rougher terrain ridden, it’s much less likely to occur on mountain bikes.
It usually develops on descents at higher speeds, such as over 20 mph. But I’ve seen it at less than 10 mph, too. My scariest wobble happened racing down Monitor Pass in the Sierras at over 50 mph during a triathlon.
What Causes Speed Wobble?
What happens is something causes the front end of the bike to start oscillating from side to side. Only the front of the bicycle, the steering is affected. This condition usually begins as a slight wobble and then gets worse and worse because as the wheel moves side to side the elasticity of the frame tubes encourage the wobble to accelerate.
It can become so violent that you might lose control of the bike and crash. The late cycling technical guru and author Fred DeLong wrote about a speed wobble on a tandem in the Alps so bad that he suffered severe bruising on the inside of his legs from the frame.
Now, if you’re lucky, you may never experience a speed wobble. But, rather than be surprised if you’re not lucky and it suddenly happens, today I’m sharing the technique to keep in mind that usually stops speed wobble in its tracks. Even if you never use it, you might ride with someone who can and you can share it.
That’s what I did and why I decided to cover speed wobble this week. While riding with my friend Tom, he said his bike had a small wobble. I told him what to do if it happens again. And that very weekend on our club’s big century, he was able to avoid crashing when his bike wobbled even worse on a fast descent in the Santa Cruz Mountains (photo). He thanked me by text immediately after the ride.
Stopping a Speed Wobble
When a speed wobble starts your first reaction might be to slam on the brakes, but that doesn’t usually help. It might slow you down enough so you can avoid crashing so badly, but it might not help you regain control of the bike enough to avoid crashing.
1. Don’t let go!
The best bet to avoid crashing is to be sure to keep holding on to the bars even as they begin to thrash from side to side. I know that’s obvious but people do panic and let go of the bars, which is the worst thing you can do. That’s what the analysts think happened to the person who was seriously injured riding the e-bike I mention under wobble causes.
2. Clamp the frame with your knees!
As you’re hanging onto the handlebars, simultaneously move your pedals to 3 and 9 o’clock, which will bring your knees to the same height. Once there, bring both knees together and hold them against the top tube of the frame and keep them there, i.e. pinch the frame with your knees. These 2 steps effectively brace the frame and will stop even serious wobbles so that you can regain control.
3. Unweigh the seat
Don’t stand up from the seat to do this. Instead, as you’re holding onto the bars and clamping the frame with your knees, try too, to put more weight on the pedals and less weight on the seat. This is something to practice when you’re not experiencing a wobble. But, it’s similar to lifting yourself a little off the seat so you don’t feel a bump so much. If you can do it when experiencing a wobble it’ll transfer some weight lower and help stifle the shimmy.
Resting a Knee Against the Top Tube
Once you recognize that a bike has a tendency to wobble, if you stop pedaling on descents and position your foot so that you can rest one knee against the top tube that will usually be enough to prevent a wobble from starting. And if it can’t start it won’t worsen and put you at risk.
What If Your Bicycle Doesn’t Have a Top Tube?
Bicycle frame designs vary tremendously so the knees against the top tube won’t work on every bike. But, most step-through frames should have a tube that you can hold with your legs and that’s worth trying to prevent the wobble. And, unweighing the seat will help, too.
Why Do Bikes Speed Wobble?
Many things can cause speed wobble including road conditions. But if it’s bad pavement, the wobble will usually stop once you clear that dangerous part. With one caused by the bike or components, the wobble will continue and worsen.
To help when searching for what causes a bicycle wobble, below is a list of things I’ve seen and experienced that caused wobbles. The way I test for speed wobble is to try riding a bicycle down a slight grade to pick up a little speed and then letting go of the bars and riding no-handed. DO NOT TRY THIS unless you are an expert rider who can ride no-handed – and realize that even then you could crash if the wobble is bad enough. Be sure to wear safety equipment, too, starting with a helmet.
You may have heard to never ride a bicycle no-handed, which is a good safety rule to follow. But know that bicycles should not wobble with or without your hands on the handlebars. And some will wobble even at lower speeds if you ride them no-handed so it’s a good test.
The following are actual issues I’ve dealt with when finding and fixing bikes that wobble. The solution is to fix the problem, so for example for the first one you would lower the stem. The only cure for the superlight frame tubing is to switch to a frame with stouter construction.
- Stem too high lightening the front end too much
- Excess weight on wide dropped handlebars
- Tire not seated (front or rear)
- Defective or damaged tire with crooked tire treads (front or rear)
- Out of true or round wheel (front or rear)
- Seat positioned too high for the rider (incorrect seat height)
- Superlight frame tubing under a rider too heavy for it
- 25-pound e-bike battery mounted on a rear rack (the tail wagging the dog)
My Friend Tom
So far we haven’t figured out what’s causing Tom’s bike to wobble (it’s a Specialized Roubaix equipped with SRAM eTap). I tried my no-hands test and it did not wobble for me. But, it has wobbled several times now for him and he’s losing confidence in the bike. When last we spoke, I told him to try lowering his tire pressure which was slightly above the recommended range. I also asked him to try dropping his stem a little since he said the front end felt too light to him.
If you’ve experienced a speed wobble, it would be helpful if you shared your story, what happened and how you resolved the issue if you did. Together hopefully we’ll prevent others from ever crashing due to a speed wobble, which would be a great thing. And, you might even help Tom solve his wobble.
Jim Langley is RBR’s Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his “cycling aficionado” website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim’s cycling streak ended in February 2022 with a total of 10,269 consecutive daily rides (28 years, 1 month and 11 days of never missing a ride). Click to read Jim’s full bio.