My worst crash happened the first year I rode seriously. East of my Colorado town, Cerro Summit rises 4 miles to a long ridge. Today the road is wide and gently curving, but back then it was narrow with tight bends. I was following an experienced rider on the descent and figured the way to learn how to go downhill was to do what he did.
What he did was tuck his nose onto the stem and blast through the corners at a frightening speed. I had neither the skill nor the experience to emulate his daredevil descent, but of course that didn’t stop me.
Coming out of the third curve, my entry-level bike began to shimmy violently. I had no idea what to do, got thrown off the bucking bike and catapulted over the bars. Only the ability to fall, honed by contact sports, saved me.
Ask a dozen experts why bikes sometimes shimmy and you’ll probably get twice as many explanations. Theories abound, but it’s agreed there isn’t a universal cause. Sometimes a bike that’s been a model of rectitude for years will suddenly shake violently for no apparent reason.
Possible Culprits, and Fixes
Here are some common causes of shimmy (also called “speed wobble”) and how to troubleshoot them. We’ll continue this topic next week with more causes and solutions for bike shimmy, and tips for how to handle shimmy if it happens on a ride.
Check the bike. Mechanical factors such as a tight or loose headset, a front wheel (or rear wheel) out of true or a frame out of alignment can make the bike shimmy. A headset that binds means the bike always wants to go straight ahead and can’t handle the continual slight steering corrections that even straight-line riding requires. The resulting vibration is transferred to the frame and amplified. But in my Cerro Summit crash, a loose headset was the likely reason. An out-of-true front wheel can create the same situation, as can play in the front hub bearings.
Badly aligned or poorly designed frames are often the culprit. The bike wants to track straight but the rear wheel doesn’t follow the front in exactly the same line, leading to instability. Suspect poor alignment if your bike shimmies after it’s been crashed. Have it checked by a reputable bike shop. Tell the mechanic what you’re experiencing. Headsets and front wheels are easy to adjust. Most frames can be re-aligned, too, although it’s a bigger project.
Check for operator error. Sometimes, shimmy is not the bike’s fault but rather the rider’s. Tense arms and shoulders can keep the front wheel from turning slightly in response to balance requirements, and the resulting vibration gets passed into the frame, where it sets off shimmy.
An extreme riding position can induce shimmy, too, such as having your weight too far forward or back off the rear of the saddle when trying for aerodynamic benefits. Some riders report that if they are cold and shiver while descending, their shaking goes from the handlebar into the frame, and the bike shimmies at the same frequency they’re shivering.
Don’t forget to check back next week for Part 2, when we discuss more causes and solutions for bike shimmy.
Coach Fred Matheny is an RBR co-founder who has four decades of road cycling and coaching experience. He has written 14 eBooks and eArticles on cycling training, available in RBR’s eBookstore at Coach Fred Matheny, including the classic Complete Book of Road Bike Training, which includes 4 eBooks comprising 250 pages of timeless, detailed advice and training plans. The Complete Book is one of the many perks of an RBR Premium Membership. Click to read Fred’s full bio.