Shimmy. When your bike shakes as you ride.
An inconclusive discussion of speed-wiggle.It's one of the few Mysteries du Velo to ever halfway stump cycling’s Three Wise Wons—Sheldon Brown, Jobst Brandt, and the Great Greek Jim Papadopolous—at least in the sense that I don’t think they chiseled their explanations of it in granite—is s-s-s-s-shim-m-m-m-m-m-y.
Shimmy is democratic, thank goodness. Every builder I’ve spoken to dreads the topic like Superman dreads kryptonite, because it comes up in one of these variants:
- “I love my new bike, it’s just great, thanks a lot…but I notice a funny thing. When I’m going about 20mph with my hands off the bar, the bike shimmies. Other than that, it’s great! What should I do?”
- I just got back from a tour on my ten year old Connoisseur Selectissimo. I’m wondering—do frames go soft after a while? Because, funny thing, but when I take my hands off the bar and give a little shake, the bike shimmies. My friend’s didn’t do it, and it’s a Cheapy.”
Some of the time it happens when the bike is loaded heavy and high up front, and when it does, the method of loading or the weight is blamed. But sometimes it happens on unloaded bikes. Jobst said it happens more in cold weather, brought on by a shaking rider with a death grip on the bars; and that is has nothing to do with weight distribution or hand-on/hands-off the bars. He says bike length and saddle height contribute to it.
Jobst died a few years ago, he was smarter than me/Grant, but I don't buy that.
Your saddle height is determined by your leg length so you can’t really do anything about that. And wheelbase is a result of design targets (chainstays, top tube, head angle, fork rake, even seat tube angle and drop), not a target itself; at least that’s how I see it.
And big bikes (lots of vertical height) always have disproportionately short horizontal length as it is. Reeling in the wheelbase while the bike gets bigger and the saddle height grows will result in a bike that won’t fit or work well.
Jobst also suggested that perfectly aligned bikes may shimmy more or more often than misaligned ones, but wasn’t sure on that one. It's nothing you can use.
Here’s what I think, not what I know:
- If the top tube is too long, skinny, or light for the rider and load, and the speed is just right (usually about 18 to 20 mph), and you’re doing the wrong things with your hands, the bike may shimmy. I b-b-b-b-believe this because the 1992 62cm RB-1s shimmied, the 1993s didn’t, and the only diff was the top tube, which fattened to 1 1/8-inch. That’s not good science, but that’s what happened.
- If the headset is adjusted perfectly, the bike may shimmy. A reader wrote about this once, and suggested the fix is to snug it some. It seems to work.
Other accused culprits include untrue wheels (they have to be super wobbly to have this effect), asymmetrical loads, and too light wheels. I’ve accused them myself, but now I recant.
My Atlantis, built in 1997 or early 1998, has been ridden a lot and especially a lot heavily loaded on dirt trails in the local hills; and commutes, and regular old rides. There couldn’t be a better-riding bike on earth, but I can induce shimmy if I have a big load in back and I hook my thumbs over the bar next to the stem and pointing toward me (it’s as awkward and unnatural as it sounds, and painful). I came upon this position when I was goofing off, and the bike shimmied within three seconds. I changed positions and it stopped. I can’t make it shimmy with a normal grip, or even another abnormal grip anywhere else on the bar. I even hooked my pointers right next to the stem and hung on, but no shimmy.
That’s not the only time a bike will shimmy. Loaded bikes ridden no hands will often shimmy at speeds greater than 16 mph, but that’s just Apollo telling you keep your hands on the bars.
This needs an abrupt conclusion, no matter how unsatisfying it may be. A satisfying one would leave you with knowing, but I’ve already said that I don’t know, so how could I pull that off? My unsatisfying conclusion:
If the bike is reasonably stiff and straight and nothing is clearly wacky about the geometry—and here I’m not talking about a little more or less trail than you may think is absolutely proper, or a degree or even three of head tube angle; I’m talking about stuff so wacky I can’t even put it into words—then I’ll bet you a million amoebas it won’t shimmy 99.9 percent of the time you ride it. When it does, change your grip. Shift your weight. Load it differently. Hold the top tube ‘tween your knees to make it stop. Do not think your bike has a mysterious perfect storm of nuancical details that culminate in a bugaboo. No bike in a Third World country shimmies.
I’ve talked with people who’ve had super stiff, oversized aluminum bikes shimmy, which suggests frame stiffness isn’t a factor; but then why did the big RB-1’s stop shimmying when they took on thicker top tubes and nothing else changed? Jobst—and I realize I’m quoting him out of proportion to Sheldon and the Greek, but he’s had a lot more to say about it—he says when the bike is leaned slightly, the gyroscopic force of the front wheel twists the top and downtube. Would a double-top tube bike resist shimmy? I don’t know.
Last thoughts: To stop shimmy, change your grip, pedal, or touch the top tube with your knees to stop it. Be the commander of your ship, master of your bike, slayer of shimmy. Also, don’t ride a loaded bike at high speed, no hands.