High-Speed Shimmy

BIG AL:  I have a 56-cm Trek 5200 with Ksyrium wheels. I've ridden it for a few years without problems, but in the last few months the bike has developed a shimmy, wobble wiggle (whatever) at speeds of 40+ mph.

Here's the kicker: I haven't changed anything on the bike, nor is my body or riding style different. The shop has pulled the bike apart and can't find anything that could cause this problem. Am I crazy? -- Lorri Lee L.

UNCLE AL FIRES BACK:  Assuming everything is adjusted properly (no loose headset or head cups or brinelled cups) and your frame is not misaligned from a crash, and your tire pressure is normal, and your hubs are not loose and you are indeed not crazy and descending with no hands at 45 mph, I have no idea.

Shimmy usually happens because a frame lacks lateral stiffness. Oscillation occurs between the two gyros (the wheels), most often at speed. Clamping your knees against the top tube with your crankarms horizontal will usually stop it.

Normally, this problem occurs when coasting, but it's been known to happen while pedaling, too. Weird crosswinds can cause a lightweight frame to shimmy on occasion. But if it's happening every time the bike goes over 40, take it to a framebuilder to get his or her very expert opinion.

A severe shimmy struck to my wife's bike during a windy time trial last spring. It scared her to death. But it hasn't happened again, even at higher speeds. Go figger.

INSIGHT FROM  AUSTRALIA:  I've cured shimmy on a road bike by slightly tightening the headset bearings. Slight, not overtight! -- Tozer

FEEDBACK FROM RON R.:  I experienced three severe episodes of high-speed wobble and then found the solution. To explain, here's a little background.

I ride a 60-cm Trek 5200. The size is a compromise because I'm 6 feet tall with a 35-inch inseam. I could have taken a 63-cm frame in terms of  leg length, but the top tube would have been too long. So I went with the smaller frame. The guy who fitted me also put on a shorter stem, feeling that my reach was a bit too long.

The three severe wobbles came at different fast speeds while descending. Dave, the shop owner noticed that my weight looked a bit too far to the rear. He suggested that I move forward when descending, keeping weight over the handlebar, and get a slightly longer stem. He also suggested putting at least one knee against the top tube as a damper, especially at speed.

This all made sense to me, as I recalled that in all three wobbles, I had pushed back on the seat, getting my head low and just over the bar. And in each case, I was going into a headwind, too.

My education was further enhanced by reading an article by Jobst Brandt. He suggested getting off the seat if a wobble developed, as the fulcrum of the oscillation is typically around the seatpost.

Armed with this new approach to body positioning, I have never again experienced a wobble. This includes a trip to Europe this summer, where I descended Mont Ventoux (took me a lot longer than Lance to get up the beast!) and numerous other Alpine passes without incident.

I hadn't realized that body position could be so significant, or that just a few inches change in position could have such a stabilizing -- or de-stabilizing -- effect. Obviously, this info isn't going to help anyone who has a damaged headset, cracked frame, badly out-of-round wheel, and so on. But for those who encounter the "mystery" wobble, which seems to occur for no obvious mechanical reason, consider your own contribution as the possible cause.

AL'S RETURN FIRE:  Ron's experience proves that shimmy is unpredictable and can't be laid to one cause for all cases. Jobst Brandt, an engineer and real smart guy about bikes, even says that a crooked frame might be less prone to shimmy than a straight one.

Watch what happens as superbikes decelerate on Speed Vision TV when they use onboard cameras. Same wobble, only those guys just hit the throttle to come right out of it. Those factory bikes have the finest design standards there are for two-wheeled, high-speed travel.

So I don't think shimmy is so much a design problem as it is dictated by bizarre conditions we encounter. No one has really come up with a definitive explanation so far. Maybe it's just a problem with two wheels?

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