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The Dreaded Shimmy(8 posts)

The Dreaded Shimmyridewt
Nov 25, 2003 12:44 PM
I've got a 2001 61 cm aluminum Lemond with a 140 stem and the cane creek "speed bars" that are legal for mass start races. I've mounted them right next to the stem. Here's the problem: Since day one, I've noticed that if I ride down a gentle incline and go "no hands" the bike will shimmy like crazy once it hits a certain speed - say 19 mph - I cannot remember the exact speed. As soon as a grab the regular bars, it stops. I never really gave it much thought a chalked it up to the big frame syndrome since I never race or ride hard with no hands and the bike never did that otherwise until ...... I installed the speed bars. For some reason, being tucked into the speed bars also generates a shimmy at certain speeds, maybe 28 mph. Once again, it was a really pronounced, bike shaking shimmy. It seems to vary with the front wheel on the bike - the worst is with a Rolf Sestriere, less frequently with a 36 spoke training wheel. My question is whether anyone has had experience with the Speed Bars inducing shimmy? I've had people suggest getting a new fork but I've also heard that it most probably emanates from the inherent flexibility of the frame so I'm not sure I want to drop $300 on fork and not fix it. I kind of hate to quit using the speed bars in races because I do think they help. Any thoughts are appreciated.
re: The Dreaded Shimmyeddie m
Nov 25, 2003 2:24 PM
Shimmy occurs when the front wheel resonates at the same freqency as the frame, or more accurately at the same frequency as the frame/rider combination. Typically, a flexible(large and light) frame with a relatively heavy front wheel is the most likely combination to cause shimmy. It's interesting that the problem is worse with the Rolf wheel. I bet that it has a heavier rim then the 36 spoke wheel. The weight of the rim/tire is the only part of the wheel which would have any effect on this.
One solution to try (I assume you checked the headset, etc.)is to put one pedal down when descending and lift your butt off the saddle. This changes the resonant frequency of the rider/frame combination and prevents the shimmy from starting. You might try another fork, but I suspect that a fork rigid enough to prevent shimmy would be unacceptably harsh and heavy otherwise. I wouldn't know which particular fork ot buy anyway. You could also try a wheel with a lighter rim and tire, but that would probably mean using sew-ups.
re: The Dreaded ShimmySpiderman
Nov 25, 2003 3:06 PM
A lot of times shimmy has to do with weight distribution. The speed bars add a lot of weight to the very front of the bike, hanging off a fairly long lever (the 140 stem) Try taking those off and see if you can recreate the shimmy (if you crash it is not my fault though:)

Another theory, based on teh frame resonation idea, is that the stiffer the wheels the more vibrations they transmit. That is possibly why the rolfs are "shimmier" than the training wheels

I don't have your answer and shimmies will stump even the greatest of frame designers but I hope that helps.
re: The Dreaded Shimmyeddie m
Nov 25, 2003 3:34 PM
The effect of weight distribution supports the resonance theory of shimmy. Moving your weight around on the frame has the same effect as changing the position of the weight on a metronome, or changing the length of a pendulum: it changes the resonant freqency. If you can create a large difference in natural frequency between the wheel and frame/rider, the bike won't shimmy.
re: The Dreaded ShimmyGregJ
Nov 25, 2003 6:00 PM
I don't know how to address the cause of your problem. But if you find yourself in a dangerous situation with a worsening problem, remember to clamp the top tube between your knees, this may stop the shimmy. Not long ago I was doing a modest descent on a very windy day. I came out of a sheltered area behind some rocks and got hit with a very strong gust from the side which put my bike into a radical shimmy immediatly, as I braked it got worse and right before I was about to hit the floor I remembered to clamp onto the top tube, stopped it right away. Normally my bike is very stable, even with no hands on the bar.
re: The Dreaded Shimmybikenj
Nov 25, 2003 6:25 PM
Here's an article from slowtwitch. I have the same problem.

Speed wobble

When a customer brings his bike into his LBS and complains of speed wobble (death wobble, front-end shimmy) often the typical bike shop mechanic immediately starts farting around with the headset adjustment. There is no adjustment problem, nor are the wheel bearings loose, and in fact it is most likely that nothing is wrong at all.

Speed wobble is just an inherent "flaw" of bicycles. Bicycle frames are amazingly stiff and strong in a vertical plane, but horizontally they are not. They are like ladders, and when you stand on a ladder it is strong, but when you and a friend stand a distance apart, each of you grasping one end of the ladder, and you twist, the weakness of the ladder is apparent.

In that same way, when a bicycle is moving in a line straight ahead, and gyroscopic effects of the wheels weigh on a unit as elastic as a bicycle frame—with its fork and wheels—you might get shimmy.

Shimmy occurs at a certain speed, depending on the bike and the rider. I have heard it said that every bicycle is prone, depending on its geometry, materials, and rider position, a particular resonant frequency, at which the bicycle oscillates. All that occurs when a given speed is reached on a descent.

The shimmy will stop if the rider unloads the saddle, because the mass of the rider is the anchor about which the oscillation operates. It may also be helpful to place the inside of your leg against the top tube on a descent when you experience a shimmy.

Certain elements will make the bicycle more prone to oscillation. All other things equal, those elements that render the bike more elastic will aid in its ability to shimmy. Therefore, a longer head tube will make the bike more likely to shimmy. Lighter, thinner tubes will make the bike more prone to shimmy. I suspect that shimmy might be more prominent now than in days past because bikes are so light, and their components somewhat more susceptible to lateral flex.

Shimmy is more likely in cold conditions, or when a rider is nervous and grips hard on the bars, because the vibration of a shivering rider is roughly similar to that of a frame's shimmy frequency, and this can help bring on the shimmy.

Therefore, while speed wobble is disconcerting, if you can adopt the habit of unweighting the saddle during a long, straight descent any shimmy problem should be largely abated.
re: The Dreaded ShimmyKeeponTrekkin
Nov 25, 2003 7:22 PM
Many good suggestions above. The fundamental fact is the resonant harmonic of the bike rider combination. Clamping the top tube between your legs (or even pressing it with one knee) changes the vibration dampening, usually stopping the vibration immediately. Shifting weight between the saddle and the pedals also changes the natural harmonic frequency (but probably is not as effective as the knee on the top tube). Braking is least effective, until you slow so significantly that you wish you hadn't. IMHO, a more forward weight distribution and tortional forces exerted on the "speed bars" are at the heart of your problem. It's no fun when it happens.
re: The Dreaded Shimmyridewt
Nov 26, 2003 8:28 AM
Thanks for all of your responses. I know when the bike will shimmy, so I'm not worried about the danger factor. I do agree that maybe the combination of a large, light frame might be the problem. I don't know this for sure, but I get the impression that if I had a custome frame built and told the builder I wanted to avoid the shimmy effect at all costs, the builder would build a beefier, heavier frame. I guess lightness has many costs!