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Bike shimmy on Descents

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Bike shimmy on Descents

Old 11-04-18, 08:22 PM
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In this thread and others I've talked a bit about my battle with this. I never have entirely regained confidence, to the point where there are races I skip solely based on having descents (eg Ft Hood). In my case it's boiled down to feeding back into the bike through my hands more than I used to, for whatever reason, doesn't matter how relaxed I ride. A significant realization in my case was that rough or uneven surface, like chip seal for example, greatly magnified the effect and lowered the speed of onset. I had a couple of short rough downhills on well worn training routs that I would start shaking down at <25 mph. I started to avoid them too.

It may sound simple but the most effective thing I've done to date is go to 25's and lower air pressure. Less road vibration transmitted to my hands and the issue is all but gone. Confidence is slowly returning.
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Old 11-09-18, 10:39 AM
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Originally Posted by Garfield Cat
The knees pressed against the top tube works well before the shimmy really gets worse. Once you sense the shimmy, then do the press. My understanding is that the knee pressure against the top tube does not need to be like a clamp. At the same time, the pedal/crank position should be horizontal and foot pressure on pedals is equally important, if not more important.

What you are doing is creating a triangle (Isosceles). But this knee lock thing is compensatory. Its just trying to address the shimmy effect and not the root cause. Some responses have suggested some causes.
This is what I do. Squeeze the top tube between my knees and put weight on the pedals. I'm not one who bombs down hills at crazy fast speeds, but I do it all the same and it seems to work for me. Others have suggested checking the bike for mechanical issues, and that sounds like good advice too.
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Old 11-09-18, 11:16 PM
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While it is certainly true that shimmy is often caused by frame geometry, fork geometry, and position on the bike there are also more mundane causes. Loose bearings or cones, insufficiently tightened front quick release and a very loose or defective headset can all cause shimmy or wobble. The worst wobble I ever experienced was on a bike with a head tube that had been worn into a slight oval shape by years of use with a J.I.S. headset (30mm) in an I.S.O. head tube (30.2mm).
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Old 11-13-18, 11:34 AM
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Originally Posted by Shimagnolo
No need to ditch the bike.
See my earlier posts.
I solved it by reading Dave Moulton's writings, taking some measurements, doing a little math, and ordering a custom fork to put the trail into a reasonable range. Bottom line is it is all about the trail.
if some of the suggestions given don't work, move down South. We have no hills so you will rarely see that speed.
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Old 11-13-18, 01:35 PM
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Clamping the top tube between your knees works to dampen shimmy, but not for a fixed-gear bike.

Jan Heine of Bicycle Quarterly suggests that a roller bearing headset provides enough damping to prevent shimmy. FWIW, I've not had a shimmy problem on my fixed gear bike using a roller bearing Stronglight A9 headset.
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Old 11-13-18, 02:06 PM
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Originally Posted by Voodoo76
In this thread and others I've talked a bit about my battle with this. I never have entirely regained confidence, to the point where there are races I skip solely based on having descents (eg Ft Hood). In my case it's boiled down to feeding back into the bike through my hands more than I used to, for whatever reason, doesn't matter how relaxed I ride. A significant realization in my case was that rough or uneven surface, like chip seal for example, greatly magnified the effect and lowered the speed of onset. I had a couple of short rough downhills on well worn training routs that I would start shaking down at <25 mph. I started to avoid them too.

It may sound simple but the most effective thing I've done to date is go to 25's and lower air pressure. Less road vibration transmitted to my hands and the issue is all but gone. Confidence is slowly returning.
I was in a crash in June on a descent and I'm still recovering. Recently, I went out once for a bike ride around the neighborhood.. I was a total mess.. Extreme confidence issues.. especially on the small descent I had.. Haven't been out since and now it's winter, so I won't be back out until it warms up, but I'm in the same boat and it's comforting to know I'm not alone. Although, I need to find a way through the anxiety so It doesn't cause me to crash again.. I'm going to keep an eye on this thread for suggestions.
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Old 11-13-18, 02:18 PM
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I have had this when have been cold at the beginning of a ride and I am shivering—not enough to be noticeable before the descent—but with enough subliminal tremor to harmonize with the natural frequency of the bike. Worse yet, static muscle activity seemed to make the tremor and the shimmy worse.
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Old 11-13-18, 04:45 PM
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Originally Posted by rpenmanparker
... When you have the problem, the solution is to ditch the bike.
Ditching the bike while the shimmy is happening and you are on it has its own consequences. (Often lowering the resale value for one.)
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Old 11-13-18, 04:57 PM
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Originally Posted by MoAlpha
I have had this when have been cold at the beginning of a ride and I am shivering—not enough to be noticeable before the descent—but with enough subliminal tremor to harmonize with the natural frequency of the bike. Worse yet, static muscle activity seemed to make the tremor and the shimmy worse.
I've had this happen on my good bike on a very hilly ride. Not fun. I knew what was going on but I was too cold to relax. I've also gone down a fast hill on that same bike with a screaming tailwind and probably went as fast as I have ever done.

Comment on quick steering bikes - yes, that can add to the problem. But I have descended Smuggler's Notch in Vermont twice in the race, no brakes and in a tuck. First time, the pace car driver (a local cop) said he had to go 60 between corners to stay ahead of us. I had no issues at all (besides hitting a frost heave I didn't see and getting my butt kicked 1 foot into the air at 50 mph; bending my cheap seatpost). I was riding a Fuji Pro, 59 cm, with its 75 HT angle, real fork rake and very little trail. That bike was quick! I never saw shimmy on that bike. It may be because I trusted it completely.

Ben
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Old 11-13-18, 05:13 PM
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Originally Posted by 79pmooney
I've had this happen on my good bike on a very hilly ride. Not fun. I knew what was going on but I was too cold to relax. I've also gone down a fast hill on that same bike with a screaming tailwind and probably went as fast as I have ever done.

Comment on quick steering bikes - yes, that can add to the problem. But I have descended Smuggler's Notch in Vermont twice in the race, no brakes and in a tuck. First time, the pace car driver (a local cop) said he had to go 60 between corners to stay ahead of us. I had no issues at all (besides hitting a frost heave I didn't see and getting my butt kicked 1 foot into the air at 50 mph; bending my cheap seatpost). I was riding a Fuji Pro, 59 cm, with its 75 HT angle, real fork rake and very little trail. That bike was quick! I never saw shimmy on that bike. It may be because I trusted it completely.

Ben
It happened on my road bike, a 1983 Peter Mooney, in fact. Short wheelbase, but nothing extreme or exotic.
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Old 06-04-19, 05:54 AM
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An update. I was working with another bike fitter early this year & he moved me foreword about 20mm based on some pedal dynamic data. Trying create a little more pedal pressure and less heel drop at the top of the stroke, which it did. Actually had to go to a zero offset seat post. No changes to the front end so this was also a significant reach change. Put up pretty good power numbers all spring. He thought this might also put me in better balance with the bike although it was not the objective of the fit change.

This was all done on a trainer. The revelation was how much more stable I felt out on the road, instantly. The slight tendency I still had to wobble downhill on rough pavement was gone. Back to bombing short descents tucked at 35+. It still has not entirely cured my confidence, my front wheel catching a little cross wind downhill still triggers a little anxiety for example, but much better. Entirely unexpected.
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