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#1
05-01-2006, 08:25 AM
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I've had it occur on several occasions, different bikes too. I was able to determine that a particular bike had a tendency to shimmy at a particular speed. The shimmy usually (but not always) started after hitting a small irregularity in the road surface and sometimes would turn violent. What works for me is to loosen my grip on the bars. I found that I had a "death grip" on the bars, and that was a major contributor to the problem. Since becoming aware of the "grip" issue, I've had nary a problem, but the knee trick works. Sometimes it takes a leap of faith to lighten your grip as the shimmy increases. Try and set up the shimmy on purpose and see if this helps.
Birddog 
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#2
05-01-2006, 12:27 PM
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Pilot error not all of the picture, in my experience...
So, if I can quote e-richie:
"for the rest of us, when it comes to the issue of shimmy, and descending speeds, and the related issues, i say it's all pilot error." I suspect that there are cyclists that can conquer all vibrational instabilities. Not me - I do possess some imagination as to what can happen at high speed, and thus will brake and thus tighten up going down many of our Colorado mountains. On an earlier version of my Ti Rapid Tour (custom geometry w/ 72 degree seat tube), braking and other "pilot errors" caused my bike to shimmy. Serotta, to their credit, worked with me on this, getting me to increase the spoke tension on my custom wheels (their low tension made the shimmy life threatening to me), get a stiffer fork, and finally, when my bike was happily shimmying away while being passed by Huffies on a MS150, replaced the frame with one with a stiffer tube set (paticularly the rear triangle and top tube, I think). Since that time, with the same semi-incompetent pilot, my bike has not shimmied - even with the sort of hard. panic braking like when a parked car pulls out in front of you on a downhill - this would bring it on the earlier frame. Enjoy, Eric
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#3
05-01-2006, 12:57 PM
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With all due respect to e-Ritchie, I don't believe shimmy is exclusively due to pilot error, from personal experience.
I once owned a Trek 5000 which was exceedingly nervous over 35 MPH. Behavior was independent of wheelset, position on the bike (with a caveat, see below), force on the bars, crosswinds, etc. Same road, same hill, same rider, three other bikes - no problem. There's a theory that the front end can only oscillate if anchored by a large mass, and lifting your butt off the saddle temporarily will stop the oscillation. Since my current bikes don't exhibit "death wobble", I haven't tested this theory. My $0.02 TimD
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#4
05-01-2006, 02:40 PM
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Quote:
Yeah, me too... just over 20 mph, no hands. Knee against the top tube stopped it. Hasn't happened with any other bike I've owned. Of course, I could keep my hands on the bars and avoid that particular manifestation, but there were the times when it felt like it was trying to shimmy in a corner, a really squirelly feeling, causing me to exit the corner wider than planned. I ran an F1 fork on it while Serotta re-raked the fork and didn't notice any problem during that time. Now with the fork raked to 43mm (73 deg head angle), I've only had one bad corner which _could_ have been explained by the road being slightly bumpy... but the same corner and line hasn't produced the same squirelly feeling on other bikes. Orin.
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#5
05-01-2006, 09:32 PM
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As a born again coward
Quote:
As I healed and began riding normally again, relaxation regained, speeds increased, life is good. One thing to keep in mind wrt relaxing your grip on the bars: you've got these two big gyroscopes (wheels) spinning like crazy that really want to go straight, you just need to relax and let it happen.
__________________
Greg
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#6
05-01-2006, 10:59 PM
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I had a bike that I could not keep from shimmying at speeds over 32mph.
I had one that would always wobble at 35mph but clamping the top tube with my knees would always cure it. I had one that sometimes would wobble and sometimes not. Knee on the tt or locking my elbows to my side would cure it. None of the 9 bikes in my stable now wobble--period. Some of my wobblers didn't wobble for other riders and some did. I tried different wheels on the worst wobbler of the bunch and it no longer wobbled at 32mph but at 35 mph it did. There seem to be many factors that affect whether or not a bike will wobble including rider size, weight, position, and technique. I think my worst wobblers had forks too flexy for me at 200-215lbs. The one that might/might not wobble was a fairly flexible ti frame.
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#7
05-01-2006, 11:18 PM
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To say that almost absolutely the problem is the rider is an insult to the many good riders that are on this forum. As was stated by Telenick there are a variety of other possible explanations.
In my case I had a shimmy on my Kestrel ONLY when it had one particular wheel on the front (a Rev X in true) and it ONLY started at 45 mph. The shimmy was so bad if I had been afraid it would have been face plant time. I had it happen twice on the same hill and then I came back with a different wheel the following week and all was fine and it has been on that hill forever and again on that and other bikes. Top speed so far on that hill has been 53. I did learn one thing from this thread...I'd never buy a Sachs bicycle...anyone that egotisical I rather not have anything to do with. It amazes me that someone with his expertise would dismiss virtually all problems of this type to the pilot. The ego of the man is unreal. I wonder what his story will be when the right set of circumstances turns his bike into a bronco? Maybe he never goes that fast... 
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#8
05-02-2006, 06:43 AM
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There is a long and interesting piece about the role of rake and trail and front loads on the handling of bikes in new issue of Vintage Bicycle Quarterly. I am not sure I agree with all of Jan Heine conclusions (in fact, I am sure I do not...) but it is a serious look at what _might_ cause shimmy.
There is a post here regarding e-Richie's comments that confuses a gifted builder's hardwon experience and expertise for egoism. That writer should be so lucky as to own a Sachs. dbrk
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#9
05-02-2006, 07:01 AM
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Quote:
from your personal anecdote, i believe you are saying the exact thing - "In my case I had a shimmy on my Kestrel ONLY when it had one particular wheel on the front (a Rev X in true) and it ONLY started at 45 mph. The shimmy was so bad if I had been afraid it would have been face plant time. I had it happen twice on the same hill and then I came back with a different wheel the following week and all was fine and it has been on that hill forever and again on that and other bikes." that i said in my off-site text (one which i introduced as being "highly opinionated") from 2002: "if your bicycle shakes, it usually only shakes on one hill at one time. that is, it's not a day-to-day problem. personally, i see it as a momentary incident, one in which the rider's fore-aft shift came at an inopportune time. a poorly designed frame, or a rider on a frame that doesn't allow for proper balance, or a combination of both, 'could' be the culprit. i've seen all combos of head angles, rakes, front wheelbases, stem lengths...and there is no constant to the issue of shimmy." i'm also kinda' surprised you didn't snip my 2002 text in which i say write: "i think that notion of 'straighter' frames shimming more frequently than crooked frames is bull****." did you even see/read that? i was referencing the fact that the original velonews article mentioned that misaligned frames had less of a chance to speed wobble than properly aligned frames (do). wouldn't you also find that odd, especially in light of the calfee article (posted by Telenick) in which speed wobble is ascribed to fork misalignment?
__________________
Atmo bis Last edited by e-RICHIE; 05-02-2006 at 07:04 AM.
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#10
05-02-2006, 07:18 AM
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Never happened to me yet, don't know why it happens, but I was behind a guy one day in a charity ride when it did happen. It was impressive.
I also think that case might have been the rider causing it more than anything because the whole ride, a longer one, he was the one guy in the group that kept doing something where you went "huh?" a lot.
__________________
Enjoy yourself. It's later than you think.
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#11
05-02-2006, 07:25 AM
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Egotism?...
Quote:
I think that it is a mistake to judge any of Mr. Sachs comments as egotistical. I can't imagine anybody better qualified to make comments on anything related to the biomechanics of cycling. He is our generation's Ugo DeRosa ATMO, BRO
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#12
05-02-2006, 09:32 AM
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There's lots of wisdom out there on this subject and I'm not sure I have anything really new to add but...........
Every bike on the planet wobbles. There I said it and I feel better. Every bike ever made will wobble. Here's the catch - most only wobble at speeds we never see. If your bike only wobbles at 75mph who cares? If it wobbles at 28mph I know who cares. When you have a bike that wobbles at an inconvienient speed you can't actually get rid of the wobble but you can change the speed at which it occurs. A different front wheel might change the "wobble speed", a different fork rake might too.......there are lots of ways to influence it. Back in my day at Serotta "wobblers" were treated very seriously ( as I'm sure they are today). One of my lucky jobs was to get my arse hauled up to the top of a good sized hill (Lake D. or Spier Falls) and then I'd go like hell down the hill and observe the wobble. We would then make small controlled changes to the bike and see how that affected the wobble speed. I made a lot of trips down those hills. Generally speaking going with stiffer and or lighter stuff ( fork, front wheel, brifters, bars...etc) raised the wobble speed. Heavier of softer stuff ( fatter tires, looser spokes, longer stem, heavier shifters, heavier bars) will lower the speed. Sometimes to an acceptable speed (above normal riding speeds) and sometimes not. Sometimes I would test a wobbler and it wouldn't wobble for me. It wobbled for the owner but not for me. That was a hard thing to deal with because I couldn't fix what wasn't there for me. The impact of the rider on the system is huge. A different weight or center of gravity will totally change the deal. Sometimes for the better and sometimes not. And a tense of loose rider would change the deal in a big way. During a test I would tense my body, move it fore and aft, sit up tall and tuck down low. It would make the wobble come and go like a switch. Yes some bikes will wobble if the rider is nervous and has a death grip on the bars. A tight body doesn't damp vibration well while a loose one does. Years ago I owned a bike that would wobble hard enough at 18mph to throw me to the ground. It was hands off stable at 50mph but 18 was not good. If I held on loosely at 18 it would stop. So there are plenty of things one can do to lessen the chance of wobble or to move it to a more acceptable speed range but there is nothing one can do to eliminate it completely. Dave
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#13
05-02-2006, 12:11 PM
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At least my case wasn't due to a death grip on the handlebars... no grip at all! A light grip on the bars would damp it, but what really worried me was that the bike was trying to shimmy at the speeds I normally ride at.
Perhaps I should send it back to Serotta and let them try it as it's not built up at the moment. It is the best rando bike I've had, but I still don't trust it. Orin.
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#14
05-02-2006, 12:18 PM
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Quote:
are we talking wheel flop from low trail measurements or actual shimmy vis-a-vis* high(er) speed oscillations? *french. very classy aclu.
__________________
Atmo bis
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#15
05-02-2006, 12:43 PM
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Quote:
Yes, the trail was a little lower than I'd expect and the increase in trail after re-raking the fork has improved things. The difference with my old Koga-Miyata Full Pro was marked. I could ride it no problems at the same speeds, hands on the bars or not. Imperfections in the road would momentarily deflect the front wheel, but it would recover with no oscillations. The Rapid Tour would start oscillating and with the oscillations getting bigger quickly. Either with the re-raked fork or the F1 (I don't remember which), it would still oscillate a little, but would damp out rather than get worse. I swapped wheels, took the rack off, did all the usual checks to no avail. I swapped wheels with the Koga-Miyata etc. with no change. Orin.
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