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Old 01-10-09, 07:00 PM   #1
darksmaster923
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Bike wobble

today, when I was going down a hill at about 30 mph, my bike began to slightly wobble. Since I get scared going down hills really easily, this freaked me out. Is there any way to fix this, or should I have the lbs look at it
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Old 01-10-09, 08:38 PM   #2
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called something like the bike shimmy, and happens "naturally."

Edit:

From SB site:

Shimmy

Shimmy is a term for a harmonic shaking of the bicyle, which usually occurs at a fairly high speed. Shimmy can be very scary, and can lead to loss of control. All bicycles are subject to shimmy under the right (wrong) cirucmstances, but it is more pronounced in some than in others.
The rider feels shimmy mainly through the handlebars, so it is often assumed that the cause relates to the front wheel or the headset. This is most often an illusion.
When shimmy is related to faulty equipment, it is more often the rear wheel that is at fault, especially if the spokes are too loose.
Serious shimmy is also freequently caused by floppy, poorly supported luggage on top of a rear luggage rack.
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Old 01-10-09, 10:08 PM   #3
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Alignment of the frame and fork are the usual suspects. Sounds like a visit to a pro bike shop or even a local frame builder is in order. A hard fall at 30 mph is just not worth saving a few bucks.
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Old 01-10-09, 10:33 PM   #4
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Quote:
Originally Posted by darksmaster923 View Post
today, when I was going down a hill at about 30 mph, my bike began to slightly wobble. Since I get scared going down hills really easily, this freaked me out. Is there any way to fix this, or should I have the lbs look at it
Did you come up off the saddle a little? Are you running panniers or a trunk bag? Otherwise, take it to your lbs for a good going over. A shimmy at 30 mph is no joke.
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Old 01-10-09, 10:33 PM   #5
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First thing I'd check... and easiest fix. Are there reflectors on your wheels? Tear those off and try it again. If it still does it, or you don't have them on your wheels.... take it to your LBS.
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Old 01-10-09, 11:39 PM   #6
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There are wheel reflectors. I also noticed that my handlebars were a little loose, so I tightened them.
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Old 01-11-09, 12:26 AM   #7
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Every bike I have ridden tends to develop a shimmy at a particular speed. One was 50mph, the other 45, and another around 38. Nothing was wrong with any of the bikes. Clamp your knees down on the top tube to stabilize things.
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Old 01-11-09, 12:36 AM   #8
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There are wheel reflectors. I also noticed that my handlebars were a little loose, so I tightened them.
Go to a LBS bro. Did you tighten the headset or the handlebar? Some things can't just be tightened, they need to be adjusted. But ditch the wheel reflectors.
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Old 01-11-09, 07:38 AM   #9
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If a wheel is out of true enough, it'll shake the bike. It's just like having an unbalanced wheel on a car.

Reflectors can throw off a wheel's balance, too, just like others have posted already. Either get rid of them or put both on one wheel so they balance each other out. You can also fulfill reflector requirements by using reflective tape on the wheel rims (make sure you apply it so it's not on the braking surface, of course).

Anyway, those are the easy fixes.
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Old 01-11-09, 08:29 AM   #10
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My bike has always done that at around 40mph and there is nothing wrong with the frame or the wheels, and like someone said, just clamping my knees on the top tube stops it right away. I have gotten to the point where if I am going to go down a hill really fast, I just put one knee on the top tube to prevent it. Scared the hell out of me the first few times it happened.
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Old 01-11-09, 08:46 AM   #11
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Same here. I almost messed my shorts the first time it happened. Now if I feel it starting to occur I move a knee to the top tube. No biggie.
I think that method has something to do with breaking up the harmonic oscillations but that is a guess.
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Old 01-11-09, 10:10 AM   #12
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Go to a LBS bro. Did you tighten the headset or the handlebar? Some things can't just be tightened, they need to be adjusted. But ditch the wheel reflectors.
The stem
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Old 01-11-09, 10:28 AM   #13
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happens on my bike at ~35mph.
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Old 01-11-09, 02:02 PM   #14
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I found this over at velonews...

Jerry

Shimmy has a host of possible causes

Dear Lennard,
I was doing a bit of pre-holiday drooling at the Calfee website when I came across the information pasted below. Seems that I recall a recent string of Q&A relating to speed shimmy and thought I’d share this. Seems to make a lot of sense, but I still have a question: Would switching out a fork cure shimmy?
Bruce

“Calfee Design has identified a cause of speed wobble (a.k.a. shimmy) and instability that can be prevented. Speed wobble is a dangerous condition that can cause the rider to lose control of the bicycle and crash. While a skilled rider can prevent and stop speed wobble, it is better if it doesn't start. Some experts state that speed wobble is caused by the rider, which is technically true because the rider responds to the steering dynamic, initiating a resonance that causes the frame to act as a spring. While loose headsets and out-of-true wheels and frames can contribute to starting a speed wobble, we have found that fork asymmetry can also get it going.

“Fork symmetry is defined as the symmetrical position of the fork dropouts in relation to the steering axis. Specifically, the equality of the distances from the dropout faces to the steering axis must be within a certain tolerance for the bike to ride in a stable and confident manner.

“Traditionally, steel forks were cold set after welding or brazing to realign them after possible distortion caused by the heating and cooling of the metal. A diligent steel-frame builder can align the fork blades to within a millimeter of symmetry. Certain well-known builders align them to within 1/2 mm.

“Carbon fiber forks cannot be cold set. They must be molded straight to begin with. We have found that a small percentage of carbon forks by various makers were molded with asymmetrical fork blades. Some are off by a little over a millimeter and others are off by two or more. Forks that are off by over 1.8 millimeters in symmetry have a good possibility of being prone to speed wobble. A symptom of a fork that is off by 1.8 mm or more is a noticeable difficulty when riding no hands at a slow speed (less than 10 mph). One has to lean to the side slightly to keep going straight. A bike with asymmetrical forks seems to corner better in one direction but not so well in the other. At speeds of 30 mph or more, the bike can develop speed wobble.

“If your bike has the above-mentioned symptoms, the fork should be measured for symmetry. This is difficult to measure without proper tools. Calfee Design measures all forks for symmetry and is equipped to measure any fork. If any Calfee customer wishes to have their fork checked, please send it to us with a letter requesting a fork inspection. Non-Calfee customers may send their forks for inspection for a nominal fee. Replacements may be available for asymmetrical forks, depending on the individual fork maker's policy.”

Dear Bruce,
I ran that recommendation from Calfee here a couple of years ago. I have many times seen an improvement in fork alignment improve a high-speed shimmy problem. But I have also seen bikes with perfect fork alignment that had high-speed shimmy. It’s not a cure-all. I’ve also seen bikes with shimmy problems fixed by interchanging one perfectly aligned fork with another one with more rake (offset).
Lennard
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Old 01-13-09, 10:47 PM   #15
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Interesting... never heard of this before. Obviously a front wheel wobble is involved in the process before the frame resonates...

Just curious, as my MTBs have low top tubes and the knee technique might not work... would lightly feathering the front brake also stop shimmy??

Or for that matter, is this purely a road-bike anomaly??

Thanks...
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Old 01-14-09, 03:14 AM   #16
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My friend on his lugged carbon fiber LOOK had a bad wobble on the flats while racing. All the riders around him seemed to notice it and they gave him a wide berth. He was oblivious. Turned out the downtube was cracked through.
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Old 01-14-09, 01:33 PM   #17
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First road bike I had was an OCR3. Previous to this I was Mountain biking. The wheels were the culprit on this one. OM wheels that flexed too much.

Then I got a TCR-C and had Mavic Aksiums on it. Downhill was nightmare with speed wobble coming in at 30 to 35mph. Tried a lot of things and eventually changed to a different wheel. Not as stiff as the Aksiums but a good wheel in any case. Once again it cured the problem. Check out the wheels as an alternative.
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Old 01-14-09, 02:36 PM   #18
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That's the "death wobble". I researched that when I first experienced it. You tube has some videos of it. You have to get up on the pedals and squeeze the top bar with your knees. It is better to do this when it first starts rather than wait for it to get very bad.
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Old 01-14-09, 04:03 PM   #19
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^ Wow, I did a search for "speed wobble" in YouTUbe... the guy on the motor bike is just too spooky!

Interesting that the guy on the road bike induces it deliberately and stops it at will...

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Old 01-14-09, 04:09 PM   #20
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Interesting that the guy on the road bike induces it deliberately and stops it at will...
This one?

http://www.youtube.com/watch?v=xODNzyUbIHo

(hope you don't mind that I went and found the link to post here so others know what you're talking about...)
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Old 01-14-09, 04:34 PM   #21
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^ Yes, that one. I should've known better and posted the link.
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Old 01-14-09, 05:35 PM   #22
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^ Yes, that one. I should've known better and posted the link.



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Old 01-14-09, 06:15 PM   #23
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I am not sure this is the same problem that bicycles have but...

Once upon a time I had a large heavy motorcycle and I was rounding
a slightly off camber curve and the handle bars started oscillating slowly
from side to side, within a couple of seconds the bars were rapidly
swinging from lock to lock as I struggled to keep the bike upright...

I could actually feel the frame flexing back and forth.

I was able to regain control without crashing, fortunately there
was no other traffic on the road.

The next day I replaced the tires and added a steering damper

I found that I could 'feel' the beginning of this phenomena and found
that if I unloaded the front wheel by adding throttle I could prevent
the undamped oscillations from recurring.

Motorcycles have a much more flexible frame, swing arms on the
rear and shocks on the front so I suspect that resonance is easier
to achieve then on a bike...

Jerry
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Old 01-14-09, 06:36 PM   #24
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Quote:
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I found that I could 'feel' the beginning of this phenomena and found
that if I unloaded the front wheel by adding throttle I could prevent
the undamped oscillations from recurring...
Funny you mentioned that! While I was studying the YouTube videos, I noticed that the guy on the road bicycle was doing a no-hander at the time, so probably sitting upright and by default had de-weighted the front wheel... and kept control throughout.

Poor guy on the motorbike though, he didn't have a chance of de-weighting a bike like that at that speed... it almost looked like he tried to accelerate out of it though...

Oh, here's the motorbike link BarracksSi
http://www.youtube.com/watch?v=RmtoOmq3ppw


So, using the front brake would probably make things worse. Sheesh, I hope I don't ever experience that.

.
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Old 01-14-09, 06:42 PM   #25
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Maybe I just don't go fast enough to experience a speed wobble, then. I've never had anything like that happen.

(and I hope it remains that way!...)
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