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Bike wobble/shimmy

Old 08-01-20, 06:26 AM
  #1  
Dan NH
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Bike wobble/shimmy

I ride a 2012 Giant TCR carbon fiber 20 speed. Twice I have experienced a very uncomfortable bike wobble while riding long downhill slopes at speeds above 35MPH. The bike is very well maintained, no frame cracks, wheel bearings no play, front fork headset no play, Continental Ultra Sport tires not too worn, spokes tuned, wheels true, axles secured.

The wobble started both times when coasting (not pedaling) and braking had no effect on the wobble which continued until nearly all forward motion was stopped from braking. The two events were separated by a period of 3 or more years and both times the issue happened I was riding in the drops.

I have ridden many long downhills at higher speeds, in both the drop position and above the shifter/brakes without any issue.

These two wobble experiences were terrifying and I am looking for ideas that might prevent this from ever happening again.
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Old 08-01-20, 06:42 AM
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What has worked well for me is to get my butt off the saddle and have the lightest of grip on the handlebars. In other words, almost all weight on the pedals. Start my descent this way and never a shimmy. If I forget, as soon as I sense any shimmy, relax grip (counter intuitive) and lift butt and shimmy stops immediately.
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Old 08-01-20, 06:55 AM
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It's only happened to me a couple of times and I agree, it's pretty scary. Lots of opinions but there is no consensus as to the cause. The traditional response is to grip the top tube with your thighs. IME I think this helps.
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Old 08-01-20, 07:59 AM
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Originally Posted by L134 View Post
What has worked well for me is to get my butt off the saddle and have the lightest of grip on the handlebars. In other words, almost all weight on the pedals. Start my descent this way and never a shimmy. If I forget, as soon as I sense any shimmy, relax grip (counter intuitive) and lift butt and shimmy stops immediately.
Good suggestions, I had thought that maybe I was gripping tighter as the shimmy started and continued cause it sure is disconcerting when it happens.
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Old 08-01-20, 08:14 AM
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Dan NH , bike wobble or shimmy when riding downhill at high speeds can be caused by either

1) a misaligned frame....and/or....2) wheels not dished properly

either of these conditions can result in the front and rear wheels not tracking in a single line
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Old 08-01-20, 08:53 AM
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I find being cold makes shimmy far more likely. Also scared or nervous. I have a 1000' fast descent on a favorite ride that can touch 50 mph (for this guy as aerodynamic and weight driven as a feather). I make it a point to bring an extra layer of clothes to put on at the state park at the mountain top, just to warm and relaxed for that descent. Makes the ride far more enjoyable even though I have to lug that stuff up about 1500'.

Ben
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Old 08-01-20, 08:59 AM
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The reason for this high speed wobble is not well understood. There are many variables, frame design which is most likely, weight on the bike and distribution front to rear, wheels, accessory weigh on the bike, and several others I havent mentioned.
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Old 08-01-20, 09:01 AM
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Try clamping both of your knees against the top tube the next time it happens. This may significantly reduce the wobble. It always worked for me back when I had a bike that was prone to high speed wobble.
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Old 08-01-20, 11:29 AM
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Originally Posted by Dan NH View Post
Good suggestions, I had thought that maybe I was gripping tighter as the shimmy started and continued cause it sure is disconcerting when it happens.
Many years ago my brother, an aeronautical engineer, had mentioned something called pilot induced oscillation to me. I wondered if shimmy might be rider induced, decided what I mentioned before would be the best way to reduce rider input, tried it out and it has been fool proof for me. I had an extended three way email exchange with John Forester and my brother trying to convince them that shimmy was rider induced. I never did succeed at that but, as an accountant, I stand by my position to this day. I have never bought all the bicycle related explanations. I can well imagine those may make shimmy worse or more likely but I still believe it is the rider that gets it started.
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Old 08-01-20, 02:20 PM
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So why is it only certain bikes have this issue?

You guys are giving good ideas to mitigate it once it happens, but hard to believe the rider is the one starting it in the first place.

I used to ride sitting upright with no hands for hours. One day the same bike started shimming when no hands for no apparent reason. Resting a knee, still no hands, damped the oscillations. some years later it stopped doing the shimmy.
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Old 08-01-20, 03:32 PM
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Originally Posted by Pop N Wood View Post
So why is it only certain bikes have this issue?
My understanding is that all bikes have this issue, it's just that many only exhibit it at higher speeds.

I have a bike that does this when I get cold enough to shiver. Putting a knee on the top tube is the only way to get it to stop.
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Old 08-01-20, 03:36 PM
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Originally Posted by Pop N Wood View Post
So why is it only certain bikes have this issue?
You guys are giving good ideas to mitigate it once it happens, but hard to believe the rider is the one starting it in the first place.
No, no, no. Nobody says that the rider is the one starting it.

The concept of "pilot-induced oscillation" does not in any way imply that the oscillation originates from the pilot. It simply implies that the pilot and their "corrective" actions act as an amplifier, which only amplifies externally imposed oscillations (instead of negating them). The oscillations still originate in the vehicle and in the external environment. In some vehicles/environments they occur. In some they don't.

That is where "only certain bikes have this issue" part comes from.
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Old 08-01-20, 03:49 PM
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Death wobble is a real thing! From my understanding, basically enough vibration at the right frequency and off it goes. It seems you have to be doing at least 50 kmh and it can occur naturally or be induced by something as simple as tap on the handlebars (I've had it happen both ways). Clamping the legs on the top bar has worked for me; changes the frequency of the vibration for the frame. By and large, a terrifying event when it happens....

A good article on the subject https://cyclingtips.com/2020/07/bicy...-to-stop-them/
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Old 08-01-20, 04:10 PM
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Originally Posted by AndreyT View Post
No, no, no. Nobody says that the rider is the one starting it.

Actually, I did, but remember I am an accountant and I’ve been told I’m wrong, by my brother no less. Regardless, right or wrong, my method has worked for me for many years. I’ve never tried the knees on the top tube method, it just seems too awkward to me.

From Sheldon Brown blurb on shimmy: “Jobst Brandt reports that unloading the saddle without standing up will stop shimmy.”

More from Brandt: “Shimmy that concerns riders the most occurs with hands firmly on the bars and it is rider generated by muscular effect whose natural response is the same as the shimmy frequency, about that of Human shivering. Descending in cold weather can be difficult for this reason. The rider's "death grip" only enhances the incidence of shimmy in this situation. Loosely holding the bars between thumb and forefinger is a way of avoiding shimmy when cold.”

Last edited by L134; 08-01-20 at 05:23 PM. Reason: Added another quote
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Old 08-01-20, 04:33 PM
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Try a fork with a slightly different rake
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Old 08-01-20, 05:37 PM
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Originally Posted by unterhausen View Post
My understanding is that all bikes have this issue, it's just that many only exhibit it at higher speeds.

I have a bike that does this when I get cold enough to shiver. Putting a knee on the top tube is the only way to get it to stop.
I ride 8 different bikes. Not a single one of them shimmy at high speed.
Has to be something in the geometry or tires.
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Old 08-01-20, 08:05 PM
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Originally Posted by Pop N Wood View Post
I ride 8 different bikes. Not a single one of them shimmy at high speed.
Has to be something in the geometry or tires.
Itís an oscillation. Stiff frames and light front ends (bars, wheels, tires and forks) tend to minimize it. Also low saddle height and small frames.

Otto
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Old 08-01-20, 08:43 PM
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Agree with what most have said but I would add I had a bad wobble on my current bike until I got it fitted correctly, learned how to ride on the drops with my butt off the seat and used 28mm tires with lowish pressure. With 23mm tires at max psi the same bike is very jittery on decents.
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Old 08-02-20, 05:58 AM
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Originally Posted by shelbyfv View Post
It's only happened to me a couple of times and I agree, it's pretty scary. Lots of opinions but there is no consensus as to the cause. The traditional response is to grip the top tube with your thighs. IME I think this helps.
I do that all the time in downhills because it's also more aerodynamic!

Originally Posted by branko_76 View Post
Dan NH , bike wobble or shimmy when riding downhill at high speeds can be caused by either

1) a misaligned frame....and/or....2) wheels not dished properly

either of these conditions can result in the front and rear wheels not tracking in a single line
There are more factors involved and it can happen with neither of those conditions present.

One thing seems certain, it never happens to MTB's with either full suspension or hardtails with suspension fork. Suspension dampers and wide knobby tires provide enough motion dampening to avoid speed wobble in the first place
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Old 08-02-20, 06:28 AM
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5 years or so ago I experienced a high speed shimmy for the first time. At 35mph it started and I had to clamp down on the binders to bring the speed down to settle down the bike. Inspected the entire bike and found the front wheel spoke tension was very low. Re-tension the wheel and the shimmy was gone. Never experienced it again.
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Old 08-03-20, 05:31 PM
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I lean one leg into the top tube. Damps it enough. I ride a noodely old hi-ten steel bike, it can happen around 45-50km/hr on that bike.
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Old 08-03-20, 11:04 PM
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yah...in temps under 70 degrees on a decent (usually in the morning near the start before warmed up), i will occasionally experience "shiver shimmy." not fun.
have to scrub the speed to 25 mph or less to negate. now actual shimmy-not including cooler temps-is not a good time. i've had serious crosswinds seemingly bring on the shimmy
at 40+ mph and it was all i could do to avoid crashing. so front-end wobbly. had it happen on aluminum/carbon and steel frames.
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Old 08-03-20, 11:41 PM
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it happened once to me.... just after i had recovered from a clavicle fracture, scared me to hell.
bike has always been the same on 100's of rides down that hill. That day (i think) was high bottom2top cross wind and i was holding the bar too tight and my butt was planted too hard on the seat.

here is what i have told myself in case i need it...
- (made practice) pedal even if at low cadence, except on/close turns
- (made practice) knee or thigh in a position to dampen along the top tube
- butt raise/shift slightly back to change pivot point -- i know this can be scary, but, fingers crossed i dont have to do it
- dont over grip the handle bar --- now, this is where i think i made it worse on that ride, i gripped it harder... thinking it was the front wheel.

luckily on that ride, i was able to slow down.. first shaking bike,.. then shaky rider!
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