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Discussion Starter · #1 ·
Just got back from a nasty local climb, and thought I was going to be roadkill on the descent! I've heard of this happening but never actually experienced it before... it's quite a scare to say the least:eek: . I read some of the archives but didn't get any definitive answers so...
Here's the story. I was breaking on a very steep descent when my front wheel starting shaking uncontrollably... I held on with all I had until eventually stopping on the side of the mountain (probably took about 30 - 45 sec to stop... seemed like 10 minutes!) My top speed was 45.7 mph so I was probably going about 40 or so when this happened. I never had anything like this happen on my old allum bike, but now I'm riding an all carbon Synapse. I've topped over 50mph on my other bike with no hint of any problems before. Does the carbon bike and fork have anything to do with the wobble? Any other thoughts?
I would really like to get some answers before I head out on a descent like that again on this bike. BTW - I'm 6'2" and 200lbs... if that helps with any thoughts. Thanks in advance for your input!

Mike
 

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You're Not the Boss of Me
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Too hard to tell, based on the info. First thing to check, is the wheel out-of-true? First you can check the spokes for tension and see if you find a loose one (pluck 'em like guitar strings). Next, while the wheel is seated in the drop outs, just give it a spin, eyeing the gap between the rim and the brake pad. Does that distance stay uniform throughout the spin?

If your wheel isn't out of true, then the next option sounds like fork or headset... but check the wheel first.
 

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Discussion Starter · #3 ·
Thanks for the fast reply... actually that was the first thing that I did once I got stopped! I wasn't going another inch down that mountain until I checked everything I could think of at least 3 times. The wheels spun fine... thanks for the advice though.
 

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Speed wobble or shimmy is one of the scariest things on a bike. The first time it happened to me, I remember wondering just how bad it was going to hurt and how many bones I'd break when I hit the pavement. Fortunately, managed to get it under control.

If you've found some old threads on it, you know that there's no definitive explanation. Some of the guys who've never/rarely had it happen will be happy to chime in and tell you it was rider error. Technique does seem to make a difference - too far forward on the bike, or a death grip on the bars seems to exacerbate the problem. Looser touch on the bike and clamping the top tube between your knees seems to help (though the latter is hard to do on many compact frames).

Wheels out of true can contribute to the problem - including being out of true vertically (we tend to check horizontally and ignore vertically many times). Crosswinds or broken pavement often seem to be involved as well. And the geometry of some bikes and the way they fit partcicular riders may play a role, too. I've had some bikes that seemed far more prone to it than others.
 

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Mine was caused from a top tube that was too small. I had another frame built with the top tube the same diameter as the down tube and I don't have the problem any more. I like the way the other frame looks, but that doesn't matter much when it is trying to buck me off. Mine had nothing to do with braking.
 

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alot of times it is just caused by riders grip on the bars. gripping on tighter is the wrong answer. loosen up some apply even pressure and here is the most important part of pulling out of it. push one peddal to straight down postition and apply all foot pressure on that peddal. Doing so puts the center of gravity lower to the ground which in return will cause your bike to straighten out of the wobble. Again usually a wobble at high speed is caused by rider error of somesort, positioning etc..
 

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Discussion Starter · #8 ·
Thanks for the advice...
I must say that I do tend to put a death grip on the drops when doing very steep descents. And once the wobble started I really had a strangle hold on them. If this happens again I will try the pedal move as you described.
So should I have loosened the grip on the bars once the wobble started?? The handle bars were shaking so badly that it seems that if I loosened my grip it would have bucked me right off.
Another thought: Is it possible that the wheel heated up enough from braking to cause something like this to happen?
At any rate, there was some serious PUCKER factor going on! I think I'm still pretty tight... I just tried to get up from the chair and it stuck to my arse!:D
 

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The death grip on the bars is the cause. See http://www.sheldonbrown.com/brandt/shimmy.html

On descents you have to force yourself to ride with your elbows bent and to relax your upper body. Also "float" your butt over the saddle and keep your pedals level,so that bumps won't bounce you around. If you do that you'll be more stable and feel less need to death grip the bars.

Next time you get the shimmy, grip the top tube with your knees. That damps the motion.
 

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I had this happen to me today too, first time ever. I was on a long, not particularyly steep downhill at about 55-60 KM/h. Mine started when a semi passed me about 5 feet to my left, I don't know if this was the cause, or if it was merely coincidence. I'm pretty sure I would have gripped the bar harder as the truck passed so that probably contributed to it. The wheels were fine, headset etc all fine.
I'd heard of this happening before, and for those who've never experienced it, it sucks as bad as you'd fear. If felt pretty much like both tires were flat, the bike was wobbling all over the place. I clamped the top tube between my knees, sat up, and feathered the brakes. The shimmy stopped by the time I slowed down to ~45km/h.
The interesting part is that I'd previously in the day hit 83.2 KM/h with no drama at all(other than pass the guy on the Harley).
 

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Wheels?

What kind of wheels are you running. A good friend had this problem and swears that it is his Campy wheels. He has even had it wobble on the trainer. I don't recall which model it was but I could find out if you are running campys
 

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Not your fault...

In spite of opinions to the contrary, it was probably unrelated to rider technique.

All engineered structures (buildings, bridges, cars, airplanes and bicycles, to name a few) have a natual harmonic frequency, just like a tuning fork. Most complex structures have some natural damping due to the construction techniques and materials. However, under the right conditions with a bike, an external stimulation creates vibration at the natural harmonic frequency and SPEED WOBBLE occurs.

A particularly graphic example of this dynamic resonant vibration is the Tacoma Narrows Bridge which self destructed when wind induced oscillation stimulated vibration at the natural harmonic frequency. No damping was provided and the bridge deck twisted and heaved 12 - 20 feet until it fell into the river.

With a bike, the external stimulus is probably pavement unevenness. No meaningful damping is naturally present in a bike (like a tuning fork). The speed wobble can be aggrevated by poorly adjusted components which give the bike more freedom to vibrate at greater amplitudes (e.g. a loose headset will allow more fork movement.)

Changing position on a bike changes weight distribution which might change the forces on the frame to the point that the speed wobble will stop. However, I am skeptical of the effacacy of this response, alone. The frame vibrates so fast under you that your weight does not follow the oscillations so weight shift is unlikely to be very benefical.

Braking is sensible because is slows you down in a dangerous situation. It may provide damping of the vibrations which is also a good thing.

Either clamping the top tube between your knees or pressing one thigh firmly against the top tube is the single most effective response to speed wobble. This provides meaningful and rapid damping of the vibrations and will return you to the world of stability.

Check the bike and components for tightness and proper adjustment as others have said and remember that your leg(s) is(are) your best friends when speed wobble occurs.
 

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Isn't your new Frame is higher & shorter than your old one? Weight distribution may be an issue particularly as you say the wobble was front ended. You may need to get low and stretched when decending.

Regarding the " Death grip" I have experience of this ( as any of my club mate will testify).
Anyone doubting this is a real cause only need to try this experiment. Ride on a quiet flat
road at about 15 miles an hour. Put the " Grip of Death" on the bars and push down ( lock your arms). Any bike will begin to wobble almost instantly. If you lock one arm and push the bars away you turn in that direction - agreed? Well when you lock both arms and push your bike wants to turn both ways and begins to wobble.
Its interesting how many people who report wobbles also report a scare or tense moment ( i.e the poster who had a truck pass closely...) I had experiences of wobbles on different bikes for several months ( I came very close to giving up the bike)until it was pointed out to me that I was locking my arms. Once I allowed my arms to "pivot" at the elbows the wobbles went away and have not come back.
 

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To rebuild your confidence, try to make it happen again. One way is to employ the "death grip" on a descent in an attempt to make the bike shimmy. Then try realeasing your grip, yeah I know, it takes a leap of faith. Have one leg (or both) ready to press against your top tube, and remember to drop one pedal with force. Do all these after your grip release. I think you will find the source and more importantly the cure for the problem. It has been my experience that a slight bump, even a small pebble ( though not always) introduces the shimmy, usually at around the same speed. Each bike/rider combo seems to have a speed where this can occur. It really helps to know how to overcome the problem and it's a great confidence booster. My own experience demonstrated that the "death grip" was the primary cause of the increasing violence of the shimmy. YMMV

Birddog.
 

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KeeponTrekkin said:
All engineered structures (buildings, bridges, cars, airplanes and bicycles, to name a few) have a natual harmonic frequency, just like a tuning fork. Most complex structures have some natural damping due to the construction techniques and materials. However, under the right conditions with a bike, an external stimulation creates vibration at the natural harmonic frequency and SPEED WOBBLE occurs.
(snip)
With a bike, the external stimulus is probably pavement unevenness. No meaningful damping is naturally present in a bike (like a tuning fork).


This is actually one arguement for carbon frames which I rarely see bike manufacturers touting:

all things being equal (which they never are, but bear with me), composites have a lower "Q" than metals. This is not the same as "Q Factor", the distance between a bike's pedals; "Q" in this case refers to how pronounced a resonant frequency is...technically it is defined as the frequency of a resonant peak divided by the bandwidth of that peak.

Having a lower "Q" basically means the material is more inert; it is less likely to be excited into self-oscillation because the amplitude of its resonant frequency is not significantly higher than any other frequencies. In a sense, carbon is self-damping, because it's "frequency response" is far flatter than a more resonant metal.

This is one reason carbon fiber gets used in some stringed instrument designs: Having a flatter resonance profile allows the natural sonority of the vibrating strings to be unimpeded by acoustic peaks or dips in the supporting material's response.

Which is not to say that an all-carbon bike won't or can't exhibit shimmy, but I would expect them to be less likely to exhibit that problem. (At least I hope so, since I just bought a carbon bike!)
 

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Discussion Starter · #19 ·
Thank You

Thank you all very much for all of the input.
As scary as it sounds I like the idea of trying to recreate the wobble, and then implementing the techniques described to steady the bike again. This way if and/or when this ever happens again, I can respond to it and not let it slow me down in the future.
I also talked to my mechanic today and I'll be taking the bike to him tomorrow just in case.
To answer a few of the questions:

The bike itself is only about 1 month old... I've barely put 200 miles on it so far.
The wheelset is Ksyrium Elite
The only other thing that I changed on the bike was new tires Vredestine Fortezza SE... which I've run on my aluminum bike for the past 2 years w/ no issues.

I hope to get this sorted out with changing my grip as well as recreating the wobble and using the fix... I love this new Cdale... it's fast soaks up nearly all of the road vibrations and is a pleasure to ride.
 
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