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Discussion Starter #1
Hi

I recently purchased a three year old Specialized Roubaix Pro with American Classic Carbon 58 wheels.
After a bike fitting and tuneup and on my first ride, on a sweet down hill @ 45+Mph I experienced extreme wobble / shake originating from the front of the bike yet causing the entire bike to shudder. Under 45 mph. The bike felt smooth, yet twitchy.
The day was warm and windy and I was on a very smooth down hill.
It was all I could do to slow without going down. Once stopped, I checked the spokes. All were tight. Wheels roll true .... using the brakes as a guide.

Does anyone have suggestions for what caused this? LBS is bewildered.
Note: I continued to ride another ten miles without incident. All subsequent rides since have been fine as well. However, I have been reluctant to approach 40+ mph again.

Thoughts please.
 

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I can't find the thread were this was discussed on another forum. It's not uncommon and from what I recall of the discussion the cause is not fully understood. I've experianced it once and it was quite unnerving. Assuming there is nothing wrong with your bike the recommended way to control it (IIRC) is to loosen your grip on the bars and clamp your legs to your top tube.

here, found a link...
http://www.sheldonbrown.com/brandt/shimmy.html
 

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Most seem to think it's an inherent frame misalignment or an issue with frame stiffness. I've had it happen to me on two bikes, both carbon. And the feeling is as if I did hit some resonant frequency of the frame itself. This is also why gripping the frame with your legs, thus damping the system, helps stop the issue. One bike I had would do it repeatably on the same hill at about the same speed. I never got far enough into diagnosing the problem before the bottom bracket shelled out and the frame was warranty replaced. On my current bike it happened a few times also. I was totally bummed, because it is scary as hell, as I'm sure you now know. I found on this bike it wasn't the frame at all, but it was that I didn't have my front skewer tight enough. I felt pretty dumb but glad that's all it was.
 

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I haven't experienced the shaking at 40, but if I did go faster I would be worried. The couple times I've gone this fast I did briefly imagine how extremely horrible it would be if something happened to my bike and I wrecked, but man was it exhilarating.
 

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One thing that pops up in my mind is, did you buy the bike at the LBS or used?

Most people can be "fitted" to most frame, however, there are some inherent problems that can crop up from a mismatch.

IIRC, being on a too small frame that is offset by a longer/taller saddle set-up can create a shimmy at speed.
 

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Discussion Starter #6
Thanks to everyone for their thoughts and experiences.
Prior to this experience, I went through a fitting at the LBS (bought bike from seller on eBay).
We are still fine tuning the fit.
LBS says the wobble happens sometimes, but the causes are hard to track and usually remain undiagnosed.
LBS advised, as one or more of you did, to grip the bike with my knees. To date, I haven't had a reoccurrence. Then again, I have stayed away from speeds above 40, until I have more time in the saddle.
Funny though. I have ridden bikes for decades. With the exception of motorcycles. I have never experienced nor heard of this happening. I have enjoyed more speedy downhills than I can remember.
I pray it was a one time occurrence....but it will never be far from front of mind awareness as I approach each peak.
Thanks again!!
 
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