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Discussion Starter · #1 ·
I took my old Colnago down a steep road today and got uo to 70 kph. At that speed, the bike wobbled all over the road and it felt like the wind was actually pushing me around. So instead of going faster (which I wanted to do) I had to start braking. Is this normal? I would think not. What could be the cause of the instability? My wheels arent perfectly true but they are good enough.
 

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Just Riding Along
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Causes of wobble...

All structures vibrate. Wobble occurs with a bike when the natural resonant frequency of the bike coincides with an externally applied stimulus (think of a tuning fork being rung with a hammer.)

Most likely, it was a unique combination of road forces which stimulated the wobble. If it recurs, I would check the trueness of the wheels, tightness of the headset and hub bearings, etc.; possibly a little wiggle from a wheel induced the wobble.

Your body is part of the system when wobble occurs. Lots of things influence it,e.g. position, weight distribution, etc. The quickest way to stop it is to press one leg firmly against the top tube (or both if you can.) This dampens the vibrations in the bike and changes the natural resonant frequency, both of which destroy the wobble.
 

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A wheelist
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Bike shimmy at high speeds is well docummented and is very scary. Lennard Zinn, VeloNews tech writer and framebuilder, has written at length about it. If you do a search here and at Google for Zinn and shimmy you will turn up lots of info.
 

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Death grip

flankwood said:
I took my old Colnago down a steep road today and got uo to 70 kph. At that speed, the bike wobbled all over the road and it felt like the wind was actually pushing me around. So instead of going faster (which I wanted to do) I had to start braking. Is this normal? I would think not. What could be the cause of the instability? My wheels arent perfectly true but they are good enough.
The fix, as others have said, is to clamp the top tube with your knees. It is also possible that you are gripping the bars tightly, thereby creating that resonant structure that wants to oscillate. Also, check your headset adjustment.
 

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Discussion Starter · #6 ·
Thank You

Kerry Irons said:
The fix, as others have said, is to clamp the top tube with your knees. It is also possible that you are gripping the bars tightly, thereby creating that resonant structure that wants to oscillate. Also, check your headset adjustment.

Well I guess I don't feel too bad. I always thought I could go faster on my roadie than my mtb but the weight, tires, and shocks of my mtb probably help me ride better. (I got up to 84 kph on the mtb). Im glad to hear this is a normal thing. The wind is a big factor most of the time as I speed up. Thanks to all who posted!!!!!!!!!!!!!!!!!
 
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