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Old 02-06-09, 03:06 PM   #1
darkroast
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Shimmy problem

Hello all,

I am encountering the shimmy problem on my LD bike (Columbus Spirit steel with carbon fork). With hands on the handlebar, the front end feels a little skiddish. With hands off, I definitely see the front end wobble. In reading the following article: http://www.sheldonbrown.com/brandt/shimmy.html, it seems to be 'inherent to the geometry and elasticity of the bicycle frame'. I got the bike built last summer for long distance events. I do not recall such front end looseness on a 300km brevet. The only major changes I have made after that event is tires. I had 700x23 Maxxi Refuse and changed to Conti 4-season 700x28. And a generic rear rack. Minor changes have been cosmetic: handlebar tape and bar plugs. I should add that I have noticed the shimmy on my commute runs - so the bike has a handlebar bag with things like wallet, cell phone, and glasses case. And the rear rack has one pannier (left side) with days clothes and food. I carry a similar load on my regular commuter (Kona Jack the Snake; alu frame with carbon fork) and I experience no wobbles.

Appreciate any assistance on how I can get a more stable front end for the LD ride.

Thanks.

Hanif.

Last edited by darkroast; 02-06-09 at 03:34 PM.
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Old 02-06-09, 04:00 PM   #2
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Attach a pair of visegrips to your bars (with duct tape under the jaws), to see how moving the centre of gravity of the sreering assembly forwards or backwards affects the problem. Do your test runs on long smooth downhills.
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Old 02-06-09, 08:58 PM   #3
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Check your headset too. Squeeze the brakes tightly and rock the bike back and forth to see if there is any movement in the headset. You can also lift the front end about 6"and drop it and listen for a rattle or loose sound.
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Old 02-06-09, 09:03 PM   #4
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If it's a new problem, I'd suspect headset issues. There isn't much that can really cause a speed shimmy. If all else fails, try a fork with different rake.
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Old 02-07-09, 09:33 PM   #5
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I'd start undoing the changes, one at a time, that you made before the bike started shimmying, starting with the most suspicious. That would be the single pannier, because its center of gravity is well off the line of lateral symmetry and behind the rear axle. After that I'd try going back to the Maxxis tires.

A road bike will be more sensitive to load imbalances because of its shorter wheelbase and altogether lighter weight.
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Old 02-08-09, 07:11 PM   #6
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Quote:
Originally Posted by darkroast View Post
Hello all,

I am encountering the shimmy problem on my LD bike (Columbus Spirit steel with carbon fork). With hands on the handlebar, the front end feels a little skiddish. With hands off, I definitely see the front end wobble. In reading the following article: http://www.sheldonbrown.com/brandt/shimmy.html, it seems to be 'inherent to the geometry and elasticity of the bicycle frame'. I got the bike built last summer for long distance events. I do not recall such front end looseness on a 300km brevet. The only major changes I have made after that event is tires. I had 700x23 Maxxi Refuse and changed to Conti 4-season 700x28. And a generic rear rack. Minor changes have been cosmetic: handlebar tape and bar plugs. I should add that I have noticed the shimmy on my commute runs - so the bike has a handlebar bag with things like wallet, cell phone, and glasses case. And the rear rack has one pannier (left side) with days clothes and food. I carry a similar load on my regular commuter (Kona Jack the Snake; alu frame with carbon fork) and I experience no wobbles.

Appreciate any assistance on how I can get a more stable front end for the LD ride.

Thanks.

Hanif.
My experience is that a roller bearing headset (e.g. Stronglight A9) will supress front end shimmy quite effectively.
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Old 02-09-09, 01:14 PM   #7
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Thanks for all the responses.

I have taken the lazy approach and asked the original builder to have a look - it coincided with a pre-season check. Before I gave the bike in I did check to see if the headset was loose and it did not seem to be. I neglected to mention that the front hub is a Schmidt dyno. I also grabbed the tire and fork and gave the tire a shake and their seemed to be some play. So I wonder if there is some play in the hub. I should get the bike back in a couple of days, so we shall see.

Thanks.
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