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Discussion Starter · #1 ·
A few weeks ago on my first real steep descent my Look 555's front wheels all of a sudden began to flutter back and forth and the whole bike shook uncontrollably. I had managed to bring the bike to a stop but I was a bit shaken. It's funny I don't notice it all the time going downhills. The shimmying is worst on bumpier roads and strong cross winds going downhill.
Has anyone experienced this with their bikes?
The last bike I had was the Cannondale R1000 which is of course a lot less of a racing frame than the 555. Now I understand from website research the "trail" (one factor of shimmys) of my 555 is a lot less than the R1000 and all other bikes I had. So am I to reteach my technics decending hills on my 555 if possible or do I question my frame structure? Headset? Heck! Maybe it's all of the above!
I will take this situation to my bike store/mechanic but I am curious to what you riders make of this shimmy thing esp. for Look frames.
Thanks!
 

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You said the shimmy is worse on bumpier roads. It sounds like the bumps could be triggering some sort of resonance that shouldn't be there. Check for cracks (frame, fork, steerer tube), set the headset tension and try different wheels and tires. Make sure your skewers are seated and firmly clamped and look at the wheel alignment. I don't know if I've done decents as steep, bumpy and windy as you but my 555 has gone downhill whithout incident.

I don't know where the mad face above came from.:thumbsup:
 

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Discussion Starter · #6 ·
hairscrambled said:
You said the shimmy is worse on bumpier roads. It sounds like the bumps could be triggering some sort of resonance that shouldn't be there. Check for cracks (frame, fork, steerer tube), set the headset tension and try different wheels and tires. Make sure your skewers are seated and firmly clamped and look at the wheel alignment. I don't know if I've done decents as steep, bumpy and windy as you but my 555 has gone downhill whithout incident.

I don't know where the mad face above came from.:thumbsup:
After getting a better understanding of what shimmy is and possible causes I am going to ride my bike to see it shimmys slightly even when not on hills. Also from the forum reponses it seems to be a mechanical or structure problem not my riding technique. At first glance my headset seem fine, no looseness there. No cracks or fractures visible to me also. Frame and fork look fine but I don't really have a keen eye for that kind of thing. The owner and his brother have both been very good with me on this bike and the previous bikes I have bought so I see some solution to this shimmy problem from them when I take my 555 Look in to their store.
 

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Good decending technique (butt off the saddle, loose grip) can help to minimize shimmy but the cause is usually some multiple of the wheel's rotation being amplified by something else. What is your speed when it happens? What wheels and tires are you using? Are they true? It could be as simple as the bumpy asphalt triggering an out of round wheel or tire that's been flat-spotted.
 

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Discussion Starter · #9 ·
Wheels are trued and aligned. No imperfections.

I come to the conclusion that something is out of whack that's not visible (headset?) and will take it back to the bike store for a going over.
 

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apart from headset

DoubleT said:
I come to the conclusion that something is out of whack that's not visible (headset?) and will take it back to the bike store for a going over.
I had something similar occur on my Madone and it turned out that the handlebars were just a tiny bit "not dead straight" in relation to the wheel/frame. Aligning the handlebars made all the difference at higher speeds. And yes, check the head-set and inspect the frame/forks too. Let us know what happens.
 

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Discussion Starter · #11 ·
acid_rider said:
I had something similar occur on my Madone and it turned out that the handlebars were just a tiny bit "not dead straight" in relation to the wheel/frame. Aligning the handlebars made all the difference at higher speeds. And yes, check the head-set and inspect the frame/forks too. Let us know what happens.
Like you said it could be something minor as misaligned handlebars.
I will definitely tell you riders what the culprit was! :confused:
 

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Overtightened headset?

Well, it happened to me when I first got my kg381. I over-tightened the expansion nut and the steering was a bit tight when moving side-to-side. During my first few rides on a long descend, I felt I was losing control. Once I re-adjust the headset, everything worked just fine.
 

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Discussion Starter · #13 ·
Shimmy problem solved!

After not riding my bike for two weeks (family vacation and sea-kayaking) I finally took my bike into shop today. Like a few of you riders had said, it was the headset, slightly loose. I should of notice the looseness myself but being a numbnut and the world worst mechanic, I didn't. I have tested the bike after the fix and it travels fine at descents and actually an improvement on straights.
Back to watching the TDF.
 
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