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TCR/compact shimmy/twitchiness?(10 posts)

TCR/compact shimmy/twitchiness?fbg111
Sep 8, 2002 1:30 PM
On my past two rides, I have had disconcerting experiences on my TCR2 and need some advice/input. In these instances I was going about 22 and 25 (with a tailwind) when I hit some unusually rough patches in the road. This gave the bike an extreme shimmy that felt like the rear wheel had come untrued and was wobbling at high speed. The first time I immediately stopped and walked the bike to the end of the ride b/c I was concerned something had come loose. Checked it out, but everything was tight and both wheels were still true and there didn't appear to be any problem. Went on a few short (5 - 10 mi), slow test rides to check it out and had no problems.

Then today I was out on my usual ride and was going about 25 on a narrow road. Hit a rough patch and the shimmy happened again. This time I was more accustomed to it and just breaked and it went away almost as soon as it occured. But when it was happening, the bike felt like rubber. Very strange, disconcerting feeling.

Anyone ever get this? Is this just the standard response of running a bike off the road or into a rough patch at higher speeds? Or is this a characteristic of the compact frame? Or is there something wrong with my bike that I should have checked out by the lbs?
shimmy/twitchiness?Spunout
Sep 8, 2002 3:46 PM
My Lemond Zurich does this when I ride no hands at 50km/h. Other than that, it is stable in descents to 70km/h.

Zinn has a great discussion on possible fixes in the Velonews.com tech area:

http://www.velonews.com/tech/report/articles/2993.0.html

Read back in the archives for the full discussion on this topic.

Sounds like your problem is serious though, with hands, while seated. Check: Wheels for true and dish, tires are true (they can be off-kilter too!), frame alignment, etc. If the bike has adjustable dropouts, try changing the wheelbase. If you are descending, pinch the top-tube with your knees. Change your weight distribution. Get a new frame with regular shape(okay, just kidding there).
re: TCR/compact shimmy/twitchiness?JohnG
Sep 8, 2002 9:18 PM
need more details.

Tell us your size and bike size.

Plus, stem length. ====> if shortish stem then that might be part of the problem.

Also, how much stem stack and carbon or al steerer?????

JohnG
dimensions:fbg111
Sep 9, 2002 4:39 AM
I'm 5'11 190, the bike is a compact medium. I'm not sure what exactly stem length is, but according to Giant's site it is somewhere in the range of 108-120 mm. What is stem stack?
dimensions:JohnG
Sep 9, 2002 6:20 AM
You sound like a good fit for the medium.

Not sure what to say. Maybe you're just a bit too heavy for this frame???? These frames aren't the most rock solid, especially for heavier riders.

BTW, a shortish stem will make a bike a bit twitchy. It sounds like you may have a normal length stem.

Loose HS??????

good luck
JohnG
re: TCR/compact shimmy/twitchiness?aliensporebomb
Sep 9, 2002 2:48 AM
Totally weird. My TCR2 is solid as a rock, at least so far.

I'm at about447 miles since I bought the bike.

Rough pavement is always going to be annoying but I definetely haven't felt
any wobbling to speak of.

Rear wheel still true? Front wheel? Have you had the LBS take a peek at it?

Seat still firmly attached to the post? I'd noticed my seat was a wee bit loose
from the post about 50 miles into riding it. After tightening it, no problem.

Bar/Fork seem to be on there okay?

Any other ideas?
re: TCR/compact shimmy/twitchiness?fbg111
Sep 9, 2002 4:47 AM
I was hoping you'd answer, since we have the same bike. It could just be my riding, I don't know. I've checked every nut and screw and attachment point, but everything seems tight. I think I'll take it by the LBS this morning and have them check it out. Thanks.
Took it to lbs...fbg111
Sep 9, 2002 8:20 AM
and they found that the front wheel was slightly untrued. They adjusted a few spokes that were pulling the wheel out of true, and then tightened them all around. The frame, fork, seat and everything else were ok. Part of the problem may also have been that when I hit the rough spots I gripped the drops very tightly, and I may have overcorrected instead of letting the bike drive itself out of instability. Thanks for the advice, I guess I'll chalk this up to inexperience and perhaps a slightly misadjusted front wheel.
Took it to lbs...GMS
Sep 9, 2002 9:32 AM
I think you'd have to nail something pretty hard to knock Mavic CXP 21s out of true (they are heavy and durable).
Took it to lbs...fbg111
Sep 9, 2002 9:48 AM
I kinda did, twice. And they were just barely out of true. I couldn't tell by eyeing it, but mechanic at the lbs put the wheel on a true tester (don't know the actual name for it), and the clamps scraped the rim of the wheel in several places when he spun it around. Adjusted the spokes and the scraping was gone. I don't know if there was enough misalignment to cause my problems, but I'll see for sure over the next few weeks as I ride.