How round should a FD clamp match the seat tube?
09-13-12, 01:41 PM
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How round should a FD clamp match the seat tube?
I have just picked up an Ultegra 6600 FD 34.9 clamp on for a carbon frame. I put the clamp around the seat tube & notice the clamp is not fully round (true?) with the seat tube, I can see a slight ovalness and a small gap between the I.D. of the clamp and the O.D. of the seat tube. But when I tightened down on the clamp screw, the gap disappeared. I wonder if this is normal? When I tightened the clamp, is it spreading the clamp to fit the roundness? I wonder if I got a bad FD with a clamp that is out of round? And I don't want to over tightened the clamp to make it fit. Any suggestions? thanks.
09-13-12, 02:24 PM
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As long as it tightens up you don't have a problem. They have to deform some way when you loosen them and they expand.
09-13-12, 02:29 PM
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I think the nominal OD of the seat tube is correct before paint is applied. Afte a few thousandths of an inch of paint are added, the clamp on the derailleur is not a perect fit, but easily flexes to match the actual painted tube diameter.
As long as it is tight then it will be fine.
As long as it is tight then it will be fine.
09-13-12, 02:56 PM
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Take the clamp off the bike put the bolt in make the ID of the FD 34.9 or35 and measure it in a few spots is it round? if its really close it should be OK if its off 2-3mm it could pose a problem when you go to torque it down on a CF post.
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09-13-12, 03:16 PM
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A clamp that is slightly larger in curvature is OK, and will generally conform nicely as tightened. But an undersized clamp can be a problem and can gouge into a carbon frame if not spread open first.
It can also make a difference whether it's the front of back half. Many FDs have far stiffer back halves to the clamp, and these won't flex down as nicely as the front half. Over-tightening mis-kmatched clamp (even to torque spec) can ovalize and crack a carbon seat tube, so use care, and don't just blindly torque to spec.
It can also make a difference whether it's the front of back half. Many FDs have far stiffer back halves to the clamp, and these won't flex down as nicely as the front half. Over-tightening mis-kmatched clamp (even to torque spec) can ovalize and crack a carbon seat tube, so use care, and don't just blindly torque to spec.
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An ounce of diagnosis is worth a pound of cure.
“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
