Bicycle Mechanics - How round should a FD clamp match the seat tube?
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09-13-12, 01:41 PM
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
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.
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.
09-13-12, 02:56 PM
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.
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.
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