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How to fix a broken off seat tube collar, or how to put a seat post clamp on propery

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How to fix a broken off seat tube collar, or how to put a seat post clamp on propery

Old 05-02-12, 03:42 PM
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How to fix a broken off seat tube collar, or how to put a seat post clamp on propery

First a little background. I built this bike myself, with little prior experience, but plenty of mechanical experience elsewhere. I figured I could tackle this on my own after reading several books. This is a Chinese carbon build, and when I first discovered the issue I yelled at the top of my lungs "THIS F#@kIN! CHINESE PIECE OF $#1T!!!" After calming down and sleeping on it for a night I decided to look for a way to fix the problem, as well as try to discover what caused it. Here is a picture of the problem.

You can imagine how I was feeling, especially when I found it 15 minutes before a race in front of a bunch of my peers. I was already hell bent on proving to them this Chinese frame was worth riding and then they see this! After some research on the net I found that this problem comes up more often then you would think, I could not find any reference to it happening on the Chinese frames, but did see several high end frames that it had happened to.

With the particular frame, there is a thin aluminum tube that is inside the carbon seat tube. The collar that sticks up for the clamp is thinner than the rest of the seat tube to accommodate the clamp. This aluminum tube continues into the carbon for about 4 to 5 inches. As you can see in the picture above, there was still quite a bit left below the break, and the slot was still there as well. The only issue was that the carbon came up on that part so it was too big around to fit a clamp over. To test my idea for a fix I put some tape around the remaining seat tube, and used a beefy hose clamp to tighten around it. It was not perfect, but it worked so I moved onto my fix.

I masked off all but the area where the new collar would be to accommodate the Thomson clamp. I then pain painstakingly ground down the carbon with a Dremel tool and file, little by little, measuring with a micrometer until I had what looks exactly like the original collar. It took about 2 hours of work, as you can see there is a lip that stops the clamp, and there is a lip on the top side of a Thompson clamp as well, I measured very carefully all the way around so that it was the perfect size.

Now for why it happened. When I bought the Thompson clamp, the instructions insisted that I put the clamp on backwards, or what I felt was backwards, with the slot facing the front of the bike, so that is what I did. Less than 3000 miles later this happens. Though you wouldn't be able to see it in a picture, the broken off collar actually has a slight pucker in the front of it where the gap from the Thompson clamp was. Not seeing that right away I put the clamp on the same way on the new collar and noticed a slight front to back movement in the post, as if the clamp was grasping the sides of the post by not the front and back. Didn't matter how tight or loose the clamp was. This got me thinking about why the collar broke in the first place, it is likely that this very slight moment was there from the beginning and eventually broke down the metal and snapped off the collar. I turned the clamp around against Thompson's recommendations and lined up the slot in the seat tube with the slot in the clamp, tightened it to specs and it does not move at all.

Lessons learned? There is more to building your own bike than what you read in a book. Though this could have very well happened at a LBS, it is likely something they have seen before and would have put it on differently. Chinese frames? Jury is still out, I can't really blame this on them, and this is not a review. Will this crazy fix hold? I will update when I get some miles on it, though it should be no different than how it was, and now that the clamp is on properly and supporting the post all the way around, I expect it to hold and not cause anymore issues. Are you stupid? Please be courteous in your response to this post, I did not post this to start a Chinese carbon flame fest, I posted this to help others that may find themselves in the same situation, or maybe prevent this by closely inspecting their post and clamp.
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