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A thick crack has appeared on the rear side of my carbon frame near the seat post. The crack formed after adjusting the seat post to the maximum recommended height.

How might this have happened?

Is this possible to fix this with epoxy or something else?

Could a much longer seat post reduce the risk of further damage?

EDIT: the strange thing is that it doesn't actually look conventional like a crack at all. The the end of the 'crack' is very blunt, as though someone took a hack-saw with a 3mm blade and cut right down it. There is also a slight bulge as though the seat post were slightly too big for the tube.

enter image description here

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  • Worth a read. Won't flag as a dup and theres no uncertaintiy. bicycles.stackexchange.com/questions/76022/…
    – mattnz
    Commented Apr 24, 2022 at 9:48
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    This should be easy to repair for a professional. They’ll probably just wrap a few layers of carbon fiber around the tube to stop the crack propagation. But I’m wondering if it’s even a crack and not just the normal slit with a bit of paint peeled off? It looks long but maybe that’s just the way it’s supposed to be? I can’t see the point where the slit stops and the crack begins. Hard to tell.
    – Michael
    Commented Apr 24, 2022 at 10:53
  • But strange for a carbon frame to have a slit/cut-out in the seat tube to begin with. (I assume it has because of the seat post clamp we can partly see in the top of the photo)
    – Michael
    Commented Apr 24, 2022 at 10:59
  • I would be interested to know the size of the post and frame make/model (specified seat post size). I wonderer if someone has cut a slit to fit an incorrectly sized post.
    – mattnz
    Commented Apr 24, 2022 at 18:49

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I would not ride that frame as is, the crack will continue, and failure of the frame would be almost inevitable. It is very hard (impossible) to perform an effect repair DIY, so I would not attempt it.

It might be repairable by a specialized carbon repairer, and if so, a long seat post would be highly desirable (and may be specified by the repairer).

As far as how it happened, likely the crack formed before thee adjustment, and adjusting the seat post (more specifically, releasing the clamp) allowed it to open up. A wild guess could be overtightening of the clamp crushed the Carbon Fibre creating a weak spot allowing it to propagate down the tube from sideways loading. Another possibility would be sideways impact/shock load - crash or dropping the bike so the seat hit the ground hard.

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