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I have a carbon Trek Madone 4.7 road frame with a nonexistent clamp interface. At the top of the seat tube, there should be a smaller-radius section of notched aluminum on which to place a clamp. That part of my frame is missing.

The top of the seat tube is much thicker than any clamp I've seen, not notched, and irregularly shaped. The interior surface has an irregular radius for at least 2cm; I assume that when the bonded aluminum clamp interface failed, it took a layer of the interior with it.

The rest of the frame is in fine shape. Is there a way I can create a new seat clamp interface?

I was considering leveling out the inner surface with Shapelock, creating a notch, and sanding down a short external section at the top of the seat tube to be sufficiently regular to fit a clamp.

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    did you try to contact a CF repair specialist?
    – Klaster_1 Нет войне
    Commented May 3, 2018 at 3:38
  • No. Cost-prohibitive.
    – Alex
    Commented May 3, 2018 at 4:29
  • Did you buy this with intent to repair? Do you have access to a machine shop?
    – justathought
    Commented May 3, 2018 at 4:30
  • I bought it as part of a bulk lot of frames, this problem wasn't immediately apparent. No machine shop, just what I can manage with typical handheld shop tools. I'm shooting for functional. If the repair is apparent, that's fine, so long as it's structurally sound. The frame will be binned otherwise.
    – Alex
    Commented May 3, 2018 at 4:34
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    This: bikehugger.com/posts/… is an older article but it seems to apply to your frame that has some part missing. Some kind of an insert for the seat-tube.
    – Carel
    Commented May 3, 2018 at 15:45

2 Answers 2

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It's a bad idea to sand down or modify the carbon fiber of the seat tube. You don't know what you will do to its integrity or strength.

I would do some research to find out if a replacement aluminum seat 'cap' that accommodates the clamp can be bought, and the proper technique and epoxy type for bonding it in.

If you choose not to fix the frame, don't junk it. Stick it on Ebay with the appropriate disclaimer about the seat clamp. Maybe someone with the required know0how will but it up and fix it.

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This is conjecture, but one avenue for a pretty simple fix achievable with hand tools and no real special skills could be making a seatmast by bonding a pre-made carbon tube of a suitable OD into the hole with epoxy, (presuming the ID of the seat tube is consistent enough for this to make sense) and then using an off the shelf seatmast topper on the top of that tube, possibly with some intermediary aluminum or carbon shim material in the way as needed. Or if the IDs worked out and you were willing to make a non-adjustable seat mast, you could steal a bonded-in saddle clamp from an old seatpost and bond it back into the ID of the mast tube.

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  • A permanent seat post would probably be more secure than any bodged clamp system.
    – Criggie ♦
    Commented May 4, 2018 at 3:36
  • The challenge there is that inside of the tube is a bit irregular and I don't have the machine tools to bore it out consistently. Interesting thought, though.
    – Alex
    Commented May 8, 2018 at 0:14

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