Does this crack look cosmetic or structural to you? It's tiny hairline thick.

UPDATE: I took the bike to trek store and their technician looked at it (I think he simply did tap test) and said he beleives it is only paint crack.

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    This is relevant, although not really an answer: Carbon Bike Assessment and Repair - The Limitations of Tap Testing Commented Apr 3, 2021 at 1:34
  • 1
    Could you add more information about how this happened? Has the top tube been clamped in a repair stand? Does it line up with the handlebars if they were to get turned into the frame? it looks like the paint is discolored around part of it- is it rubbed off or chipped?
    – Pisco
    Commented Apr 3, 2021 at 2:31
  • I recently bought this bike and did not notice the crack when buying. Also know nothing about history on how it happened. paint is also discolored a bit. I did coin test but did not notice tone change. I’m beginner though.
    – moe
    Commented Apr 3, 2021 at 2:51
  • 4
    No one can tell from a photo is carbon is compromised or not, especially if the mechanism that caused the damage is unknown. The fact you have asked suggests you are concerned enough to seek help though a physical inspection by someone who knows carbon products. That said, carbon, especially carbon consumer grade frames, is infinitely more robust than most believe, lawyers would have a feild day if it shattered at the first minor contact with a real world object.
    – mattnz
    Commented Apr 3, 2021 at 5:20

2 Answers 2


This isn't an area I would expect to find a purely cosmetic crack from normal riding forces. that leaves damage from an outside force, or a manufacturing defect in materials or workmanship.

If the bike has been in a crash, impacted, clamped by the top tube in a work stand, or if the crack's location is consistent with the handlebars turning forcefully into the frame, or some other outside force, or if you don't know the full history of the bike and you're thinking about buying it used, I would err on the side of caution and assume the damage is more than cosmetic.

If the damage is not due to an outside force, but rather the result of a defect in materials or workmanship (e.g. issue in the carbon layup), and you're the original owner, go get that warranty. If not, and you haven't already, I would recommend you talk with the shop anyways, as you might get a loyalty credit for the non-warranty damage. this looks like a model year 2016 or 17 Boone 7, but I'll also note that subsequent (used bike) owners of model year 2020 and newer bikes are covered for three years from the original retail purchase, with the original proof of purchase.

If you're asking because you haven't bought the bike yet, I would assume that it sustained more-than-cosmetic damage from an outside force, and proceed with your negotiations, if you still wish to buy it, as though the frame will need repair or you're buying for the parts.

Edit: Since you already own the bike, and you got it used, I would tread with plenty of caution, since I'm told carbon fiber can sustain structural damage without showing any visual signs, but that's a function of my risk tolerance, which may differ from yours. Even if you can't rule out structural damage, you can still check for affirmative signs of damage with checks like the coin test, feeling for "give" when you push on it, and monitoring the crack for changes or growth over time. The traditional spiel from a liability-conscious bike shop is that we can't be 100% sure it's safe to ride from a simple external examination, but that doesn't mean that everybody in your position immediately stops using the bike altogether.

  • You're right. It's boone 2016. I've already bought this from previous owner. Bike had pretty low usage. What do you suggest now? Should I keep riding it and mark the crack and look if it grows or not? Or it is unsafe and I should get the frame repaired. I still need to take the bike to get tested for crack. I don't know why I missed this when buying although I checked the frame for any cracks. Thanks
    – moe
    Commented Apr 3, 2021 at 4:33
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    @moe I concur with others that it is more likely to be paint damage than structural. You are correct that you can just keep riding it and see if it grows. Few people will be able to actually test that properly - that requires ultrasonic or other imaging. In the worst case, find a reputable carbon repair shop. Appleman Bicycles in Minneapolis says that most repairs are in the $3-400 range, so hardly ruinous. Carbon is definitely repairable by someone competent.
    – Weiwen Ng
    Commented Apr 3, 2021 at 13:21
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    @moe Does it line up with handlebar damage? The location and angle look about right. I’d just mark the crack and keep riding it. CX bikes are built very strongly anyways.
    – MaplePanda
    Commented Apr 3, 2021 at 21:01
  • @MaplePanda I'm not sure what do you mean by crack lining up with handlebar. I added more picture that shows crack position to handlebar and a more focused picture of crack itself. Crack has irregular pattern and not linear.
    – moe
    Commented Apr 4, 2021 at 2:32
  • I also asked the previous owner and he said he does not know. He said this bike is ridden less than 100 miles and has not been in any accident. Although he said he has used bike rack to carry bike ( but he said he used soft cloth). Does bike rack can cause cracks in frames?
    – moe
    Commented Apr 4, 2021 at 2:39

that is not a normal area for paint crack, paint crack usually happens in the fork where there is continuous of flexing of the carbon. That crack most likely came from external impact and there is a high chance the fiber is damaged underneath. The only way to know 100% is to sand the paint down

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