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During a recent fall, my chain came off my front chainring, and scratched up my frame. It looks to me like it's just the paint that's damaged but would appreciate a second opinion. The bike is a Trek Domane SL5 2021.

  1. Is this really just the paint, or more serious damage?
  2. Either way, what should I do to prevent it from degrading further?

The following pictures are unfortunately the best I could do w/o taking apart the bike.

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    Does this answer your question? What do I do when I am unsure if a carbon fiber component is safe to use or needs replacing or repairing? - 99% sure its cosmetic, personally I would probably cry about it over a nice single malt for a night (I am always looking for a good excuse), then ignore it.
    – mattnz
    Jan 3 at 5:00
  • @mattnz makes sense, that's kinda what I figured. If more than just the paint was damaged, I suppose you'd see black carbon fiber shine through at the very least? Jan 3 at 5:15
  • Any idea what those fuzzy things are? Overall doesn't look like much of a problem. I'd recommend pulling the cranks off at some point — it's a 5 minute job for Shimano cranks and we are potentially talking about a several hundred dollar problem here.
    – MaplePanda
    Jan 3 at 5:39
  • @MaplePanda no idea, unfortunately. I figured it's probably some kind of plastic strands from the coating? I don't really have the tools (or expertise for that matter) to take the cranks off... Might just take it to my bike store and have them take a look at it. Jan 3 at 5:53
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    Okay, they can’t be CF strands. Those are held in the epoxy much too firmly for that to be the case. I’d recommend a little bit of white nail polish or model paint to cover the damage, and then a sticker (basically the same as Nathan’s answer).
    – MaplePanda
    Jan 3 at 19:17

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It's very likely only cosmetic, but the fine hairline-looking scratches (hopefully only scratches) are not the most common thing to see in damage that occurs from a dropped chain.

The other thing to be aware of here is that most of the time, damage from chain drop affects the chainstays and bottom bracket. Here you've got it up into the down tube. That area is going to be much thinner-walled than a chainstay. It's reasonable to be cautious in this case, particularly if the chain not only rubbed that area but also got forcibly stuck and ground against it.

What you should definitely do is pull the cranks off and do a "tap" test. Starting up higher on the down tube, tap it gently with something like a plastic tire lever and observe the sound. Keep tapping down until you get to the damaged area. What you're looking for is a sudden duller, less resonant sound in that area. This test is good at not providing false positives; if it does get dull and less resonant sounding right in that spot, yes you probably have something to worry about.

What I would also do personally, even in the hopeful case that there's no sound change and you decide to not worry about it, is use a very fine grit sandpaper and smooth out the scrapes in the painted surface until you can see that it was just in the paint. Then I would put a thin frame protection sticker over the whole area, rationale being that over the life of the bike, there are decent odds this won't be the last time.

Finally, the bike appears to have a chain catcher. Make sure it's adjusted right, i.e. as close to the chain as possible without rubbing. Usually they should be able to prevent what happened here.

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    @TimoStaudinger The only special tool you need is a Hollowtech 2 preload cap tool, which is very simple and cheap. Then it's just a 5mm for the binder bolts. It's a good thing to be equipped to do by yourself anyway because the same tools are needed for normal BB adjustment. Jan 3 at 6:45
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    I would also recommend a torque wrench so that the pinch bolts are torqued both properly and equally. Here is the procedure in great detail youtube.com/watch?v=QqBtB8Kyl2U IMHO, anyone who owns a carbon frame should own a torque wrench. Even for stuff like the stem or seatpost on an aluminium bike it is preferred. Canyon ships a simple one with their bikes. Jan 3 at 8:52
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    @NathanKnutson I went to the bike store, and had them confirm that it's just superficial damage, and the carbon fiber is fine. I'll spend some time with it over the next few days to make sure the chain catcher is set up right. Thanks again! Jan 3 at 18:58
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    @VladimirF I was on the fence on whether to buy a torque wrench, since they're not exactly cheap, but then again, neither was the bike. I finally ordered one, so I can properly do maintenance myself. I see your point! Jan 3 at 18:59
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    @WeiwenNg Factors that invalidate the tap test would be sections with rapidly varying wall thickness or curvature. This downtube is boxy, and the damage is pretty well away from the joint so any wall thickness change is likely to be more gradual in that area. In my post I was trying to point out that false negatives in the tap test on bikes are uncommon. I will stand behind that. In this case the other side of the bike is very highly unlikely to have meaningful asymmetry, so if there was a dull sound at the damaged spot but not the other side, it probably means something. Jan 3 at 19:08

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