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Is it likely that the carbon is fractured based on my pictures, making it unsafe to ride?

I discovered an odd, oval-shaped scratch on my chainstay. I remember my bike being knocked over on the asphalt a couple of months ago, but apart from that, I cannot explain the damage. When comparing it to damaged carbon frame pictures online, it doesn't resemble typical carbon damage. However, I am unable to determine if it is merely cosmetic or something more serious.

Finding experts in my area with the necessary tools and experience in working with carbon is proving to be very difficult. I have attached pictures with a marker indicating the oval shape.

Scratch from side Scratch from above Scratch from above marked

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  • Internet photos can be hard to diagnose. I concur with Ted that this looks more like a scratch from what we can see. For a general treatment and further reading, you can refer to this answer. bicycles.stackexchange.com/questions/76022/…
    – Weiwen Ng
    Jul 3, 2023 at 23:19

2 Answers 2

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That does not appear to be damage, in as much as it conspicuously appears to be the shape of where a chainstay chain-slap protection sticker was originally placed. what you are possibly seeing is the remnant of some adhesive and/or road grime that has not been cleaned off. Try using a little bit of rubbing (isopropyl) alcohol on a cloth/towel/paper towel to dissolve the remnant off and see if it can be cleaned off. If the alcohol does not remove it, you might also try some traditional WD-40 instead. Just be careful to not use too strong a solvent, just enough to remove the adhesive/grime.

It could actually be the actual chainstay protection sticker that is still in place. If that is the case, then I would leave it. Alternatively, you could replace it if you can find a replacement.

Your bike is perfectly safe to ride based on what is visible in the photos.

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    Very likely just the protective tape, you can even see it peeling off on the top left corner if my eyes are not fooling me (the small white mark).
    – DoNuT
    Jul 4, 2023 at 11:35
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    A chainstay protector was my initial thought when I viewed the photos.
    – Jeff
    Jul 4, 2023 at 20:53
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Based on the photos provided, I would agree with the above comment that it looks a lot like a plastic protective layer that is often sold on higher end carbon bikes, especially around the areas most susceptible to wear (in this case rubbing on the chain).

I can also tell you that based on my understanding of the material properties of carbon fiber that it is extremely hard to predict fatigue and failure of carbon fiber visually. And when carbon fiber does fail, it almost always fails catastrophically (ie, one bicycle becomes two unicycles) So if you can sit on the bike and the chain stay doesn't snap, then most likely the carbon is in the same condition it was in before you dropped it.

Hope that helps

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    From a few stories I’ve heard and photos I’ve seen, a broken chainstay or seatstay are actually surprisingly benign because you still have 3 other tubes holding the rear wheel and apparently you can feel the rear end getting soft before it actually breaks.
    – Michael
    Jul 8, 2023 at 8:17
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    @Michael having broken a seat tube personally while riding, its not immediately obvious while a crack is forming. I noticed a strange sound and attributed it to a shoelace or something occasionally tapping which stopped when I coasted. However when the frame tube parted completely, the bike felt really "soft" and weird immediately, like riding on a half-flat tyre but not. It was a very clear "STOP NOW!!!" feeling. I've had similar from a coworker whose chainstay parted while riding on the flat and level, it went from "something's a little odd" to immediately "I'M RIDING A SLINKY"
    – Criggie
    Jul 8, 2023 at 11:57

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