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New carbon road bike (~1 month), rode less than 200 miles. When cleaning the bike, I discovered several scratches on the chain stay, near the cassette. Could this be due to a manufacturing defect, a bad assembly, the way I ride, or just normal and caused by the chain? Should I inquire at my lbs that I purchased the bike from? I didn’t notice these scratches until today, and now I’m wondering if they were there when I purchased the bike.

One thing to note is I did fall off the bike once (clipless fail) on the side that the scratches are on. But I have no idea how the scratches would be on the inside part of the chain stay that didn’t hit the ground.

enter image description here

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  • Btw, are you sure there is no plastic protector already in place? What is the line running parallell to the chainstay? Looks like perhaps the edge of some clear protecting film/tape. Have you felt if there is an edge there?
    – WornChain
    Commented May 25 at 21:06
  • From the chain when you remove the rear wheel. Poor quality lacquer.
    – Noise
    Commented May 26 at 16:18

5 Answers 5

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Chains can slap against the chainstay. With the condition of roads in the modern United States, it’s almost guaranteed - I can only speak to this country and to Singapore, which doesn’t have freeze-thaw.

This is just paint damage. Possibly just to the clear coat.

To mitigate this, you can put on a chainstay protector, or buy 3M helicopter tape and put some of that on the chainstay.

7

The regularities and shapes of the scratches suggest IMO that it might be from an interaction with the chain's outer plates. Looks completely superficial/cosmetic to me. I would probably just put on some duct-tape or something else like weiwen suggest, but I know nothing about fragile plastic bikes.

enter image description here

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  • 12
    Carbon bikes aren’t fragile
    – Paul H
    Commented May 20 at 14:56
  • 6
    Nor made of plastic… Commented May 20 at 17:19
  • 4
    @WarrenBurton "Carbon fiber composite materials, also known as carbon fiber reinforced plastics (CFRP), are composite materials made of carbon fibers and resin (mainly epoxy resin)". If plastics is what mitsubishi chemical group considers it then that is a good enough authority for me. You can also ask google what the resin is in the carbon-resin-mix is and it will answer that it is a plastic. m-chemical.co.jp/en/products/departments/mcc/composite-products/…. Fragile is my opinion.
    – WornChain
    Commented May 20 at 20:23
  • 2
    @WornChain Safety glass is a composite material which includes plastics (which makes it much less fragile, BTW) but it would be strange and confusing if you went around saying cars have plastic windshields. Carbon fiber can be 'brittle' but so are a lot of things we don't consider 'fragile'.
    – JimmyJames
    Commented May 20 at 21:19
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    @JimmyJames You'd better send a letter to Mitsubishi Chemical and educate them as well while you are at it. GL.
    – WornChain
    Commented May 20 at 21:56
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Like others have said, the markings look like they've resulted from the chain scratching the chainstay. This is not uncommon, and protecting the frame with some type of tape is the easiest and the most common solution. Mountain bikes, where this problem is more pronounced, often have very sturdy chain stay protectors (and a clutch mechanism in the rear derailleur to reduce the chain movement).

That said, you can also do a quick check that your drivetrain setup does not contribute to the issue. Switch to the smaller chainring in the front (assuming you have a two-by system) and to the smallest rear cog. The rear derailleur should be able to take all chain slack, i.e. the chain should feel taut, like in any other gear. If this is not the case and there is slack in the chain, your chain might be too long. If the chain is not too long, it is possible to adjust the rear derailleur to keep the chain more taut. (Also, it's not recommended to cross-chain like that; if you have a two-by system and you often ride with the small chainring and a small sprocket, try to make a habit to switch to the larger chainring earlier.)

Checking the chain length and/or adjusting the B screw is not hard. Your rear derailleur manual should explain the process, but here is an example of one Shimano manual. Page 12 explains how to measure the chain length (note that you have to remove the chain, so this is way easier if you have a quick link in your chain). Page 17 explains how to adjust the "end adjust bolt" (aka B-tension screw), mentioning its use to remove the slack.

You mentioned your bike is new. If there's any slack in the chain, it should get fixed in warranty/as a part of the first maintenance. On the other hand, even if the main cause is too long chain, the protective tape will help keeping the frame in better condition and it will protect the chainstay also when the chain is dropped so you might want to consider it in any case.

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Either caused by chain slap (see other answers, get a chain stay protection) or maybe by being really careless when installing the rear wheel. When you take out the rear wheel, make sure you are in the smallest sprocket, this will make taking it out and back in much easier and avoids hitting the chainstays with sharp cassette teeth.

In any case, it looks like superficial damage.

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This is from removing the rear wheel. Modern road bikes have this issue, maybe because of the the sharpness of 11 and 12 speed chain links. Trek for example have a sticker on the chain stay advising bike builders not to remove the protective wrap until after the bike is built. If you haven't removed the wheel yourself then this is assembly error and paint damage caused by your LBS. Tell them so they don't keep making this mistake on other people's bikes.

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  • Personally, I think a LBS would have to go out of their way to create that kind of damage. If yours is doing this, then I would find a different LBS. The other answers are pretty spot on with the damage being chain slap that can occur during normal riding due to several different scenarios.
    – Ted Hohl
    Commented May 24 at 14:48

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