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Mudguards have worn away where they contact the seat stay on carbon frame. A little deep, wondering if cosmetic or more serious. The one on the right stay doesn't look as deep but you can see 3 colours which worries me, black paint, then light silver, then dark grey. Thoughts? In need of repair or helicopter tape to protect is enough?

I tried to capture the images as accurately as possible but the lighting does cause some reflections that hide some detail.

enter image description here

left stay

right stay

right stay

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    You need to stop using that style of mudguard right now. Its eating your frame.
    – Criggie
    Commented Dec 4, 2023 at 22:39
  • Stop buying "plastic bikes", they are bad for the environment.
    – WornChain
    Commented Dec 4, 2023 at 23:04
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    @WornChain I mean, compared to aluminium that is broadly speaking true. But unless you never fly, drive a car or eat meat, the environmental impact of using a carbon bike is a pretty minor part of your footprint, provided you don't buy a new one every other year. Since (contrary to popular opinion) carbon has better longevity than all metals, it's actually not a bad choice. That of course assumes you take good care of your bike, which doesn't seem exactly what the OP did... Commented Dec 5, 2023 at 11:38
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    @WornChain: I wouldn’t be so sure that the resource usage and emissions involved in the production of a carbon frame are actually greater than that of a comparable metal frame. Besides that, it’s only <1kg of material, as long as you don’t buy a new one every month it’s probably a tiny part of the average European’s footprint.
    – Michael
    Commented Dec 5, 2023 at 17:49
  • I bought this bike second-hand, the marks are not my doing. Big mistake! Quoted £220 for repair. The bike is probably worth it to repair as I got it for a good price, but it's certainly not the good deal it was and I wouldn't buy it in retrospect. Commented Dec 6, 2023 at 1:48

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You have worn right through to the carbon, and possibly into it.

There is a school of thought (stress testing by Reynolds) that the seat stays don't do very much. This may go against logical thought.

If it was mine, I'd fill the holes with epoxy resin (araldite) and cover with helicopter tape when cured.

If it was a customer's, I might suggest they consider a new frame. It's a great excuse to upgrade your kit.

There are professional repair services for carbon but the damage is barely worth bothering with unless it's a valuable frame.

Cue everyone else's differing opinions.

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  • +1 Summed up by risk tolerance, liability and who is paying for the fix.
    – mattnz
    Commented Dec 4, 2023 at 21:01
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    It's definitely not true that "seat stays don't do very much", they're actually carrying most of the rider's weight. What is true is that they're loaded almost exclusively in compression - meaning that unlike on most other places on the bike, small cracks are relatively uncritical since the loads tend to press them back together, rather than rip them further apart. Commented Dec 5, 2023 at 11:29
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    @leftaroundabout You can see from stress analysis that they are the most lightly loaded part of the bike, even when pedalling uphill in the seated position. This is why they can be made so thin and the position is not so critical (dropped stays being so common now). I hope it's obvious to all that they serve a function other than enabling conformity with UCI regulations.....
    – Noise
    Commented Dec 6, 2023 at 13:12
  • @Noise Thanks, I'm tempted to take your advice. I bought this bike because it was going for a good price, I did not need it and was actually shopping for an indoor trainer! Alas, I am bike and deal-obsessed, which can make for impulse buys. But I will get it repaired. I'm 85kg, pushing 90 all loaded up and I won't be able to trust pushing the bike to the limit on the side of a busy road, down a decent, or in a race. If the frame was to fail at 60+kmh it could be catastrophic. I'm just wondering if a seat stay failure is likely to lead to me losing control, but I'm not sure I want to find out. Commented Dec 6, 2023 at 14:00
  • @leftaroundabout would you then say that it is not so great of a risk to ride this frame given the location of the damage compared to say the toptube? i.e. if it was to crack/fail that the compression forces on the seatstays would mean the rear triangle is likely to stay intact and not cause a massive loss of control like a top or downtube failure? I know no one can be sure of this and no one wants to put their name to safety and risk over the internet but I'm just curious. I am going to get this repaired professionally. Commented Dec 13, 2023 at 10:41

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