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So I purchased this wheelset a while back for a CX build. When they arrived I could see that the previous owner had tried to run them with rim brakes, leaving a mark on the carbon rim.

The rim brake mark seems superficial to me, having just scuffed the layer of finish and not penetrated much further, but someone has said any marking on a carbon disc rim makes them dangerous to ride. Thoughts?

Oblique view of a section of the rim, showing abrasion caused by the rim brake Side view of a section of the rim, showing the track of the rim brake

These are model Prime RR-28 and I intend to run with discs, not rim brakes.

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    Previous owner was ... misguided. If you look really closely, can you see any exposed carbon fibre anywhere ?
    – Criggie
    Sep 11 '21 at 23:22
  • 3
    No, no exposed CF. it doesn't seem to have worn off enough material to get past the top coat.
    – soletorp
    Sep 11 '21 at 23:59
  • 4
    Are you even sure this rim is really carbon? I feel like low-profile carbon rims are very rare.
    – Michael
    Sep 12 '21 at 5:40
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Everything about the wear marks on this look like a dark anodized aluminum rim with wear marks through the ano. Worn carbon brake tracks look like dull, burnished carbon usually. I have not been around any carbon disc rims that someone has taken a brake pad to, but I imagine the effect would be similar. There's something bright grey or metallic poking through in your pictures. What is that?

Carbon disc rims often have a fairly heavily reinforced cross section from the bead seat to the edge of the rim, and go down to a very thin cross section starting from the bead seat area down. You see variations of that premise all over place when you look at different carbon rim cross sections. So yes, if they are in fact carbon, that lower band of wear could be pretty bad news. Shallow section carbon rims can be very weight-conscious components and contrary to other carbon things, I wouldn't want to make assumptions that they have a cosmetic layer to give.

For example, this is a Light Bicycle AR28, which is very much in the same genre. You can see how thin the cross section gets once it's clear of the reinforced area where the walls join. Most carbon rims are like this, to the extent that it would be the right assumption to make given you don't have an exact picture of the cross section. I wouldn't ride a carbon rim that I had any doubts of material loss in that area.

AR28

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  • If the grey your referring to is the mark in the first pic closest to the spokes, that seems to be brake pad residue/some other residue as I can remove it with a wet wipe or a by running over it lightly with my fingernail
    – soletorp
    Sep 13 '21 at 11:32
  • Excellent point about being unable to measure that area - I assumed it would be reachable on the inside
    – Criggie
    Sep 13 '21 at 23:12
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edit: Nathan's answer makes an excellent point - this answer presumes you can get to the back of the worn area, which may not be possible. Measuring through the valve hole might be a challenge.


In your position I would measure the rim's thickness and make a judgement based on how much metal is there.

Consider that the rim is under a lot of sideways stress from tyre pressure, and a weakening will allow the lip of the rim rim to flex, eventually failing catastrophically.

Rim brakes do wear out the wheel's rim over time, so this is not unknown, though it normally takes many years before a rim is worn out.


To measure your rim thickness, you could simply "feel" it between thumb and forefinger, and compare with a known-good rim.

To do a quantifiable job, you'll want some special calipers. Normal vernier or digital calipers don't do the job because the rim probably has a lip to hold the tyre's bead.

There are many great options listed at https://diy.stackexchange.com/questions/133035/how-do-i-measure-the-thickness-of-something-with-a-lip-or-frame reproduced here:

  • Dental caliper:
    ""
  • Neck caliper:
    ""
  • Variation on the above - make some pointed probes that go onto a regular digital caliper, and use the "zeroise" function:
    ""
    Or you could 3D print some plastic ones.
  • Double-ended, or Indirect caliper - these allow you to take the measurement from "outside" the part:
    ""
  • Ultrasonic thickness testers (expensive)

If there is "enough" metal then you're probably good to ride these wheels.

If there is a lot of extra metal you could consider refinishing the damaged part, purely for aesthetics. I'd consider a light sanding to add tooth, and then several thin coats of hard-wearing spray paint, alternating with light sanding, just to hide the damage. And then a clear-coat on top for durability.

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