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I have recently purchased 2nd-hand carbon rim brake wheels and noticed the front rim has what looks like residue from the brake pads stuck in a track the whole circumference of the rim, on both sides. It is only on the front rim, not the back. I wasn't sure if it was a groove into the carbon braking surface or material deposited onto it, so I scraped a small section with a knife, gently. It came away with little effort.

My first thought was to try and remove all of it, but then some researching found mixed opinions -- some people said to remove it to get back to the original braking surface, and other people said to keep it as it 'assures silent braking' and 'helps reinforce braking action and it protects the rim from premature ageing due to overheating'. quote source

Should I remove all of the residue or leave it there?

image of braking surface where residue was removed zoomed image of braking surface where residue was removed

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As you may know, disc brake rotors need bedding in. This involves basically developing a coating of the pad material on the discs. It really does improve braking power.

With almost all rim brake rims, I don't believe that bedding in is a recommended practice. I have a pair of ceramic coated (actually PEO, but that's not relevant) aluminum rims, and these did come with a recommendation to bed the pads in. That was an exception, and I hadn't heard that recommendation for any other regular aluminum rim I used. I did not notice a visible layer of pad material on the brake surface, and you don't have a visible layer on your disc rotors either.

Indeed, I fear that having a layer of pad compound on the rims might interfere with braking in this case. That may be especially true if you are using a different pad compound from what's on the rims - in general, I'd stick to the manufacturer's recommendation, if you can find out what that is. You don't necessarily know what pads the old owner used.

Based on that, I would clean the rims if I were in your position. Soap and water may suffice. Rubbing alcohol or denatured alcohol would also be fine.

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  • Thanks, good points about the different compounds. Soap and water isn't doing anything so might try some alcohol, otherwise might need to find something abrasive.
    – Jamie
    Apr 3, 2023 at 23:38

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