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During operation, disc brake pads cut grooves into the rotor. When replacing the pads with new ones, would it be a good idea to smoothen the rotor with sandpaper(e.g. grit 200 then grit 800)? What is the role of these grooves on the disc in braking?

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Unlike car disk brakes, there is not a lot of spare "meat" on bicycle rotors. "Machining" the rotors is not really feasible. If they are worn to the point grooves are causing performance issues, the rotors should be replaced.

Sanding them down evenly (without expensive machining tools) will be next to impossible to remove scoring and maintain a uniform rotor thickness. It is likely to cause pulsating or even the brakes to grab or lock up.

A light sanding probably will not hurt, but is unlikely to make any significant difference.

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A fine sanding could be good if you have some contamination from the old pads. For example, after winter commuting I notice the pads are heavily contaminated. –  Rider_X Jul 28 at 17:48

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