Check Rims and Disc Rotors for Brake Wear

Braking slowly wears away metal, and on rim brakes that metal is part of the wheel. Lay a straight edge across each rim's braking surface: if the track is obviously concave, curved inward, the rim is worn out and the wheel needs replacing. On disc brakes, measure each rotor where the pads touch it, with calipers if you have them. Shimano rotors start at 1.8 mm and should be replaced at 1.5 mm, a limit stamped on the rotor itself; most SRAM rotors are retired at 1.55 mm. Rotors are a modest expense, but worn rims mean new wheels, which can wipe out the saving on a cheap bike.