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This circular disc mounts at the hub of the wheel (front or rear). Rotor sizes are 140mm, 150mm, 160mm, 185mm, and 203mm. 160mm, 185mm, and 203mm, being the most commonly used. The rotor size will depend upon rider size, bike size, brake itself, and the terrain/riding style of the rider. Larger rotors provide more braking power and modulation and are primarily used for larger riders and more aggressive riding styles. It is not uncommon to run a larger front rotor and a smaller rear rotor. This is done as the vast majority of braking power is in the front and a smaller rear rotor prevents locking up the rear wheel leading to skidding. 140mm and 150mm rotors are frequently used by cross country racers.