CNC machined from 6061 T6511 aluminum. Machined surfaces allow for better caliper alignment and a more rigid brake setup.The Post Mount standard is universal to front and rear brakes.Frames and forks that use the post mount brake interface use a stock size that varies from 160mm - 203mm. It’s important to check your frame and fork for the default post mount size before ordering a suitable adapter.To calculate the size of the required adapter, subtract the frame/fork stock mount size from the desired rotor size. Picking the Correct Adapter To go from a 160mm post mount to a 203mm rotor, you need a 43mm adapter. To go from a 160mm post mount to a 200mm rotor, you need a 40mm adapter. To go from a 160mm post mount to a 180mm rotor, you need a 20mm adapter and so on. For 2016+ Fox 36 and 180mm rear brake mounts: You will need the 23mm adapter to work with the 180mm post mount and a 203mm rotor. Going from a 180mm post mount to a 200mm rotor requires the regular 20mm adapter. MODEL ID: DA0003-B