Post to post brake mounts allow you to run your brakes with a different sized rotor. The model you need depends on what size post mounts your fork/frame has, and what size rotor you want to run. Ultimate adaptors are XTR level and use a lighter alloy with more cutouts to save a couple of grams in weight. F 160-180mm - R 140-160mm 160-180mm: 20mm increase Allows for 180mm rotors on 160mm post mounts. F 180-203mm - R 180-203mm: 23mm increase Allows for a 203mm rotor on 180mm post mounts