Centerlock rotor lockrings secure centerlock brake rotors onto centerlock hubs. These are not compatible with brake rotors and hubs that use a six-bolt mount. The external spline centerlock rotor lockring will work with 12, 15, and 20mm axles and is the best option for most bikes. The internal spline centerlock rotor lockring is compatible only with 12mm axles and is primarily selected for road/gravel/CX bikes with limited frame or fork clearance.