XD-15 Ceramic bearings provide superior smoothness, reduced weight and extended bearing life Grade 3 Silicon Nitride ceramic balls, Chromium steel races with triple cryogenic treatment, filled with Kyodo Yushi grease for ceramic bearings Super light Delrin wheels with aluminum dust covers AB0029: Designed as an upgrade for most 11 speed Shimano Dura Ace and Ultegra derailleurs Sold as pair (16g/pair) Third party online sales strictly prohibited by manufacturer