CeramicSpeed cartridge bearings are an upgrade option for any cycling discipline. These bearings provide extremely low friction, improved power transfer, and a service life that far exceeds traditional steel bearings. Features: Grade 3 Silicon Nitride balls Hardened steel races Reinforced polymide cage and non-contact seal to keep dirt and moisture out High resistance to poor lubrication Can be used for any cycling discipline Sold as each Hand built in Denmark Specifications: I.D.: 12mm O.D.: 21mm Thickness: 5mm