Hope Pro 4 Rear Disc Hub 12 x 148mm for Boost, XD Driver, 32h, Black Equipped with stainless steel bearings Taller Flanges than the Pro 2 Additional end caps sold separately Boost Hubs have wider hub shells and are only compatible with 12mm Thru Axle x 148mm frames and are not convertible Upgraded 44T ratchet ring and 4-pawl drive system for quicker engagement