American Classic Rear Disc 225 Hub Mountain 135mm QR 32h Black with XD Driver (SRAM XX1) XD (XX1 ) Driver Body compatible with SRAM's new single-ring XX1 group New XD Driver eliminates any play between cassette and freehub body which can degrade shifting accuracy Aluminum locking tube replaces conventional cassette lockring 17mm aluminum rear axle New drive system features a 6-pawl, 24-tooth design with a cam plate that engages all 6 pawls simultaneously