RaceFace Evolve 4-Bolt 9 Speed Bicycle Chainring Set Features our X-Type external bottom bracket system. Manufactured from 6066 aluminum, this material is 30% stronger than 6061. Patented I-LOCK reinforced pedal interface features a secured chromoly pedal thread insert that WONT strip out or come loose. Evolve chainrings feature our patented S.H.I.F.T technology for lightning quick up-shifts. 6mm of chainline adjustment  ideal for chainguide compatibility.