- Chainline: 46.9mm- Bottom Bracket: Not included - SM-BBR60 / SM-BB72- Tighter gear steps than 1X drivetrains make it easier to maintain pedaling rhythm- 31-48T: Shimano's widest range gravel-oriented double crank.- GRX 12 speed, hollow arm double crankset.The new FC-RX820-2 crank balances weight and efficiency for mixed-terrain riding. Gearing options cater for the specific demands of gravel riding and to maintain tighter gear steps for maintaining pedaling rhythm.The Shimano FC-RX820 crank balances weight savings, optimized efficiency and an array of gearing options that cater to the segmented demands of mixed-terrain riding.- Gravel-optimized 48-31t gear combination ratios preserve riding rhythm on rough surfaces- The 17t range covers an array of riding conditions- Smaller gear steps for cadence control on mixed-terrain riding- Hollowtech II crank arms provide strength and stiffness with the benefit of weight savings- Compatible with HG 12-speed chains- 47mm chainline- 151mm Q-factor