Surly Single Speed Cogs.Fit Shimano freehub splinesSCM 415 hardened CroMoly steelDesigned as a singlespeed cog, but may also be stacked and used with 9-speed shifter. Multi gear use will result in slower, less crisp shifting than cogs designed for multi-gear systemsItem SpecificationsMaterialSteelTeeth21teethCassette Body TypeShimano 1Cog Teeth21teeth