Endless Bike Kick Ass Cog 21T Black Machined 7075-T6 aluminum singlespeed cogs Wide base prevents the cog from digging into freehub bodies and reduces flex Splines are machined for perfect alignment and fit on all mountain cassette hubs 100% made in the USA Shape, width and height of the teeth are CAD designed and optimized for Singlespeed use with 3/32" chains (1/8" OK)