The famous and popular 2010 series of ACS Claw Freewheels. The material is chrome molybdenum with special notches for easy installation & removal. Available sizes are: 13T, 14T, 15T, 16T, 17T, 18T, 19T, 20T & 22T to fit 3/32 and 1/8 inch pitch chains. ACS freewheels are also popular for use on single-speed adult bicycles, aka FIXIES for converting a fixed (stiff) hub bicycle into a single-speed freewheeling bicycle.