SURLY Singleator Chain Tensioner - BlackSurly Singleator mounts right up to your derailleur hanger and provides a simple, effective, and adjustable means of tensioning the excess chain slack found in vertical dropout single speed drivetrains. Triple sealed spring, large cog capacity and chain guides Stainless steel pivot bushing Includes two springs for upward or downward tension adjustment Do not use with a coaster brake, fixed gear, multi cog or multi ring drivetrains Source: Quality