Surly SingleatorSpring loaded chain tensioner for running Single Speed on non-adjustable chain tension frames.Fit to rear mech hanger position.Surly Singulator features CNC alloy body variable chainline floating tension pulley stainless steel pivot bushing Triple sealed reversible spring.Compatible with 1/8' and 3/32' chains.11T jockey and slotted alloy chain rejection side plates.All in all... the mutts nutts of SS chain tensioners.