Surly 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

Rated fully KICKASS by a very authoritative entity who will remain unnamed, Surly's cast and polished stainless steel clamps won't rust or chip paint because they can't (even the black ones!). They're shiny! They look good! And most importantly, they deliver the clampability seatposts crave.

Surly Mr. Whirly Spindles. - 100% CroMoly steel - Proprietary dimensions and spline for use only with Surly Mr. Whirly cranks