SURLY's Hurdy-Gurdy is a chain tensioner designed for open-front horizontal and semi-horizontal dropouts that keeps the wheel in place as the awkward pedaling forces pull forward.
Hurdy-gurdy can be used for Surly cross-check dropouts and dropouts compatible with many open 10mm axles. The shaft can be used with 9.5mm and 10mm solids, quick releases, and bolt-on axles (Surly Ultra New Hub) with 6mm bolts.
Stainless steel is used for the tensioner body and screws because of its proper strength and corrosion resistance when installed, but simply because the Surly staff like the shining ones.
Hurdy-gurdy allows you to quickly and easily position the wheels and adjust chain tension without the need for tools (after loosening the axle nuts and bolts, of course).