Surly Bikes is a designer and importer of bicycles, frames, parts, and accessories based in Bloomington, Minnesota, established circa1998.  The company is well known in bicycle messenger and single-speed culture[who?] and helped establish the Single Speed World Championship (SSWC). Surly specializes in steel bicycle frames. 

Distinctive components made by Surly include the Dingle cog, cranksets with separately detachable spiders, reversible chain tensioner, and the "Large Marge" an unusually wide bicycle rim. Surly also sells lifestyle items such as wool jerseys and flasks bearing the Surly logo