Surly track cogs are threaded the same as a BMX freewheel (1. 37 x 24tpi right-hand threads). This means that they will fit on single-speed rear hubs, as well as BMX freewheel hubs, but this application is not recommended. Track cogs are meant to work on track hubs only. Features and Information 415 hardened Cro Moly steel 1. 37" x 24 tpi Item Specifications Cassette Body Type Threaded, Track Chain Compatibility 1/2" x 1/8" | Surly | Track Cog | Silver | 18T, 1/8"