Surly track cogs are threaded the same as a BMX freewheel (1.375 x 24 tpi 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: 415 hardened chromoly steel 1.375" x 24 tpi Photo for color and material reference only Specifications: Cogs: 17 to 20 Size: Single Speed Material: Chromoly Color: Silver Weight: TBD