The Splined Single Speed Cogs from Surly breathe new life into Shimano SRAM freehub wheels. They’re machined from CroMoly and then heat treated and chrome plated for a durable smooth finish and the 4.35 mm spline width spreads the load out on the freehub body to avoid gouging. Tall tooth profiles help chain retention but the wide base of the cog also means several sizes can be mounted together and shifted between using a 9 speed a derailleur and shifter. Sizes 17T and larger are drilled out to save weight.