This might be the BEST spacer/conversion kit for singlespeed use on the market! The SSK-3 kit actually creates a "channel" for the chain to run in, preventing it from popping off the cog. SSK-2 is a basic kit with clean, angled spacers, lockring, and 16T cog SSK-3 is Wheels Manufacturings top-end kit, featuring an integrated "guide" shaped spacer that helps prevent your chain from falling off. Same spacers, plus the "Guide" spacers, lockring, and 16T cog both kits fit Shimano and Shimano-compatible 8/9sp freehub bodies Cog works with single speed chain only | Wheels Manufacturing | Singlespeed Kit Ssk2 Kit, W/spacers, Angled Spacers, 16T