Box One Single-Speed Chromoly Cogs are CNC-machined from long-wearing CNC-machined chrome-molybdenum. It's designed to fit Shimano(TM) compatible cassette hubs and has a 5mm wide inner base, giving a more solid interface with the cassette body. FEATURES - Chromoly Steel Offers Extra Long Service Life - 5mm Wide Inner Base to Give a More Solid Interface with Freehub Body