Shimano CS-HG51 8-Speed Cassette: High quality 8-speed cassette for Narrow 8-speed HG chains. Hyperglide sprockets have a computer designed tooth configuration with contoured shift gates, resulting in a crisp smooth shift even under load Close ratio gearing allows a more efficient use of energy through finer cadence control Sprockets are cut away to further reduce weight without reducing rigidity Corrosion-resistant steel cassette lockring