The Shimano Tourney TY300-SGS rear derailleur features SIS Rear Indexing and Double Servo technologies to provide smooth action with perfect tracking for the most accurate SIS shifting.SIS Rear Index Shifting System provides smooth actionDouble Servo enables the guide pulley to track the cogs closely while moving front to back optimizing each shiftDesigned to keep the rear derailleur guide pulley at an optimum distance from each sprocket for accurate SIS shifting13t upper and lower pulleysCompatible with HG 6/7/8 speed chains