Shimano Tourney RD-TY300-SGS Rear DerailleurThe 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 action Double Servo enables the guide pulley to track the cogs closely while moving front to back optimizing each shift Designed to keep the rear derailleur guide pulley at an optimum distance from each sprocket for