Shimano 105 RD-R7150 Di2 Rear Derailleur - 12-SpeedThe Shimano 105 RD R7150 Di2 12 speed rear derailleur completes a shift in the blink of an eye with its high performance motor and optimized link structure. This is the hub of the Shimano Di2 system, with the charging port, indicator LEDs, adjustment switches and more integrated into a compact package. Low profile Shimano Shadow RD design reduces damage from contact and falls Receive the wireless shifting signal from the shift lever Check the battery