Shimano Di2 SM-JC41 Junction B Box This Di2 B- junction box "splits" the main wire from the Ajunction box. It is a simple, small hub that has 4 female connectors and serves as the junction point in the bottom bracket area. Usually a front derailleur, rear derailleur and battery are connected to the Junction B. Please note that this junction box is specifically made for frames that accomodate an internally routed system. Shimano recommends the SM-JC40 for externally-routed frames. | Shimano | Di2 Sm-Jc41 Junction B Box Internal Frame E-Tube Junction Box B