Shimano BT-DN320 Di2 Internal Battery derailleur but they're plenty strong so

but they're plenty strong so the durability is not sacrificed

Elastic Arch Support

Fits All Bikes

The light automatically detects brake events and intelligently changes output mode to Active Alert - a rapid 150-lumen flash “brake light” mode

This includes disassembly

Shimano BT-DN320 Di2 Internal Battery derailleur but they're plenty strong soShimano BT DN320 Di2 Battery InternalFull wireless technology means an all new Battery. The BT DN320 fits snuggly and protected within the new Wireless XTR Rear Derailleur and powers the fast and smooth shifting XTR is known for. Features: Protected Battery Tucked safely inside an enclosed space, the battery wont budge even in rough terrain, while remaining completely removable for charging. Longevity 310 MaH provides for more riding on one charge.