Commonly used by Shimano, BB86 is a pressfit interface with a bottom bracket shell diameter of 41mm and a width of 86.5mm. Instead of placing the bottom bracket bearings outside the frame, this wider interface enables the bearings to be installed inside the bottom bracket shell which allows for the downtube of the frame to be wider and stronger. A wider downtube and bottom bracket shell results in a stiffer frame with better power transfer.
86.5mm bottom bracket shells are typically found on road bikes while 92mm are commonly found on mountain bikes.
Since the bottom bracket shell is only 41mm in diameter it can be problematic to install a 30mm crank spindle. A 30mm spindle fitting through a 41mm hole only leaves 11mm (5.5mm each side) of room for a bearing to fit into. The result is a very thin radial cartridge bearing with small ball bearings that wear out quickly. It's commonly recommended to use a smaller crank spindle measuring 24mm in diameter so there is more clearance for a properly sized bearing to be installed which extends the life of the bottom bracket.
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