High Quality Content by WIKIPEDIA articles! The bottom bracket on a bicycle connects the crankset to the bicycle and allows the crankset to rotate freely. It contains a spindle that the crankset attaches to, and the bearings that allow the spindle and cranks to rotate. The chainrings and pedals attach to the cranks. The bottom bracket fits inside the bottom bracket shell, which connects the seat tube, down tube and chain stays as part of the bicycle frame.