The
bottom bracket on a
bicycle connects the
crankset (chainset) to the bicycle and allows the crankset to rotate freely. It is not a bracket as such. 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.