Bottom bracket : The bottom bracket (BB) is a component on a bicycle that provides support for the crankset as it rotates. It is installed in the bottom bracket shell of the frame, which connects the down tube, seat tube, and chainstays. Depending on the design, the BB can either consist of a spindle passing through bearings or only contain bearings, which are used in conjunction with a crankset that has an integrated spindle. Based on frame design, BBs come in two types: threaded and press-fit.