The bottom bracket on a bicycle connects the crankset to the bicycle and allows the crankset to rotate freely. It contains a spindle to which the crankset is attached and the bearings that allow the spindle and cranks to rotate. The chain rings and pedals are attached 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. There is some disagreement as to whether "axle" or "spindle" should be used in particular contexts. The distinction is based on whether the axle/spindle is stationary, as that in a hub, or rotates, as that in a bottom bracket. Like all bikes and bike parts bottom brackets vary in size and quality depending on the expectations of the bike.
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