Bottom Bracket

The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely. It contains a spindle to which the crankset attaches, and the bearings that allow the spindle and crankset to rotate. The chainrings and pedals attach to the cranks.

Bottom bracket bearings fit inside the bottom bracket shell, which connects the seat tube, down tube and chain stays as part of the bicycle frame.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY-SA 4.0

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