Bottom brackets connect the cranks to the bike and allow the crank arms to rotate. There are various sizes of bottom bracket shell on the market, so compatibility needs to be checked. The three most common frame bottom bracket shell widths are 68mm (English threaded road and mtb), 70mm (Italian threaded road) and 73mm (British threaded mtb). There are also many axle shapes and styles, from JIS or ISO square tapers through to the round splined axles such as ISIS or Octalink and then there is the new external cup systems which have the axle built into the crank arms. Some frames are now using the new BB30 system which uses bearings pushed directly into the frame and removes the needs for cups to be threaded into the frame.
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