The bottom bracket shell of a bicycle is the part of the frame where the spindle of the cranks go through. The bottom bracket and spindle together form the foundation of the drivetrain on your road bike or mountain bike. All the different standard dimensions can make it hard to determine what kind of bottom bracket shell your frame has and what bottom bracket bearings you need. How can you even tell what bearings and bottom bracket shell your frame has? This extensive how-to explains exactly what you need to do in order to service and replace your bottom bracket bearings yourself.
The BB assembly may also consist of spacers and lockrings, etc. A good-quality BB, correctly installed, should serve you with years of trouble-free riding, but bearings can wear out or the BB can become loose. Find out more about the differences between BB standards and to help you choose which one is right for you. Bottom bracket spares. MTB and road bottom brackets are functionally the same as they ever were, consisting of two sets of bearings which allow the pedals and crankset to rotate freely.
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Once upon a time, all bikes were made of steel. And the bottom bracket axle was a solid steel rod. There were cups in the bearings into which loose ball bearings fitted, with the axles having a curved surface which pushed against the ball bearings. A threaded bottom bracket shell used to be the norm when bikes were made of steel.