For those who don't geek out on bikes, the term bottom bracket probably doesn't mean a lot. So what exactly is a bottom bracket?
The bottom bracket is a bicycle component used to connect a bicycle’s crankset to its frame, allowing it to rotate independently of the frame itself.
The bottom bracket is normally threaded, but sometimes pressed, into a frame’s bottom bracket shell — that’s the sleeve that connects the tubes to the frame’s front triangle. Without a bottom bracket your bike's transmission would not be able to rotate.
Bottom brackets use bearings to allow for rotation and are usually considered a service item. Some can be adjusted or rebuilt, but modern designs are more frequently designed to be replaced rather than repaired.
Some designs include an axle or spindle while others simply provide a surface upon which an axle can spin.
When a bottom bracket begins to fail, its bearings can make unpleasant creaking noises and develop play. Replacing a bottom bracket is normally a relatively quick and inexpensive job for someone who is mechanically minded.
Many different bottom bracket standards exist, for a detailed overview of these then head on over to our ultimate guide to bottom brackets.