What is a bottom bracket?

Confusingly-named component explained

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?

Advertisement

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.

Older square taper-style bottom brackets include the axle upon which the crankset rotates
Older square taper-style bottom brackets include the axle upon which the crankset rotates
Shimano

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.

Advertisement

Many different bottom bracket standards exist, for a detailed overview of these then head on over to our ultimate guide to bottom brackets.