Without it, there would be no pedals and no cranks. Yes, I’m talking about the bike’s bottom bracket. Just like a vital functioning organ, the bottom bracket can’t be seen and when it’s functioning properly can’t be heard. But just like other parts of your bike, it will wear out. When the time comes to replace it you will need to know what bottom bracket is at the heart of your bike.
A bottom bracket is a unit that allows the cranks to rotate whilst stopping them from moving from side to side. It consists of two sets of bearings that are attached on each side of the bottom bracket shell. Some of them come with a spindle, which two sides of the crankset are attached to and connect to the bearings. The bottom bracket is held in place either by screwing it into the bottom bracket shell or a perfect and tight fit into a threadless bottom bracket shell.
Threaded bike bottom bracket standards
The majority of bicycles with a threaded bottom bracket use the same thread and size which conforms to the ISO (International Standard Organisation.) It is often referred to as a “British thread” or the measurements are given, “1.375” x 24 tpi “. “TPI” stands for “threads per inch”. There is one exception and that is the Italian standard. This is referred to as an “Italian thread” or the measurements are given, “36 x 24 tpi”.
Other bottom bracket standards are much rare and generally found on vintage bikes.
Cup and cone bottom bracket
Older and budget bikes have a cup and cone bottom bracket with two threaded cups, a lock ring, two sets of ball bearings, and an axle.
By adjusting a non-drive side bearing cup, it is possible to achieve bearings that are tight enough to stop side-to-side movement but loose enough to spin.
Sealed cartridge bottom bracket
Other systems use bearings that are sealed and don’t need adjusting. This incorporates the cups, bearings, axle, and shell.
They come with the ISO or Italian thread. There are four different width bottom bracket shells, and by measuring the width you can define which one you need.
Most cartridge bottom brackets use an axle that is square and narrows slightly at the end. These are called “square taper”.
There are exceptions where there are splines at the end rather than a taper.
The axle length also varies depending on, for example, how many gears are on the front cogs, the style of frame, and the style of the crank.
These bottom brackets don’t need the bearings adjusting, but can’t be stripped and repeatedly repaired like the cup and cone bottom brackets.
External bearing bottom bracket
These bottom brackets consist of two cups with sealed bearings, a plastic sleeve, and spacers.
The threads are ISO or Italian. The axle is attached to the right or left crank. The major difference is to do with whether you have a road bike, mountain bike, downhill, or fat bike system.
There are two common bottom bracket shell widths: 68mm/73mm and 83mm.
These bottom brackets are the lightest threaded option and are easy to fit. However, the shell of the bottom bracket needs to be faced to be able to use them.
Press-Fit bike bottom bracket standards
Some modern bikes don’t have threads to hold the bottom bracket in place, the units, or sometimes just the bearings, are pushed into place and the tight fit is enough to keep them there. These are called “press-fit” or “PF”.
Just like the threaded external bottom brackets, the axle is attached to the right or left crank.
The press-fit systems are not subject to standards and manufacturers can either design their own or use one currently in use.
Direct fitting bottom bracket bearings
The frame’s bottom bracket shell is designed so that the cartridge bearings fit directly into the shell.
Sometimes they are held in place with a circlip. Replacement bearings are readily available and the salient measurements are the inner diameter of the shell and the crank spindle diameter.
Press-fit bottom bracket unit
These units usually come with two bearing cups with sleeves and spacers.
The sealed bearings sit in the cups that are pressed into the frame.
There are several different standards for press-fit bottom brackets, including BB30, PF30, BB86/92, and BBright.
Ashtabula bike bottom bracket standards
Also called a one-piece crankset, this type of bottom bracket consists of two 51.3mm cups that are pressed on each side of the bottom bracket shell, two sets of bearings, two cones that screw on each side of the crankset, a washer, and a lockring.
This system can be found on some older cruiser bikes , BMXs and budget kids bikes.
Now it’s time to find out which tool is required to replace your bottom bracket. Our article here will guide you through all the tools used to replace a bb.
Support our work so that we can keep creating more great cycle maintenance articles. Find out more about how to become our patron here.