Some of you non-bike mechanics may be thinking “Do I even have a bottom bracket?”, “Do I want someone to see my bottom bracket?”, or “Internal may be a better idea. This is just one of those things that should stay private.”
First of all, any of you that have a bike also have a bottom bracket. If you have a tandem, you have two!
What is the bottom bracket? It’s the spindle and bearings that allow your crank and chainrings to go round and round. The bottom bracket is housed at the “bottom” of the bike. Typically, it is 68mm wide with 1.37x24 tpi threading. Unlike the pedals where the left side is threaded left-handed, on the bottom bracket the right-side is left-hand thread.
Traditional bottom brackets are 4-sided where the crankset is pressed on to the spindle. Bolts that thread into the spindle press the crankset on to the spindle. This was fairly reliable, although larger riders sometimes “wallowed out” the crank arm, deforming the softer aluminum against the harder steel. However, this bottom bracket was heavier and sometimes had too much flex.
An improvement came with the “splined” bottom bracket. There are three common standards: ISIS, Octalink, and Powerdrive. ISIS is supported by a few independent companies, where Octalink and Powerdrive are supported by Shimano and FSA respectively.
The splined bottom bracket featured a larger diameter, hollow spindle, making it lighter and stiffer. The splined attachment to the crankset was more secure. This was a definite improvement to the square taper BB.
The latest innovation is the external bottom bracket. It is called an “external” bottom bracket because the bearing cups are outside the frame. This spaces the bearing further apart and allows for a larger diameter spindle. This results in less flex and better power transmission. As a bonus, it is also lighter.
Note the location of the bearings on this bottom bracket with the bearing location on the previous picture.
The external bottom bracket does not have a spindle. It is used with a new type of crankset where the spindle is part of the drive-side crank. The non-drive side crank arm is splined and attached after the spindle is passed through the bottom bracket.
Bolts are used to make a secure clamp to the spindle.
This is a worthwhile upgrade if you have an older bike with otherwise good parts. Note that the bottom bracket threading is 1.37x24tpi (English) and the width is 68mm. If you have any other threading or dimensions, you’re probably out of luck. You may be able to find a new crankset/BB combo for around $150, but expect to pay more. Or you can keep an eye out for people who have done upgrades and have these for spare parts.