It used to be the case that pretty much every bike came with a 68mm wide, BSA threaded bottom bracket shell. That meant that pretty much every bottom bracket would fit into every frame.
That's changed a lot over the years as various new bottom bracket standards (and new frame shell standards) have been introduced. PF30, BB30 and T47 have become commonplace - these standards all have a much larger diameter bottom bracket shell, with the idea being that larger bearings and larger spindles can be used.
One particularly vexing question is whether you need an internal/inboard or external/outboard bottom bracket:
- On an internal bottom bracket, the bearings sit inside the frame of the bike. That means the bottom bracket barely adds any width to the shell.
- On an outboard bottom bracket, the bearings sit outside the frame of the bike, typically adding around 10mm on each side to the effective width.
Whether you need an internal or outboard bottom bracket depends on what sort of crankset you have, and how wide your frame's bottom bracket shell is.
68mm or 73mm shell
If your frame has a relatively narrow shell, here's a simple guide to what sort of bottom bracket you need.
Shimano Hollowtech: Outboard
SRAM GXP: Outboard
SRAM DUB: Outboard
BB30/PF30: Inboard
83mm or 100mm shell
If your frame has a wider bottom bracket shell, here's what you'll need.
Shimano Hollowtech: Inboard
SRAM GXP: Inboard
SRAM DUB: Inboard
BB30/PF30: Not compatible