Help me understand Bottom Bracket sizing
I apologize in advance for asking a basic question but I'm returning to mountain biking after a long hiatus and a lot has changed! One major change I see is that Bottom Brackets aren't as "standard" as they used to be.
The frame's bottom bracket tube is simply one of many tubes on a bike. It has three dimensions: Length, outside diameter, and inside diameter. Knowing they are threaded, I assume there are also different thread pitches and that the depth of the threads can vary.
So when I see a frame's bottom bracket listed as "73mm" or "66mm," I assume that's talking about the length (width, really, since it is perpendicular to the frame's main axis).
Is that all that varies on a frame's bottom bracket tube? Does OD or ID also vary?
how are the actual bottom brackets (the bearings and axle) now sized? Do you buy them in specific lengths? Diameters? Styles?