Press fit bottom brackets have a bad name. There are a bunch of reasons for that: PF bottom brackets were / are really popular on carbon frames. That’s because if you press a bottom bracket into a frame, you don’t have to epoxy in aluminum threads, which adds weight and complexity to the manufacturing process. But it’s very hard to make a very round hole using Carbon, because as the carbon cures, it slightly changes shape, like a pie cooling. There’s no way to ream out the carbon post cure, so the frames out the door with oval bottom brackets, and in certain places the bottom bracket binds and in certain places there is play. Play = creaking. Binding = premature bearing wear and more drag. If you don't believe me on this, I suggest looking up Hambini on youtube. Be warned, his channel is NSFW, cause he swears a lot.
Manufactures tried to combat the crappy manufacturing tolerances of carbon frames by being cheap and theoretically resourceful. Instead of making the bottom bracket out of metal, make it out of plastic, and stick the bearings in plastic retainer cups. The plastic would sort of deform to the wonky shape of the cured bottom bracket shell in the frame. It would be cheaper to make, and lighter. Everyone likes lighter!
The catch was multifold: 1. Plastic PF bottom brackets have to be installed very very slowly and carefully. If they are pressed in like a headset, IE pressed till ya can’t press any more, the cups deform, bearings bind hard. Pressed in not enough, they settle over time, and the cranks bang around against the bearings. Also, creaking. 2. Plastic isn’t a great bearing support. So the steel races on the bearings would eventually (in a few months) wallow out the soft plastic cups. 3. The steel outer bearing races are stiffer than the plastic of the cups, and impose their roundness on the cups, so they bind and form gaps once the bottom bracket was pressed. Hence plastic cups being ‘theoretically’ resourceful. I’m not saying every bike mechanic out there today has thought this totally through, but they have experienced all of these symptoms and come to despise press fit bottom brackets.
Here’s the thing: in a steel or aluminum frame, you can machine the bottom bracket perfectly AFTER welding, so it’s straight as an arrow, and the cups are perfectly round. Combine that with an aluminum shell’d press fit bottom bracket and you have a light weight, creak free bottom bracket that allows for the use of a taller and wider bottom bracket shell. Wider is better because it lets you run the chainstays further out, room for more tire clearance and a stiffer rear triangle. Taller is better because it gives more space to weld the tubes without the tubes touching each other. Less tube touching means more stiffness, which means more strength (not always, but in this case).
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