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What is the purpose of a threaded bottom bracket like the kinds sold by Shimano etc.? How does this differ from a design in which bearings, spacers and seals are directly press-fit into a bike frame? Are both options useable on the same frames, or do they serve the same purpose but for different frame constructions?

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  • The answer in this terminology index should give an idea what types of bottom brackets exist: bicycles.stackexchange.com/a/2759/26917
    – Grigory Rechistov
    Nov 2, 2020 at 15:05
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    I was about to write a long answer about why press-fit bottom brackets came into existence, but Weiwen Ng did the work for me. In short, press-fit is for carbon, and using press-fit on metal is has a lot of drawbacks for basically no benefit. Why were press fit bottom brackets invented?
    – whatsisname
    Nov 3, 2020 at 2:42
  • The difference with press-fit bottom brackets? The creaks that start coming from the press-fit bottom bracket about a week after you get the bike...
    – Andrew Henle
    Nov 3, 2020 at 15:23

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What is the purpose of a bottom bracket like the kinds sold by Shimano etc.?

The purpose of any bottom bracket (BB) made by any manufacturer is to allow cranks to freely rotate in the bottom bracket shell (part of the frame).

How does this differ from bearings, spacers and seals that are directly press-fit into a bike frame?

Different frames accept different types of BBs. Some of them are threaded into it, some are pressed into it. There are many incompatible designs of threaded BBs, and several incompatible types of press-fit BBs.

Are both used on the same frame, or do they serve the same purpose?

A frame is designed to accept either a variant of a threaded BB, or a variant of a press-fit BB. Seldom it is possible to modify the frame to accept a different type of BB.

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  • Might be worth mentioning that technically, shimano HT-2 bbs are pressed bearings too.
    – abdnChap
    Nov 2, 2020 at 16:12
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    @abdnChap True, the bearings are pressed into the cups. It is the cups themselves that can be threaded into the frame, or pressed into the frame.
    – Grigory Rechistov
    Nov 2, 2020 at 16:20
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    Only on metal frames, I think it’s possible to ream a BSA shell and redo it in Italian if you had destroyed the threads. On the few metal PF30 frames that exist, I think you can cut threads and turn them into T47 shells (if you have the equipment). Otherwise, you’re absolutely correct, it’s exceedingly rare to be able to change between BB interface types, and i think its even more rarely done.
    – Weiwen Ng
    Nov 2, 2020 at 19:00
  • @abdnChap I’m pretty sure all BB’s are press fit in that regard. I’m not aware of any where you literally screw the bare cartridge bearing into the frame (although that would be a workable option too!) It’s a common argument against press-fit haters; it’s the tolerance of the interface that matters, not the interface itself.
    – MaplePanda
    Nov 2, 2020 at 23:24
  • Cup and cone bearings aren't press fit
    – abdnChap
    Nov 3, 2020 at 9:15

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