How to choose BB shell width?

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I love BB standards (sarcasm)

I wrote up some standards a while back:

Are you building a steel or titanium bike?

The first question to ask is, do you need a T47bb?

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For a steel bike, these are the reasons to use T47:

  • If you want to run the cable housings internally through the BB
  • more space for 30mm spindles (this is mostly for fancy machined cranksets)
  • If you find yourself using 38mm and larger downtubes (mountain bikes). Having a larger BB shell gives a better surface to weld/braze the downtube and seat tube to.

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For a titanium bike most people have switched to T47:

  • internal routing is way more common on titanium bikes (high end market)
  • Titanium bikes use bigger tubes (>38mm).

If you answered “yes I want T47” the next question would be, internal or eternal bearings?

Internal (T47IB):

  • internal is the “real standard”, supported by SRAM and Campagnolo
  • For gravel bikes, it gives you a little more room to land the NDS chainstay. The DS chainstay is more limited by the chainring than the bb shell space
  • it gives a bit more room for internal cable routing

External (T47EB):

  • not an official standard according to SRAM (they do not make a DUB T47EB
  • The BB cups are much easier to install and uninstall (bigger tool interface)

TLDR:

I would first establish your motivation for T47.

Pros:

  • better match for >38mm downtubes
  • room for internal cable routing
  • better compatibility with 30mm spindles

Cons:

  • It is heavier
  • you need more tools
  • It its easier to cross-thread.
  • It does not help with DS CS clearance (the more difficult chainstay)

If you decide you want T47, framebuilders have more or less converged on:

  • Gravel bikes: T47x85.5mm Internal Bearing
  • Mountain bikes: T47x73mm External Bearing
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