How to choose BB shell width?

Im building my second frame and it’s going to be a boost-spaced hardtail (first was fixed gear so nothing serious to worry about here). I want to use a bb47 bb shell but found out there’s more then one width of them (obviously it can’t all be simple). So is there one of the widths a more logical option over the others? excuse me if this has been asked/answered before but I just couldn’t find anything.

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The spindle length on your crank arms would determine the ideal BB width. A wider BB would give you more real-estate to attach chain stays and would also require a longer spindle. You can use a longer spindle on a narrower BB shell by using spacers.

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That makes sense, cant seem to find spindle length of the crank though. Ill search harder.

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You can just use the same widths as for BSA, so for MTB that 73mm.

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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?


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.

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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If the bike is built correctly, the crank choice is independent of the bottom bracket standard. Just make sure you use the correct BB for road or mountain standards:

  • Road External Bearing - 68mm
  • Road Internal Bearing - 85.5mm (most common standard)
  • MTB External Bearing - 73mm
  • MTB External Bearing - 90.5-91.5mm (?) This is not an officially adopted standard
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Wow! this is why I love this forum! Thanks for the detailed reply. The main reason I want the T47 is because it would give me more space for cables (dropper included) and the chainstays. also it will give me a bit more room to have the DT and seatstays be offset to give myself some more room to braze. As for the cons, Im not building a light bike per se, so that shouldn’t be too much of an issue. I don’t have any threading or facing tools as of now, so I would probably go to a better established friendly framebuilder in the area to ask them to help me out (looking at you @project12 ).

Ps, I guess this was a typo but im curious as to what

Eternal beearings would be? A bb you’d never have to change? sign me up!

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