[Standards] MTB Drivetrain - Cranks and BB's

Education Resources
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Drivetrain Standards:

Non-Boost

  • Chainline: 49mm
  • Rear Hub: 142x12mm

Boost

  • Chainline: 52mm
  • Rear Hub: 148x12mm

Sneaky-Boost

  • Chainline: 55mm
  • Rear Hub: 148x12mm
  • most mountain bikes in 2022 are sneaky boost
  • Q factor increases by ~6mm

Super-Boost

  • Chainline: 56mm
  • Rear Hub: 157x12mm

Drivetrain Documentation:

Sram Frame Fit Specifications: Sram Mountain Bike Frame Fit Specifications PDF

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(Todo: Daniel) re-draw Shimano’s crankset specifications

Common Bottom Bracket Standards:

Resources:

Standard Nom. Width Type Typ. OD Comments
BSA 73mm Thread 1.5in Most common BB standard. Poor compatibility with 30mm spindles. No internal cable routing possibilities.
T47 EB 73mm Thread 2in Sram does not make a DUB T47EB BB at the moment. Used by NMW, Meriweather, …
T47 IB 92.5mm Thread 2in T47 IB is much more common on gravel and road bikes. Sram makes a T47IB BB, but not at MTB width (potential hack?).
PF30 73mm Press 2in Most common pressfit standard
BB92 92.5mm Press 1.5in Also known as “Shimano Pressfit” standard. No one makes a BB shell for this anymore. Industry seems to have moved to PF30
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Super-Boost chainline is 55 - 56.5mm, right?

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I honestly have no idea what the “official” spec of superboost is. Shimano and Sram officially list the superboost as 56 and 56.5mm (respectively). But they have not updated their documentation for at least two years.

If you read between the lines, super boost got secretly replaced by “sneaky” boost 55mm chainlines. I have been testing the 55mm boost chainline and I can’t tell any difference aside from wider Q factor.

As far as I can tell, only knolly and pivot are still pushing the super boost standard.

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A quick history lesson might be in order here.

We’ll skip the pre-1990s stuff because there was a lot of odd stuff going on (126 Campy spaced MTBs!) and none is relevant now, but:

1: In the 1990s, every mountain bike was 135mm QR. Note: this is the same hub as 142x12! This was the era of triple cranks (3 rings) and the chainline was measured to the center of the middle chainring. This chainline was 47.5mm for a LOOONG time. Why? Because it’s about where the middle of an HG cassette sits - hence, perfectly straight chainline in your middle ring in the middle of the cassette.

2: Fast forward to the mid 2000s, when bigger tires are starting to be a thing (ie, bigger than 2.1", and no, this is not a joke) Frame manufacturers are suddenly having trouble squeezing everything in, especially those annoying 29er tires, so the chainline moves to 49/50mm for most bikes, though the hub/cassette remain the same. This still shifts ok, though it does run rough in the lower gears sometimes if you’re a middle-ring-only kinda guy.

3: It’s 2010, and front derailleurs haven’t gone away yet, but everyone wants a 29er. Crap! Let’s gain ourselves just a bit of room - how far out can we push things without screwing with the normal BB shells/Q factor customers expect?

The answer: 3mm on each side. Those who are paying attention will notice that bikes from this era are all 2x10 - because there’s not enough space to push the former 3x/44t big rings out any further (nor as much need for them with 29" wheels) so they have been scrapped.

4: Ok, it’s 2012 and everyone is on 1x. No more granny gears, no more front derailleurs. We can run a ring in the old 3x “big ring” position (ie 55-58mm chainline) without a problem if we move the cassette out enough to match. Boom! 157x12 superboost! Lots of space to fit ring/chainstay/tire, stronger rear wheel, etc. What’s not to love?

Superboost is what boost probably always should have been. It’s my go-to configuration for all mountain bikes. That said, normal boost will quite possibly win the standards war, though it’s obviously inferior.

If we back all the way up to the 90s, and we were aiming for a straight chainline in the middle of the casssette, the chainlines would be:
142x12/135QR: 47.5mm
148x12/boost: 50.5mm
157x12/SB+: 55mm

In practice it’s all over the place and you can get away with going ~3mm wider chainline on any of those options without real problems, even with pretty short chainstays. With really long chainstays you can go as nuts as you want, since the angle that the chain and cassette meet is the limiting factor.

YMMV.

Hopefully that’s useful.

-Walt

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Also, from a builder/rider’s perspective, you can achieve a lot of different chainlines (on most cranks) by just swapping chainring offsets. SRAM MTB cranks, for example, can be set up for anything from 49 to 59mm just by ordering a different chainring. So if you do want to experiment with boost/superboost/whatever, you can do it without even buying new cranks.

-Walt

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What BB width are you using for SB?

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Whatever Super Boost is, the chainline isn’t 46mm as stated in the first post.

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You can use 68/73 or 83mm BSA (I probably wouldn’t do 83 unless you have a really good reason, crank selection kinda sucks) or the normal 74/84 T47. Or any of the pressfit stuff if you’re into that.

-Walt

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If you are going to do Super-Boost do it right. Pair a 52mm chainline with the 157mm rear end.

Just don’t build Sneak-Boost. A 55mm chainline with 148 and 50,51,52t cogs give horrible chain angle in the upper cogs.

This might not matter if most of your climbing isn’t done in the largest rear cogs, but it makes a difference if you do.

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Ah, thanks for catching typo. I updated it.

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Regarding possible BB shell lengths, I vaguely remember that @Daniel_Y suggested using a 91.5 T47 bb shell for internal cups instead of a 74 shell for external. The bearing size is the same on the wheels mfg cups I looked at and the overall length, seal to seal, should be within 1mm. It frees up some space and I can’t think of a downside.

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Oooohhhhhh boy! BB Standards! It’s been a while.

We’re missing a few on your list. Since I’m a huge proponent of T47 let’s start there.

This is a screenshot of the original release drawing for T47

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As we can see, this includes 68, 73, 83, 86.5 and 100mm. Included after this launch are 91.5, 85.5mm Asymmetric (thank you Trek), and BBRight T47a which is at roughly 79mm and 77.5mm for Cervelo and Factor respectively.

BB Standard Name Thread Width Internal/External Asymmetric Common
T47 68 T47 68 External No Yes
T47 73 T47 73 External No Yes
T47 83 T47 83 Internal No No
T47 86 T47 86 Internal No Yes
T47 92 T47 91.5 Internal No Yes
T47 85.5 T47 85.5 Internal No No-ish
T47 BBRight (Cervelo) T47 79 Internal Yes No
T47 BBRight (Factor) T47 77 Internal Yes No

The most common sizes would be 68 and 73mm (matching the BSA spec) as well as 86 and 92 to work as analogues for BB86 and BB92.

NOT included in this list but also existing are 386EVO and 392EVO which are technically still 86 and 92mm shells just offset. I bring this up as 386EVO and 392EVO standards are what many 30mm crank spindles are designed to and the other sizes are spacer-adjusted versions of the same.

This does also mean that your chainline may vary based on the spacers that you use (See all Cannondale Hollowgram cranksets) and so a 52Cl or 55Cl may actually measure in at 51 or 56 in practical application. Does it matter? A bit. It will mostly affect your chain retention on the cassette during backpedaling but that is also subject to component spec.

So, as with everything that is engineering related; “It depends”

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@WHilgenberg Thanks for posting that. It is very helpful.

A few comments:

  • In my limited survey, I found most road/gravel bikes use the T47IB (86-85.5mm) standard. The only production bike I could find that uses the T47EB is the Ibis Hakka.
  • One piece of supporting evidence is that SRAM does not make a T47EB
  • I don’t know any production mountain bikes that use T47, neither external nor internal
  • However, I think most custom mountain bike frame builders who use T47 bb’s choose T47EB, including @Neuhaus_Metalworks .

Can anyone find more examples of:

  • Gravel bikes using T47 EB
  • Production mountain bikes using T47
  • Anyone using T47IB on mountain bikes?

I personally really like the T47EB on mountain bikes because of the large tool purchase. I have too much trauma from removing seized square taper BBs. Unfortunately, the industry seemed to have settled on T47IB (for now). I hope we (as framebuilders) can have more leverage and convince SRAM to produce a T47EB DUB bottom bracket!

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I’ve personally never really bothered with T47 as I don’t typically lack real estate on the shell to attach things, but I could see how it would make bending/manipulating tubes less of a problem in a lot of cases.

I mean, it’s better than BSA in basically every way on paper. But BSA doesn’t have any real problems, so it’s a hard sell for me.

-Walt

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I don’t know of many/any Gravel bikes with T47 EB aside from maybe Litespeed, Seven and possibly Moots as well but you are definitely straying into custom territory there.

You won’t find many if any production MTB frames with T47 simply because of the space constraints it puts on suspension platforms, VPP frames especially, and an OEM will find it difficult to spec one component if it isn’t compatible across their lineup due to warranty support etc.

I’ve used T47 IB on an MTB frame or two but it doesn’t make a huge difference to me. It can afford a bit more area to attach a chainstay yoke and make it stronger and/or lighter but it’s not something I’ve pursued.

Enduro sells a DUB bearing kit which consists of a 6806 bearing with a custom-ground inner race. Since Praxis uses 6806 bearings in their T47 external BB, you could swap the bearings out. You would likely need to make your own spacers though.

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Trek was using T47 on some bikes, that’s why Sram started making T47 BB’s, so I heard at least. I don’t know whether they’re still using them for their metal bikes or not, but here’s an article from 2019:
https://cyclingtips.com/2019/06/trek-t47-threaded-bottom-brackets/

I love T47 for Ti frames and short chainstay frames in general since the 86 or 92 can use the internal bearing BB’s and have lots of real estate. But the boom in available yokes in the last couple of years has made this less necessary at least for the frames I’ve been making. Less companies are making internal bearing BB’s too, such as Cane Creek only offering the EB version.

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I went with a 73mm T47 shell on my custom frame hoping it’d work better with 30mm cranks. Most quality cranks are 30mm spindle these days and I’ve cracked a couple 30mm BB cups for BSA shells in the past. That may have been user error but I think it also has something to do with the large step up in the shape of the cup. Since I wasn’t building this particular bike manufacturing wasn’t a huge concern but so far so good from an end user standpoint. An added benefit I’ve noticed with T47 is that bearing swaps have been easier since I now have better access to the 30mm ID bearing with the larger shell.

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Attached are the drawings for the Easton and Raceface aluminum crank arms. They list ALL spindle options with frame clearances for the chainring and crankarms. The Easton one includes 1x and 2x chainrings from sub-compact to full size.

For whatever reason the RF one is easy to find online, but the Easton one is not publicly posted.

easton ea90 clearances.PDF (863.0 KB)
rf turbine clearances.pdf (3.0 MB)

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Anyone have drawings of Shimano GRX 1x 12 speed cranks? Need to check if I have to mount a MTB crank and try to find a 42t chainring for that, or that the GRX will fit the frame.