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Bottom bracket 'standards' and why I might go insane


Tony Harrison

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Unlike when I was a lad, and it was all easy, and only BMX frames started to get different bottom bracket standards, now we seem to have gone mad with different specs. Even the naming from one to the other isn't conventional, with some putting the emphasis on spindle diameter, others on shell diameter.

What the bleeding f'ck is going on?

I have bought a frame with a 'BB92' shell, so it's 41mm inside diameter with a 92mm length. Currently fitted is an XT bottom bracket.

I'd like to fit an SRAM Eagle GX crankset. I understand that to do so I will need to buy a GXP BB, which SRAM do make for the BB92 interface. However, what about spindle lengths? There are options here too I think, and I don't want to buy a crankset only to have the spindle too short or long.

As my frame has a 148mm 'BOOST' spacing I know that I probably need the SRAM BOOST chainring, which has an additional 3mm offset to give a 52mm, rather than 49mm chainline. But... if I end up with a longer spindle and have to use spacers, that's all going to go out the window anyway...

So does anyone have a clue how I can solve this? Or do I just buy some Shimano cranks.

(frame is a 2018 Rocky Mountain Altitude)

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Ok, a guide I read says:

"Because the bearings are housed inside the shell, the spacing remains the same as it would be with an external bearing configuration, which means a wider spindle length is not necessary for riders switching from a 68/73mm threaded shell to a BB92 set up."

So presumably the SRAM cranks I've seen for sale for 68/73 will fit...

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3 hours ago, Tony Harrison said:

Ok, a guide I read says:

"Because the bearings are housed inside the shell, the spacing remains the same as it would be with an external bearing configuration, which means a wider spindle length is not necessary for riders switching from a 68/73mm threaded shell to a BB92 set up."

So presumably the SRAM cranks I've seen for sale for 68/73 will fit...

Pretty sure you’re on the right track there. Standards are a real pain in the arse.

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I need to decide which groupset before I buy the wheels, because if I'm going for the 12sp SRAM setup then I need the XD driver.

I'm not even that keen on 12sp, 11sp would be fine, and as I have an XT BB I could just buy Shimano cranks and a Hope cassette/driver...

Not in a rush though, got some time to work it all out.

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I've only done a few hundred miles on my GX 12 speed setup and it has been great so far. Does as it should really, I have heard some people saying it can be a bit problematic and clunky at times but for the most part it's pretty reliable and solid. 

I don't think it's fully necessary though, get the front chain ring size correct and 11 speed is more than adequate and would fulfil my needs perfectly if I had it. 

I'd say just get the stuff you can get the best deal on, you can't go far wrong but getting the chainring size right is critical. I see so many people barely using the smaller cogs on the rear yet they have a big ass chainring and end up spoiling the lower end.

Edited by Duncy H
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Still on 9 speed 32 by 34-11, does fine for all of the riding I do. Much stronger chain as well.

Next time I replace drivetrain I'll probably up the chainring to 36ish and cassette up to 42 or so, give a little more at both ends of the range :)

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Plates are thicker I believe, certainly I've heard of plenty of people preferring less gears for a more reliable chain.

Edit : 9 speed is 6.7mm wide, 12 speed is 5.25mm, that's 22% wider. Roller width is the same but the links need to be narrower to fit the cassette spacing.

Edited by forteh
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On 15/01/2019 at 8:13 AM, Tony Harrison said:

I'd like to fit an SRAM Eagle GX crankset. I understand that to do so I will need to buy a GXP BB, which SRAM do make for the BB92 interface. However, what about spindle lengths? There are options here too I think, and I don't want to buy a crankset only to have the spindle too short or long.

The Eagle GX cranks are weird in the sense that the have relatively long machined races (they're not races but I don't really know what you'd call them...) on the axle, and basically the BB bearings can sit at any point along them.  It seems you can play fairly fast and loose with the BB cup spacing/positioning without it being an issue.  I'm more used to setups where the axle/bearing position is a little more fixed, so it took a bit of getting used to that drive-side there's nothing between the outer race of the BB bearing and where the bashring starts other than fresh air.  Fairly sure the BB bearing preload is handled by witchcraft or something.

I suspect that the current standards clusterf**k is probably why they've done this though - SRAM generally seem to gear stuff towards making it easier to sell as OEM, so if you've got a crankset that's compatible with almost everything out there without needing to play around with spindle lengths and so on you'll make life easier.

The bit about the width in comparison to a 68/73mm setup makes sense though.  Most BB bearings are around 7-10mm wide, so if they're within the frame with a 92mm shell then it's not really much different to external BB on a 73mm shell.

 

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18 hours ago, Mark W said:

I'm more used to setups where the axle/bearing position is a little more fixed, so it took a bit of getting used to that drive-side there's nothing between the outer race of the BB bearing and where the bashring starts other than fresh air.  Fairly sure the BB bearing preload is handled by witchcraft or something.

 

There's a shoulder on the spindle so the non-drive side keeps it in place...

 

SRAM_SHimmySpindle.png

 

...but what I don't understand is what stops a sideways impact on the drive-side crank from pushing the bearing/cup out of the shell.

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Yeah, it's that latter part with the drive side bearing that throws me with it.  It shouldn't work, but it seems to have been holding up well for a lot of riders who do massive stuff.  From personal experience they're probably the easiest cranks I've fitted to my bike, and they've worked flawlessly the whole time they've been on.

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And here's me with a big standard 73mm threaded bb and Shimano zee cranks, with an xt bb. And got 'burns on the hex. All this shite that's been foisted on us by Spesh, trek and Giant does my head in

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