So i've had gxp bbs/cranks on a few different frames with 68mm shells, and while I would not suggest that anyone go this route over, say, hollowtech ii, they've been more or less fine and not totally fuckyducky in any way. the only real issue has been the self extracting bolt not behaving (fantastic feeling: first mile of a 100k when your crank arm falls off), but that's probably my fault somehow.
anyways, my old trusty square taper bb on drunkbig (73mm shell) passed away peacefully in the night, so i swapped on a newish gxp bb i had sitting around and picked up a mtb triple to go with it. after installing, and rereading 1000 times that no spacers should be used for a 73mm shell, I'm left with this horrendous driveside gap:
I've never seen anything remotely like this on my 3 previous (68mm) gxp builds. NDS looks totally fine.
To my surprise, about 60% of the internet seems to think this is totally fine and normal. As I understand it, only the non drive side cup is preloaded on gxp, so that gives me some solace. And I haven't noticed anything wrong while test riding, but it hasn't really been put through its paces.
But look at that thing. It looks wrong, wrong, wrong, and I'm just having trouble believing this is how they were designed. It also seems to be putting the chainline farther out than I would expect (ignore the photo, I'm using the middle ring). The temptation is to put a spacer on the nds bb cup, but I've heard this is bad bad bad, and also would presumably shift the whole business out of center.
Anyone have any thoughts or experience in this department? My investigating and tinkering and thinking has sort of reached the end of the road.
edit: forgot to add, crank is a sram x5 (gxp), and we're talking plain ol gxp, not pressfit gxp.
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donavanm
Posted: Fri May 19, 2017 1:13 pm
Joined: 14 Jun 2012Posts: 1342
That looks super fucky. Its just spindle coming out? have you measured actual spindle length, they vary. I had to search for days, but I ended up finding a sram chart that covered the various bb/crank/shell/boost spacer options. Will post if i find it at home. Im guessing thats a 2.5mm gap and you need a DS bb cup spacer.
walker
Posted: Fri May 19, 2017 2:19 pm
Joined: 05 Aug 2015Posts: 1855Location: beastcake
donavanm wrote:
That looks super fucky. Its just spindle coming out? have you measured actual spindle length, they vary. I had to search for days, but I ended up finding a sram chart that covered the various bb/crank/shell/boost spacer options. Will post if i find it at home. Im guessing thats a 2.5mm gap and you need a DS bb cup spacer.
So it says gxp on the crank, and
Quote:
, PressFit cut weight by up to 40 grams and provide easier installation by allowing you to press the bearing cups into the frame instead of threading them. There's no change to the crank — spindle length and diameter remain the same.
Spindle length should be correct, even if there was such thing as a pressfit gxp crank and I ended up with it.
A ds spacer would also push chainline even further out, which can't be right.
Edit: no it wouldn't I'm dumb
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walker
Posted: Fri May 19, 2017 2:23 pm
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Spindle length should be 72mm, will confirm when I'm home
Errr double that
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walker
Posted: Fri May 19, 2017 4:24 pm
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so i just talked to the recycled guys, they also think it's fucky.
but then i read and thought and read and thought, and now i'm pretty sure it is actually designed this way. and it's actually a pretty neat design.
as i mentioned before (but didn't grasp the importance of), ALL of the preloading is done on the nds. That means that the side-to-side positioning of the whole crank/spider/chainring assy is determined by where the lip on the spindle meets the nds cup. There is no "pinching" of the ds cup (unlike hollowtech ii).
This is cool because it makes chainline adjustments pretty easy, all you have to do is add spacers between the nds cup and the shell. Doing so shifts the whole assy towards nds, so chainline moves inboard. Conversely, adding spacers to the ds does nothing at all (unless there's no more gap to fill, then it'll just bind). As it is now (no spacers) the chainline is as outboard as possible, which explains, well, why my chainline is so far outboard. Knowing this, I would definitely recommend (threaded) GXP for anyone building up a ss mtb, sscx, 1x, track or anything else where an easily variable chainline would matter. Works better with a 73mm shell, but you could get a little wiggle room with a 68 too.
Plus, that ugly gap also allows you put in a bb mounted chainguide/bashguard on the ds without affecting chainline.
SRAM could have done a much, much better job describing all this. Instead they just say "don't use any spacers."
That said, I still think that the gap is bigger than should be expected; I'm guessing the shell has maybe been aggressively faced once or twice? this frame has most likely seen some shit.
if you want to read on and on about it (maybe worthwhile if you already have gxp), here's the thread that showed me the light:
Are you sure you don't have a DH or fatbike crank meant for an 83mm or wider shell? That would explain all your extra spindle length.
Page 14 says to use spacers for 68, 83, and 100 mm shells. N/A. Pretty sure it's a standard MTB crank, don't think it would clear fatty stays and it doesn't look gnar enough for DH. Recycled (shop not floor) guys also said standard mtb. Also came set up as a triple w no guard.
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blasdelf
Posted: Wed May 24, 2017 8:42 pm
BAD NAVIGATORJoined: 01 Mar 2010Posts: 1505
dude wat how are you failing at this so hard
24mm MTB cranks on 68mm BSA shells use spacers underneath the BB Cups
most Shimanos would use two 2.5mm spacers on the DS and one on the NDS
your SRAMs more likely use one 2.5mm spacer on each side
none of this has anything to do with the GXP standard in particular
none of this has anything to do with chainline
donavanm
Posted: Wed May 24, 2017 9:49 pm
Joined: 14 Jun 2012Posts: 1342
blasdelf wrote:
24mm f'n mtb cranks on 68mm BSA shells use spacers underneath the BB Cups
I think the confusion is because
walker wrote:
73mm shell
walker
Posted: Thu May 25, 2017 10:18 am
Joined: 05 Aug 2015Posts: 1855Location: beastcake
eeesh jah love.
this thread is becoming a bizzaro world copy of the thread I posted above.
fred this isn't anything worth getting worked up about. and if you need to get worked up, just read the other thread first.
yes it has to do with chainline.
yes, the reason that it has to do with chainline is the gxp standard (specifically the "floating" side). I suppose you could move chainline via spacer placement with the shimano standard if your total spacer width stayed the same but the point is you don't need to worry about that with gxp.
just find a gxp bb/crank at the shop and play around, it will make sense.
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axel
Posted: Thu May 25, 2017 10:29 am
Joined: 15 Feb 2013Posts: 1654Location: St Johns PDX
this fixtation that one needs to adjust chainline by horizontally aligning their crank seems silly.
on every ss mtb I've owned, my chainline is dialed in by adjusting spacers on a cassette body.
this has a number of advantages, primarily in that one does not ever need to pull their crank to re-adjust chainline. just pop the wheel out, switch out the cog/spacers as needed, then put the wheel back in and re-tension the chain.
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walker
Posted: Thu May 25, 2017 10:40 am
Joined: 05 Aug 2015Posts: 1855Location: beastcake
rob-I have a ss hub, so i'm stuck with adjusting from the front. that said, i don't know why I would be readjusting chainline. sorta set and forget, yeah?
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axel
Posted: Thu May 25, 2017 11:33 am
Joined: 15 Feb 2013Posts: 1654Location: St Johns PDX
assuming all of your cogs are the same width, yes.
but this assumption turns out not to be true in practice.
I have Dimension and Surly cogs for my ss and swap between them as-needed; for some reason the Surly cogs are 2.5 mm wider at the base compared to the Dimension cogs.
so every time I swap between gears I adjust chainline accordingly. thankfully it's very easy to add/remove spacers from the cassette body.
spacing at the hub is actually super-common for chainline adjustments, even on a ss hub.
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J_Dada
Posted: Thu May 25, 2017 11:44 am
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axel wrote:
I have Dimension and Surly cogs for my ss and swap between them as-needed; for some reason the Surly cogs are 2.5 mm wider at the base compared to the Dimension cogs.
Cogs with a narrow base can dig into your freehub body, I like King and Surly cogs because they avoid (or at least lessen) that.
walker
Posted: Thu May 25, 2017 11:55 am
Joined: 05 Aug 2015Posts: 1855Location: beastcake
axel wrote:
spacing at hub
yeah that makes sense. but wait, won't that throw your wheel out of center? does the 5mm or so just not matter?
anyways, i'm not swapping gearing much. one ss is drunk bike, one is city trackish bike, and the other is sscx, none of which i really care about changing gearing for (maybe sscx for certain races but honestly i'm just lazy and if i do anything at all it will be just swapping chainring position or maybe freewheel).
Shit how did we get here
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axel
Posted: Thu May 25, 2017 12:17 pm
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walker wrote:
axel wrote:
spacing at hub
yeah that makes sense. but wait, won't that throw your wheel out of center? does the 5mm or so just not matter?
in the photo I posted you'd do this spacing before you even laced up the wheel and only do it once to align with the chainring.
J_Dada wrote:
Cogs with a narrow base can dig into your freehub body, I like King and Surly cogs because they avoid (or at least lessen) that.
yes, this is a perfectly good reason. the Dimension cogs are cheeaaap.
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walker
Posted: Thu May 25, 2017 12:56 pm
Joined: 05 Aug 2015Posts: 1855Location: beastcake
axel wrote:
in the photo I posted you'd do this spacing before you even laced up the wheel and only do it once to align with the chainring.
Gotcha. Sure would be nice to have an easy way to correct chainline after building the wheel ; )
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walker
Posted: Tue Jun 20, 2017 2:16 pm
Joined: 05 Aug 2015Posts: 1855Location: beastcake
dramatic conclusion for those following along:
put 2 spacers on nds. very little gap remains. mounted chainring on outer position, chainline looks good enough for government drunk.
on a barely related note, i've been requested to post this. I ended up servicing the rear hub because it was gross. i was surprised to find a cartridge on one side and ballz on the other.
it's some sort of 105 ss hub, fwiw.
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