SRAM GXP installation question

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G1032
Posts: 32
Joined: Mon Jul 27, 2020 7:49 pm

by G1032

I don't know what is wrong so but when I tighten the nds crank arm the cranks basically don't rotate. There's quite a significant gap. It's a few mm

eins4eins
Posts: 699
Joined: Wed Sep 28, 2016 11:49 am

by eins4eins

G1032 wrote:
Thu Mar 09, 2023 7:30 pm

Before I go emailing looking for a replacement BB can someone please confirm that this is indeed the issue and that the spindle should be able to slide all the way through so that the DS bearing touches the crank.
Thanks

No, the DS bearing shouldn't touch the crank. Like i said before = DS is floating. NDS is where the axle is fixed by clamping the bearing between the step in the axle and the crank.

Is the crank on the NDS maybe pressing on a dust cover and not the inner part of the bearing?

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G1032
Posts: 32
Joined: Mon Jul 27, 2020 7:49 pm

by G1032

Ya. Pressing onto the dust cover. Crank so restricted though it's actually difficult to rotate and I'm nowhere near the recommended torque. I've a 2mm gap between floating bearing and the crank.

G1032
Posts: 32
Joined: Mon Jul 27, 2020 7:49 pm

by G1032

I've 12.7mm of spindle protruding on the NDS with a 67.5mm wide shell.
This here would suggest I need 15mm which would make sense. If the shell is 67.5mm wide then is it possible there is an issue with the BB?
[URLImage[/URL]

TobinHatesYou
Posts: 12456
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

G1032 wrote:
Thu Mar 09, 2023 10:23 pm
I've 12.7mm of spindle protruding on the NDS with a 67.5mm wide shell.
This here would suggest I need 15mm which would make sense. If the shell is 67.5mm wide then is it possible there is an issue with the BB?
[URLImage[/URL]

No, if a BSA BB shell is too narrow, that just means the DS will be more exposed. If it's too wide, then that might cause binding on the DS.

There's two possibilities here: Either the BB cups and bearing covers are out of spec -or- you've installed everything incorrectly.

Even if the NDS bearing + dust seal combined to be too wide for some reason, the crank should theoretically still spin freely as the seal is designed to only make contact with the bearing's inner race. Are you sure you have the dust seal facing the right way?

If you think you've installed everything in the right sequence and orientation, then I suggest trying SRAM's own GXP BB.
Last edited by TobinHatesYou on Thu Mar 09, 2023 10:49 pm, edited 1 time in total.

G1032
Posts: 32
Joined: Mon Jul 27, 2020 7:49 pm

by G1032

That's how they're positioned ya. They were like that in the box and I haven't moved them since. Absolutely correct about not knowing how GXP works. I did say that in the opening post that my BB experience is largely limited to BB30 and Hollowtech 2. I don't pretend to know anything about GXP. Trying to find out as I go along here.
I can't see how anything could be installed incorrectly. I thread each cup into the frame. I put the spindle through. I attach the NDS crank arm. But when I tighten it up the cranks basically don't rotate. Recommended torque is 45nm I think.. I'm getting nowhere near that before this resistance is being met.

G1032
Posts: 32
Joined: Mon Jul 27, 2020 7:49 pm

by G1032

I might actually try and video this process tomorrow and post it. Someone might spot something that way

TobinHatesYou
Posts: 12456
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

Try installing the BB without the center sleeve. Shouldn't make a difference, but beyond swapping BBs entirely, I'm out of ideas.

Karvalo
Posts: 3444
Joined: Fri Aug 10, 2018 6:40 pm

by Karvalo

G1032 wrote:
Thu Mar 09, 2023 10:45 pm
I can't see how anything could be installed incorrectly. I thread each cup into the frame. I put the spindle through. I attach the NDS crank arm. But when I tighten it up the cranks basically don't rotate. Recommended torque is 45nm I think.. I'm getting nowhere near that before this resistance is being met.
Are you sure? This doesn't make any sense. First you installed it with an extra spacer that shouldn't have been there. It bound, ok that makes sense. Now without the extra spacer you're saying you've got more spline sticking out than you should do, but that the arm also binds on the bearing before it's fully tightened? How is it not one or the other?

TobinHatesYou
Posts: 12456
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

He’s saying the splined interface isn’t sticking out enough even though the step in the spindle is butting up against the bearing on the other side. I can see how that would prevent the NDS crankarm from being able to hit its torque spec. Essentially he isn’t achieving an interference fit because the tapered interfaces aren’t meshing fully.

eins4eins
Posts: 699
Joined: Wed Sep 28, 2016 11:49 am

by eins4eins

In the video, the dimensions seem fine. Crank sticks out enough on the NDS.

This is what a Sram BB Dust cover looks like. There are steps so that the dust cover only presses against the inner ring of the bearing and has no contact to the outer casing of the bearing. Maybe check your dust cover or show some pics of your NDS assembly.

Image

Image

Aeo
Posts: 668
Joined: Wed May 25, 2016 2:06 am

by Aeo

Do you have a spacer to step the NDS side of the spindle up to 24mm? Example here: https://acros-components.com/en/bottom- ... xp-adapter (also available on Aliexpress)

You can just run your GXP crank in a traditional 24mm Shimano bottom bracket. Tighten everything down, see how much play there is in the crank axially. Slot in 24mm spacers as needed (the Acros set has some included, Wheels Mfg sells them in 0.5 and 1.0mm width).

The tolerances on the splined taper can vary a little bit and change over time after many (re)-installations. A lot of this is covered on Hambinis blog: https://www.hambini.com/sram-gxp-an-engineering-guide/
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Karvalo
Posts: 3444
Joined: Fri Aug 10, 2018 6:40 pm

by Karvalo

TobinHatesYou wrote:
Fri Mar 10, 2023 2:25 am
He’s saying the splined interface isn’t sticking out enough even though the step in the spindle is butting up against the bearing on the other side.
Ooooh I see it now. Somehow I misread that as 17.5mm sticking out not 12.7 :doh:

G1032
Posts: 32
Joined: Mon Jul 27, 2020 7:49 pm

by G1032

delete.

G1032
Posts: 32
Joined: Mon Jul 27, 2020 7:49 pm

by G1032

Aeo wrote:
Fri Mar 10, 2023 8:35 am
Do you have a spacer to step the NDS side of the spindle up to 24mm? Example here: https://acros-components.com/en/bottom- ... xp-adapter (also available on Aliexpress)

You can just run your GXP crank in a traditional 24mm Shimano bottom bracket. Tighten everything down, see how much play there is in the crank axially. Slot in 24mm spacers as needed (the Acros set has some included, Wheels Mfg sells them in 0.5 and 1.0mm width).

The tolerances on the splined taper can vary a little bit and change over time after many (re)-installations. A lot of this is covered on Hambinis blog: https://www.hambini.com/sram-gxp-an-engineering-guide/
This is what I'm thinking I'm going to have to do. Shimano BB with a 24-22mm adapter and spacers then if needed

by Weenie


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