Sram Force GXP BB Play

Discuss light weight issues concerning road bikes & parts.
foreigngreg
Posts: 11
Joined: Wed Jun 27, 2018 7:42 pm
Location: Bern Switzerland

by foreigngreg

Hi!

Looked around a bit, but I could not find a satisfactory answer, so here goes...

I just bought a sram GXP BB and force 22 cranks. Having installed them, there was some still some play. Sram instructions indicate to dissasemble, regrease and reassebly in such a case, the play should then go away. So this I did, 8 times, torking the 8mm bolt to 54Nm each time (the max specified), to find each time that there was still play, until the 9th time, when the bolt snapped.

Now I am left to buy a new crank bolt (or get one on warranty)...

But the question of play remains, I think at this point, dimantleing and regreasing should be futile... Therefore what to do? I measured the BB shell, it is at 68.05mm, therefore in spec, I also measured the cups of the BB (12.01 and 12.48mm), and the total width at the cups once the BB torqued on the bike (92.48mm), which means everything is in spec (width at -0.02mm, sram gives a tolerance of +/- 0.5mm), so what could be the problem?

* side note: My torque wrench was calibrated in 2017. I am a Mechanical engineer, and so quite experienced in such assemblies.

I am really starting to think that this sram GXP system is shite. Look forward to any feedback!

by Weenie


Marin
Posts: 3138
Joined: Wed Jan 22, 2014 11:48 am
Location: Vienna Austria

by Marin

Did you have the rubber seal on the non drive side? It adds about 1mm.

Since the crank only clamps onto the left side bearing, having the seal washer is critical. The shaft floats inside the right bearing with GXP.

sungod
Posts: 1685
Joined: Fri Dec 24, 2010 9:37 pm
Location: it's raining, it must be uk

by sungod

gxp clamps the nds bearing inner between the crank and spindle

for there to be side to side play...

it's assembled incorrectly - as above, is the correct nds shield in place
the nds bearing is faulty/damaged - it could happen
it wasn't torqued enough - unlikely from what you say
the nds inner is too narrow - unlikely, but nothing is impossible
the nds cup is loose
the bb shell is sleeved and the sleeve has broken free - it happens
the crank/spindle is clogged/damaged/faulty - seems likeliest - i've undone/retorqued gxp cranks many times with no issue, if you snapped the bolt then possibilities include: it was faulty; torque wrench is way off; you messed up

stuff fails, people make mistakes, neither means gxp is shite

foreigngreg
Posts: 11
Joined: Wed Jun 27, 2018 7:42 pm
Location: Bern Switzerland

by foreigngreg

Thanks for the quick replies,

I have the non-drive-side shield, it was in place correctly. I do not on the other hand have a rubber seal that Marin mentionned, maybe thats what I am missing? I did not see any in the pack. What does it look like? Maybe I can use some of the O-rings that I have at home.

Hexsense
Posts: 595
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

GXP in pressfit BB needs the Wavy washer on the drive side. But your BB is just 68.05mm which suggest a threaded BB so likely doesn't apply to your case.
As other said, GXP havily rely on clamping NDS bearing so most focus should go there.

2lo8
Posts: 489
Joined: Thu Mar 31, 2016 10:32 am

by 2lo8

I think he means the NDS shield that has little rounded teeth on the internal diameter to interface with the GXP splines. It's got some rubbery bits to act as a bearing splash shield, prevent water ingress along the spines, and acts as a spacer.
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TobinHatesYou
Posts: 2066
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

As others have mentioned, the width of your BB actually doesn’t matter with GXP. The only way there can be play in a GXP setup is if the NDS bearing is moving around...either it has broken free from the being directly pressed into a BB or the sleeve it resides in has broken free.

The only other possibilities are you somehow have the wrong NDS bearing installed. Was this GXP BB purchased new?
Last edited by TobinHatesYou on Thu Jun 28, 2018 1:03 am, edited 1 time in total.

grover
Posts: 1145
Joined: Mon Jul 26, 2004 1:06 pm

by grover

I second everything already stated.

One last thing. Is the bottom bracket a SRAM part or manufactured by someone else? Many non-SRAM GXP bottom brackets do not have a 22mm internal diameter of the NDS bearing. Rather they have a 24mm internal diameter with a sleeve to bring it down to 22mm. I've seen this either have play or be missing completely be the source of your problems.

2lo8
Posts: 489
Joined: Thu Mar 31, 2016 10:32 am

by 2lo8

Doesn't SRAM now use 22.2mm ID bearings because they were cheaper?

A SRAM BSA GXP BB should have a circlip to prevent the bearing from drifting.
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kkibbler
Posts: 852
Joined: Tue Sep 09, 2014 9:30 am

by kkibbler

Also holy cow I’ve never actually gone up to 54nm on a crank bolt or anywhere else on a bike. I have the torque wrench to do it, it’s just wholly unnecessary.

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protocol_droid
Posts: 239
Joined: Sat Aug 20, 2005 7:40 am
Location: San Diego, CA

by protocol_droid

the other thing could also be that the washer kit that you have for the gxp bb is for specialized as those are thicker and wont allow you to fully crank down on the crank and it will still have play. the correct washer kit (wavy spacer and 2 bearing covers) is for mtb on the label. i had this very same issue and once i had the correct kit it went together fine. might be something to look into.

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foreigngreg
Posts: 11
Joined: Wed Jun 27, 2018 7:42 pm
Location: Bern Switzerland

by foreigngreg

BB and Crank are new, BB is "Sram GXP team" BSA thread, on a road bike.

I got one star-cut washer with the cranks that mounts on the non-drive-side. Nothing else. Looks like I am missing the wavvy spacer you talk about. Makes sense, as I can't see how the system would otherwise not have play without crushing the bearings...

Im gonna see if I can get hold of one today.

TobinHatesYou
Posts: 2066
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

Threaded BBs shouldn’t need the wavy washer since the cups and bearings themselves don't need preload. The only way the cups back out is if the threads loosen and axial precession prevents that.
Last edited by TobinHatesYou on Thu Jun 28, 2018 8:06 am, edited 1 time in total.

foreigngreg
Posts: 11
Joined: Wed Jun 27, 2018 7:42 pm
Location: Bern Switzerland

by foreigngreg

Right, thats what I understand from Sram documentation, but the fact remains: Without a spring washer, my setup has play, despite the BB being in tolerance...

I think that it has to be bloody precise without some sort of 'elastic preload' to not have any play. In the industry we never mount bearings witout preload if we do not want play, so I don't really understad what Sram intended to do here. To me a spring washer is needed...

by Weenie


2lo8
Posts: 489
Joined: Thu Mar 31, 2016 10:32 am

by 2lo8

A GXP BB clamps the inner race of the NDS bearing. There's no elastic preload, just like if you had a preload spacer between the inner races of two bearings, except there's only one bearing in this case.
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