SARM GXP Bottom bracket

Tr0nc0

Hello,

I am new to this forum, I am looking for advice about an issue I have been experiencing on my road bike.
I did find a solution, but I would like to know if anyone else had the same problem.
I own a steel British threaded 68mm frame fitting a (British threaded 68mm BB) SRAM GXP BB and a compact Sram APEX crankset.
The BB is now 3 year old and has 20000km on it.
After taking apart and reassembling the crankset as part of usual maintenance I suddenly started experiencing a problem with the chain and the big ring.
Took me a while to realize, but whenever I stood on my pedals the chain was getting caught on one of the big ring's teeth and then snapped back in place.
The feeling was similar to when the rear derailleur is not properly tuned and the chain tries to go up one sprocket and then falls back in place.

I started thinking this was due to the torsion I was applying when standing up on pedals. I managed to solve the issue by adding two 1mm spacers on the non drive side (between frame and cup), which weirdly enough corrected the chainline which is now inline with the specs (I never noticed when I first fit the BB that my chainline was 2mm out compared to specs).

Does all this make sense to you, is it just regular wear out? I was surprised about this cause I rode my bike for 3 years without these spacers and no issue whatsoever. Nothing is bent either...the issue appeared all of a sudden after reassembling the crankset last time.

Thanks a lot!

keezx

Still the same chain and chainrings after 3 years?

Tr0nc0

Hello Keezx,

the chain ring has been changed as I also suspected a warn out ring, but then comparing with the new one I did not notice any serious issue, anodization was still very good.
I also replaced the chain with the proper one for my setup (SRAM 10 speed PC1051).
All the above operations did not fix the issue...
The only thing I neglected in the past 3 years is BB maintenance, but I always been checking for side and radial play and never noticed any issue, not even when I stated experiencing this issue.
The frame BB shell doesn't seem to be showing any hairline cracks or damage that could justify flexing too much when torque is applied.
I checked several times for BB cups and crank's bolt fastening (as well as for chainring bolts fastening), but everything seemed in order.

I don't wanna annoy anyone with this topic, the fact is that is really bugging me cause everything was fit to specs and according to instructions, worked perfectly for 3 years and on last disassembly/reassembly the issue appeared out of nowhere. Took me months to try out everything I knew and finally realize what the root cause was.

keezx

Maybe it's me but I still do not know what you mean exactly.
I first assumed the chain skips on the chainrings which is very rare and only happens with extraordinary wear of chainrings /chain or both.
Apparently that's not the case with yours , so what does your chain exactly?

Tr0nc0

Hi,

I think I know what you mean, I'll try to explain better what is happening here.
Not easy though...:S

when pedaling the chain properly wraps onto the chainring but then sometimes the tooth that is about to engage the next free link doesn't catch the link properly, as if the tooth was not exactly centered with the chain but slightly on one side of it (still inside the link of course). So being, the chain is pulled up and is not sitting in the groove between each tooth and then it snaps back in place.
I tried to draw a little picture...let me know if it is clearer.

dj58

I assume you did not have this problem before you removed the crankset? Reading your posts you have made three changes?

1. Added 2mm spacers between the NDS BB cup and frame to adjust the chainline.
2. Fitted a new outer chainring.
3 Fitted a new chain.

Things to try, remove the spacers and put the crankset back to the previous position and see what happens.
Fit another new chain, new one may have a manufacturing defect, you have check it for a stiff link? Fit the old chainring, new one may have a manufacturing defect.

Tr0nc0

Hi,
I think I have been confusing the readers a bit, cause the two spacers actually managed fix the problem (even if I still have the issue from time to time, not as frequently as before though, which was an absolute nightmare...).
I submitted this post cause I can't still quite catch the rootcause and I was hoping to collect other riders' experience on this particular "chain catch" issue.
Maybe is the gxp BB that has worn out in a way that I can't detect just by visual inspection...that is the point I wanted to get to.

I will least the operations (all incremental) in the order I executed them and put on the side the effect:
1) replaced the chain (issue still present);
2) replaced the chairing (issue still present);
3)added 2x1mm spacers on non drive side (issue now only sporadically seen)

So to me the conclusion is the the spacers are what actually fixed the issue, I just can't understand what exactly the rootcause is and why it appeared all of a sudden.
I blame the torque that is applied to the cranckset system that somehow manages to disallign the chainring so that it doesn't engage the chain properly (this is just an idea...)

keezx

My opinion now after these explanations: damaged chain or bent tooth of chainring.
Never put spacers on GPX BB's, chainline is not that critical to cause faulty chainrun.

Tr0nc0

Hello Everyone!
coming back to this post after some time.
I think I finally realized what the issue was even though still not 100% sure.
I think the problem was really in the worn out, not properly lubricated freewheel body.
The feeling was really the one of chain skip, but really was a problem with the freehub.

thanks for all the tips!

F.