Tech Gremlin - Please help

[David Greyling]

Hi guys

 

I need some help please. My girlfriend's bike has an issue that I have no reasonable explanation for. It is a Specialized Jett 29er hardtail size small. Groupset is currently XT rear derailleur, SLX front derailleur, SLX shifters, Sram X7 crankset with a sram GXP BB. I have replaced the chain, cassette, front chainring, front derailleur, bottom bracket and both shifters in the last two months so this is fresh equipment. Note, the issue began before all these changes, while the new kit made a difference the route issue is still there.

 

Essentially, when shifting to the big chainring (irrespective of position on the cassette) the chain rides up on top of the teeth of the chainring momentarily (anywhere between 45deg and 180deg of the rotation) before settling into the chainring correctly. this results in at best in a click and slight jump in the chain and therefore feedback through the pedals. at worst, it derails the chain to the outside. I've had a bike shop I trust look at it. They have done their best but essentially what we are doing is using the front derailleur as a chain retention device. Once the chain is riding on top of the teeth it's a 50/50 chance of it settling into position or derailling so we've set the front derailleur to try and stop the derailling like a chainguide - not the ideal situation but workable for now.

 

I even tried running a 1x10 with a Kalbo NW chainring and the chain derailled (to the outside). I checked chain length and although it is not a clutch derailleur we were not riding bumpy or tech track at all.

 

to note, the only thing that has remained constant in the entire setup is the rear derailleur and the crankset (and obviously the frame)

 

I have my own theory, but I'd appreciate input from anyone who has experienced this, or anyone who has a good idea of what/ how to diagnose the actual problem.

 

many thanks in advance

David

[Robodog]

If you don't come right bring it here and will get Jason to sort it out.

 

PS. you know I'm not a mechanic so not going to try guess the problem. All I know is that other mechanics call Jason when they are stuck on something.

[BDF]

Sometimes the angle of the front derailleur and how high it is from the teeth on the big ring influence this. Just a possibility as I have not seen the kit. When you rplaceed the front derailleur, did you fit it exactly as the old one was? If so, it would repeat the problem if the angle was wrong. I am assuming you have the correct chain for the groupset?

[Spoke101]

DId you replace both chainrings?

[GrahamS2]

Are the chainrings fitted in the correct orientation, and is the fd at the correct height?

[Patchelicious]

Hi guys

 

I need some help please. My girlfriend's bike has an issue that I have no reasonable explanation for. It is a Specialized Jett 29er hardtail size small. Groupset is currently XT rear derailleur, SLX front derailleur, SLX shifters, Sram X7 crankset with a sram GXP BB. I have replaced the chain, cassette, front chainring, front derailleur, bottom bracket and both shifters in the last two months so this is fresh equipment. Note, the issue began before all these changes, while the new kit made a difference the route issue is still there.

 

Essentially, when shifting to the big chainring (irrespective of position on the cassette) the chain rides up on top of the teeth of the chainring momentarily (anywhere between 45deg and 180deg of the rotation) before settling into the chainring correctly. this results in at best in a click and slight jump in the chain and therefore feedback through the pedals. at worst, it derails the chain to the outside. I've had a bike shop I trust look at it. They have done their best but essentially what we are doing is using the front derailleur as a chain retention device. Once the chain is riding on top of the teeth it's a 50/50 chance of it settling into position or derailling so we've set the front derailleur to try and stop the derailling like a chainguide - not the ideal situation but workable for now.

 

I even tried running a 1x10 with a Kalbo NW chainring and the chain derailled (to the outside). I checked chain length and although it is not a clutch derailleur we were not riding bumpy or tech track at all.

 

to note, the only thing that has remained constant in the entire setup is the rear derailleur and the crankset (and obviously the frame)

 

I have my own theory, but I'd appreciate input from anyone who has experienced this, or anyone who has a good idea of what/ how to diagnose the actual problem.

 

many thanks in advance

David

[dirtypot]

It's got nothing to do with the back.  It's all got to do with the FD.  It must be set wrong.  It's either too high (there should only be 1mm clearance between the top of the tooth and the FD) or the angle isn't straight.  Or the limit screw isn't set correctly.

 

Running a NW doesn't prove anything either.  Without a clutch you will drop the chain.  I've tried it, it doesn't work.  Get a clutch if you're wanting 1x perfection. 

Edited April 5, 2016 by dirtypot

[David Greyling]

Hi All

 

Thanks a lot for the input. To answer the questions to date:

 

Robodog - Thanks, I will likely to that.

 

BDF - FD is 1mm above the closest chainring tooth and outer plate is parallel with the big chainring. Actually changed the FD too, went from high mount dual pull to a low mount down pull, prior one was the sram X5, new one is the SLX. Currently running all 10speed, 10speed SLX chain on 10 Speed SLX cassette with 10 Speed Sram crankset.

 

Spoke101 - No, only  the big ring, but the small ring looks like it was born yesterday so i'm not worried about that

 

GrahamS2 - Chainrings are correct (small bump oriented under the right crank arm and the Fd is the correct height i believe.

 

Patchelicious - There is always one...

 

Dirtypot - agreed, but as far as i can tell all of those requirements are met. Also agree with the 1x10 clutch comment, it was an experiment to try and patch a solution in the short term. long run would be clutch for sure.

 

My theory for what it is worth is that the BB threads are slightly out of line with the frame, causing too much angle of the chainring. the chain rides on top of the teeth with the teeth sitting in the miniscule gap between the roller and the plates of the chain. I cannot replicate it on my other bikes even if I try...

 

Thanks again for all the help, greatly appreciated.

[Frosty]

Try posting a photo or two of the chain rings, FD and the chain in the position you see the problem - might help (or not).

[BDF]

David,

I doubt that the thread alignment is the issue. It would have to be radically off to have the effect you are describing.

 

However, I am beginning to wonder about the compatibility of the Shimano SLX shifters and FD with the SRAM X7 chain set. It seems that the shifter is slightly over-shifting for the gap between the chain rings, or perhaps all you need a spacer/shim between the BB and the crank arm on the drive side which will move the whole drive side crank a fraction.

Edited April 5, 2016 by BDF

[David Greyling]

I'll take some pics later to share. BDF, that makes sense. to be honest I assumed (yes, stupid I know) that the chainring spacing and therefore shifter and FD spacing would match but it's entirely possible that it doesn't.

 

Forgive the description, but when I ran the shift slowly it on a stand it almost seemed that the angle induced on the chain would make it impossible for it to seat on the chainring. I'll take pics and share later...

[madbradd]

Wouldn't the spacing on a 2X crankset just be set by the limit screws?

 

Has the BB shell been faced properly? Or is it possible the crank spider is bent/skew? Similar to your "skew thread" theory.

 

Does the chain only ride up when under power or even on the bike stand? 

[Pulse]

Doubt this, but perhaps excessive cable tension?

 

Otherwise sounds like a misalignment of the chainring in some way. I know you answered this, but is the new chainring not fitted the wrong way round (inside out), so its 'dished' slightly differently?

[David Greyling]

Finally got the pics (sorry about the orientation, uploading them sent it haywire)

 

The pics are chronological, you'll see the chain get picked up, sent onto the top of the teeth and then stay there as the crank rotates. on the stand it usually drops back into the chainring, but you can imagine how this ends on the trail, usually with a derailled chain and an unhappy face.

 

to note regarding limits, the outer limit is set as close as possible, even an eighth of a turn more and the chain will not be picked up to the big ring. the limit set like this is the only thing stopping the chain from derailling every time.

 

[David Greyling]

The bike originally came specced with a Sram X7 front derailleur and shimano generic 2x10 shifters. I have now changed to shimano slx 2x10 shifters and a shimano slx front derailleur if that helps at all...

[MJ the Expat]

Not sure if anybody asked this but if you have a problem getting it onto the big chainring and you are sure the FD is set correctly then maybe the spacers on the bb is incorrect meaning you might have to much space between the chainring and the bb itself? This would mean the FD will have to be forced to get the chain onto the chainring.