Problems with new drivetrain

[autolycus]

Thanks again for the considered and comprehensive replies. But i'm tending away from chain length (and possible rear mech) as a diagnosis.

The reason is that this morning i have replaced the 50/34 GXP with the old 48/34 ISIS and i can still not get consistently into large/large. This is with the bike back to original spec, except for the cable change that i did to accommodate the stem riser.

On both set-ups i can go large/large only by dropping into the small front first and then changing to large/large using the front derailleur.

Once i have done that, still on the large front i can change up to the second largest rear and then back down to the largest. But only once!!! If i change back up to the second largest rear and then attempt to go back down to the largest again, it won't do it. It sounds crazy but i have repeated this sequence three times and it's been the same each time.

EDIT: Now it won't go on to the rear large on either front chainwheel.

 

[Ajax Bay]

If you can't push the RD across far enough by hand it's the limit screw - unscrew it a bit, but not so far that the cage impacts on the spokes. If you can then it's cable tension - need more of it right down at the smallest sprocket. But, relying on the image you posted, that chain is too short, whatever you say, for the 50-34, and on the limit for the 48-34, I'd say.

 

[autolycus]

I'm not denying the chain is too short for the new chainset, but it's not causing this problem. The bike now has the same (factory) components as before and it all worked before. I've spent about 8 hours on it today. Went back to the old cables. No change. I've tried adjusting the limit screw 100 times, often to a point where it is not doing anything, but the RD will not go far enough across without forcing/bending it. If i adjust the cable tension even to the point where it wants to skip to a lower gear it still won't go onto the large cog. And that's a change even from yesterday when at least it went into the large cog when on the small chainwheel.

Could i have screwed up the cables? Trouble is i can't see anything as they are internal and it seems like i'm clutching at straws. I did change the cassette and back again too (same 36T spec just a little less worn) could i have screwed that? It does seem to wobble a bit on freewheel but feels solid and it's torqued correctly.

I've got a special ride Monday, planned for over a year, so no time to book a LBS and i'm getting really desperate. Help!

 

[Cubist]

Check the routing of the cable into the mech. I'm not well versed on SRAM, but some shimano mechs can suffer if you put the cable on the stop bolt from the wrong direction. It could be something as simple as that. There are no dark arts to gear setup, but there are pitfalls. Be methodical, and if necessary start again from scratch. What you have described above sounds like a tension issue. One massive forehead slapper is making sure the shifter is fired all the way off before you start to try and tension it. Undo the cable from the mech, and pull it gently whilst releasing the shifter tension. it needs to be in position 0.

 

[autolycus]

Thanks, i don't now think it's a cable/tension issue.

I 'm sure in this saga i've set the RD L screw many times apparently without issue. Now, if i remove the chain and push the RD right across - even with the screw completely loosened - the jockey wheel will still not quite reach as far as directly below the large cog. i.e. where it should be set. It is actually stopped by two parts of the DR body coming together. It's as if there should be a spacer or something to push the cassette further out towards the RD.

 

[Milkfloat]

Have you removed the RD in all your fiddling?

 

[autolycus]

Nope, it worked fine before and I haven't touched it.

 

[Ajax Bay]

If the cage won't physically move across to put the guide jockey wheel underneath the large sprocket, then this is not the cable.
I think we've already established you are content the hanger is not bent.
And you've fitted the longer chain.
So the other option is to put a 1mm spacer onto the freehub before the cassette. Since you'll still need to screw in the lockring, this relies on the lockring having a decent number of turns at present to get to being tight/40Nm (ie before inserting the 1mm spacer). And with the spacer in, be careful screwing the lockring in and suggest it should not be tightened much (the lateral forces on the cassette are low especially if you avoid cross-chaining) as there will be less threads engaged; just needs to start 'clicking'. This is a short term fix for your Monday ride, only.

 

[autolycus]

Thanks again for the input. I found this morning that i had reversed the spring on one side of the QR (!) and putting that right has got me back to the point where i can get the large cog with the small chainwheel. I can now also just get the large cog on the large chainwheel maybe 60% of the time with effectively no L screw engagement and very fine adjustment of the shifter nut. One notch in or out (??) and it won't go on.

Clearly from your suggestion you really understand the problem. I had wondered myself whether there was a spacer i had missed, but as you say it's a small shim that would be required for a temporary solution. As i can now get my rock bottom gear and i am happy there is no catastrophic failure imminent (plus no idea where i'd get the right sort of spacer) i'll leave well alone now until after the ride. As it happens, my pal is coming up from east devon (lympstone) for the trip to the south coast and i was rather panicking as i didn't want to let him down.

In the longer term, any idea exactly what the problem/solution is? Worn RD perhaps? It seems now that it was nothing to do with the original modification; the chain and (large) RD are fine for the current set up. Incidentally, i believe the RD has a 45t chain capacity, so it should be fine for a 25 + 16 set up too if i eventually redo the mod.

 

[mrbikerboy73]

I had a rear wheel replaced under warranty and the bike shop forgot to put the spacer on the freehub before installing the cassette. I wondered why it was suddenly shifting so badly and this was the reason. The cassette rattled and wobbled too. If a spacer had been missed out, the cassette would be that much closer to the hub, which could result in being unable to shift onto the largest cog on the rear.

 

[autolycus]

My cassette wobbles too on the over-run but there seem to be varying opinions as to whether a bit (whatever that is) of wobble is acceptable. I don't think this set up is supposed to have a spacer under the cassette - it certainly wasn't there when i removed it - and i think a full 2.5mm might be going a bit too far. Thanks for the suggestion.

 

[Ajax Bay]

But a 1mm spacer might be OK.

 

[autolycus]

Sure, but as you said, it's not really a long-term solution and i'm not sure where i'd get one anyway.