Bicycle Mechanics - Shifting problem w/ rear derailleur

I'm having a problem shifting through the cassette while I'm on the big ring. When I'm on the road, shifting is sloppy and will often jump between cogs when I stand up while climbing. It's usually worst in the middle of the cassette but it more often than not won't shift to the biggest cog at all. When it's in the stand, shifting is just fine, works how it's supposed to, and I can't detect any problems. When I'm in the small ring, shifting is just fine. It's only the big ring where I am having trouble.

I'll increase my cable tension while on the road and that'll help shifting in the bigger cogs, but the lower cogs will then go out of whack. It'll also have an adverse effect on shifting while in the small ring.

Any ideas where to look next? I can't figure out what I'm doing wrong.

Sounds like you could have a bent derailleur hanger, or possibly the shift cables and housing need replacing.

Al

By odd coincidence, I have an extra hanger. Wouldn't be too much to swap it out and see if anything changes.



Also, I was wondering over lunch if this could be an issue with the 'b' screw? If I have that set with too much tension, could that product this effect when I'm in the big ring?

By odd coincidence, I have an extra hanger. Wouldn't be too much to swap it out and see if anything changes.



Also, I was wondering over lunch if this could be an issue with the 'b' screw? If I have that set with too much tension, could that product this effect when I'm in the big ring?

The "b" screw is there only to keep the upper pully from hitting the largest cassette cog. I don't think that is the problem.

By odd coincidence, I have an extra hanger. Wouldn't be too much to swap it out and see if anything changes.



Also, I was wondering over lunch if this could be an issue with the 'b' screw? If I have that set with too much tension, could that product this effect when I'm in the big ring?

Replacing the hanger is unlikely to help, unless the current one is severely bent. What is needed is to align the hanger, which means making sure the mounting bolt is perpendicular to the wheel plane, both up-down and fore-aft. This is done with a deraileur alignment guage (Park Tools DAG-1, for example). Your symptoms (works on bench but not on road, adjustment seems to only work for a small range of cogs) are typical of a mis-aligned derailleur mount. Sticky shift cables can also give similar results.