Originally Posted by
thingsthatgo
It doesn't seem to me that I'm pulling more cable through each time, though it could be a tiny amount that I'm not noticing. Wear on the housing ends seems like the best explanation, followed by perhaps a failure in the RD springs (about 8000mi on the RD). I'll check them and post the results.
Thanks all for the suggestions.
Don't take anything apart yet. Do a proper diagnostic first. Use a dry marker to color the wire on either side of the pinch nut. That will confirm if it slips, or if you end up pulling more through. Do this for 2 ride cycles to have definite confirmation.
If you are pulling wire through, then, as I said, either the housing run is shortening, or the wire is pulling through at the head. What I suspect is happening is that your low gear limit is a bit tight so whenever you shift to low the cable has higher than normal tension causing the slippage, which doesn't happen until then. The most common place for this slippage is at the ferrules or fittings where the housing begins to extrude through. Cutting and squaring up the housing as Retro Grouch suggested will help, but may not solve the problem. By now your ferrule has a conical bottom, and enlarged exit hole, so the newly cut housing will satrt the extrusion process anew in fairly short order.
The problem may simply be that whoever set up the bike used brake cable ferrules instead on index cable ferrules. The difference is the strength and shape of th bottom. A standard (brake) ferrule has a conical bottom left by the point of the drill, which is OK for brake cable's spring wound housing. But with index housing the conical bottom encourages the ends to pinch together and work toward the hole. Ferrules for index housing have reinforced flat, or reverse conical bottoms to keep the ends from moving to the hole.
BTW- I strongly doubt that the RD spring is implicated either way. But you might be able to solve the problem or at least slow it down somewhat by easing off the inner limit very slightly (remember this is what keeps the RD and chain out of your spokes, so triple check that you cannot overshift no matter how hard you try). Easing off the inner limit will prevent the excess tension that's pulling things apart, but the process, having started probably won't end until you've replaced tha part where the slippage is happening (housing ends, ferrules or cable head in lever).