I'm hoping someone can help me with this rear derailleur indexing problem that's driving me crazy.

Some background: I'm using SRAM Rival rear derailleur, Rival shifters, SRAM chain, SRAM 11-28 cassette, and SRAM Red 38/53 chainrings and crank.

The problem: The rear derailleur keeps going out of index in an odd way. It will be indexed perfectly for while, and then after maybe 30-40 miles it will suddenly no longer stay on the largest (28t) cog (not cross-chaining - happens while in small chainring). It will shift into that cog, but it will immediately drop back into the next largest cog. This then throws the entire rear derailleur out of index because the shifter is in its lowest click but the chain is on the second-lowest gear. Rather than self-correcting on the next shift, the whole system then stays one cog off as I shift up and down across the cassette (though it's not perfectly one cog off - it's probably about 3/4 of a cog off, causing the chain to slip and rattle all over the place).

The indexing will have been fine until this happens - it's shifting onto the cog and having it slip off that pushes the whole thing out of index. It's then so far out of index that it cannot be fixed using the barrel adjuster. I have to re-adjust the tension in the cable and re-index from scratch.

The first time this happened, I figured that the cable must be slipping in the rear derailleur pinch bolt when under the greatest tension (when shifting into the largest cog), leaving the indexing out of whack. But I've gone through this whole cycle several times now, and I've measured the length of excess cable coming out of the pinch bolt and it's not changing. The pinch bolt is firm.

Over the past week, I've had the derailleur indexed by two different professional mechanics, as well as doing it myself. It slips out of index in the same way each time, after maybe 30-40 miles. I just re-indexed it again, and once again it's fine, but I expect the whole cycle to repeat itself.

I've had this cassette, chainring, and chain for about six months with no problem until this started a week ago. Before this, my prior components were the same size and make, also with no problem. I've had the derailleur for two years. The limit screws and b-screw are in the same adjustment they've been in for two years without problem.

Any thoughts are much appreciated by both me and my wife, who has had to deal with my obsessing for the past week.

How many miles on that RD? My money is on a wear problem with it. By your description, everything else seems to cancel out. Also, have you looked at the shift cable for the RD? I've had a similar issue before, and the cable housing would seat in its boss after I set up the derailleur, causing the same problem.

Question -- afterward, are you solving the problem by pulling down more cable in the pinch bolt?

If yes, than odds are that either the cable head is slowly coming off, or pulling through the lever cam, or the housing is extruding through a ferrule, or at the lever. The other related possibility is that you didn't properly seat the housing in the lever before taping your bars, and the housing is slowly creeping forward toward the lever, effectively getting shorter.

This is a classic "raise the bridge or lower the water" scenario. Either the wire is getting longer, or the housing shorter, and a bit of tracking down will verify which and where.

Clarification question: it's not changing and then you pull more cable through, or it's not changed after you've pulled more cable through?

Check your cable housing ends too. If you see a tiny wire or two sticking out - that's it. Pull off the cable housing end, cut the housing so it's square, put everything beck together and you'll be good-to-go.

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).