jralbert

So winter approaches, and with it the inevitable switch from my delightfully-tuned road bike to the disc-equipped rain bike for commuting. Rain bike runs an 8-speed cassette and triple crankset, and when I pulled it out for the first ride this fall I noticed that the shift indexing was miserable; so I studied the relevant Park Tool and St. Sheldon articles, and went to work on the barrel adjusters until that indexing was as crisp as a September morning. But after a few weeks, I notice the indexing has gone right back to its lousy old ways: very hesitant shifts, auto-shifting back and forth under load, chain noise on certain cogs. I can adjust it again, but I'd like to know it'll stay adjusted! Do I have a problem with the barrel adjuster, or with the cables, or is this just normal cable-stretching?

s0ul_chicken

It almost sounds like a bent hanger. Check and relube the cables as well to see if that helps as well.

bikeman715

It could be just cable stretch , if you shift a lot . it can be a bent hanger , or your chain is worn out . it can be a lot of things .

dsbrantjr

Check your shift wire to be sure that it is not fraying and about to part, especially if you have brifters; catching it before it breaks will save you a world of grief. Getting a broken-off shift wire head out of brifters is exquisite torture.

jralbert

Okay, details! This is a flat-bar hybrid MTB with trigger shifters - the whole group is Shimano Alivio - so thankfully no brake-shifters to worry about. I can't rule out that it's not a bent hanger, though I'm not aware of it having taken any abuse that would suggest that; but I'll start by checking there. The cables themselves appear on cursory inspection to be in decent shape - no visible fraying or bent segments - but this bike has had more mileage put on it this year than in its entire life to this point, so I would imagine that stretching is not out of the realm of possibility. If it really is just stretching, can I assume that the cables will settle down at some point and then be reliable until someday they eventually just break?

Retro Grouch 

I've worked on a couple of bikes that would tune up just fine but the indexing would drift off after a week or two.

Check everyplace that you have a cable housing end. If you find one or two little wires sticking out, that's it. Pull the cable, square off the housing and replace the housing end cap.

HillRider

One more place to check is at the cable anchor bolt. If the cable isn't clamped firmly or the cable is greasy or oily, it can slip and change the shifting adjustment as you ride. I once installed a Teflon coated inner shift wire and could NOT hold my indexing adjustment until I scrapped off all of the coating and acetone washed the couple of inches of now bare wire around the anchor bolt.

mrrabbit 

Sometimes it's the little unknown devil in the details that getcha...

=8-)

Kotts

It sounds to me like your cables could use to be lubed. Particularly if the cables pass under the bottom bracket,the rear derailleur cable can be getting pulled a tiny bit as the bottom bracket reacts to pedalling force.

cny-bikeman

I agree to focus on the cables. One can't just look at one part of a mechanical system when trying to solve a problem. What the cable adjuster does is to control the position of the derailleur in one position. It's up to the cable/housing and shift lever to provide consistent movement between cogs. Yes, a cable hanger can be bent, but that is much less likely to be a problem, and most of the time it's just bent in, which does not necessarily cause inconsistent shifting and behavior - it usually takes twisting of the hanger or derailleur to cause those problems. The fact that it happens over time definitely points to the cable/housing - settling in (stretch), poor housing ends, lack of lube or contamination by dirt, kinks, fraying, etc.

Bill Kapaun

I'd look at the DER cable.
Specifically that section contained in the rear "loop".
You have that inch or so of cable that is exposed to dirt/mud etc. and then pulled into the housing when shifting to the large cogs.

rydabent

The inside of the cable housing may be wearing out. If all else that has been suggested fails replace it.

jralbert

Thanks everybody for all the fantastic advice! I'll get the bike up on the stand this weekend and look into the things you've all suggested; if I still can't get it fixed, I'm sure I'll be back here for more help. Almost unnecessary at this point, since every thread illustrates it, but I'll just put out there again how great a resource this forum is: I've learned more here in a few months of reading and posting than I ever knew in all the years I've ridden. Thanks!

jralbert

Just as a follow-up, I re-adjusted the RD today via the arduous task of turning the barrel adjuster 1/4 turn; indexing has returned to near-ideal. Maybe it'll stay this time? I should add that since this is the rain commuter bike, and we've just enjoyed a full-scale two-day PNW wind-and-driving-rain storm, there was an absolutely remarkable amount of detritus accumulated on the cable housing at the bottom bracket which I removed before re-adjusting the RD. Seems like that housing is an awfully critical element of the bike's control system to stuff right down on the most environmentally-exposed face of the frame - I wonder if there's a guard or fender of some kind that could be used to protect it?

Bill Kapaun

Piece of surgical tubing?

IF you bike sat for several months, the lube probably stiffened up a bit.
This causes the upshift to be slow as the spring tension has to overcome the "stickiness".
You turn the barrel adjust out a bit to compensate. Downshifting is now a little more problematic.

Just clean/lube the cables! It's normal maintenance.
Shift to the largest sprockets front & rear.
Move the shifters to the smallest sprockets position.
This relaxes cable tension so you can move the cable housings out of the stops and slide them back & forth to your hearts content to wipe off & relube the inner cable.
Replace housings and shift back to the largest sprockets.
10 minutes if you're real slow.