donheff

While I was in Florida my chain started skipping around in the two large rear rings. I put the bike up on the stand yesterday and discovered that the rear derailleur was way out of whack. It never got to the last low gear ring and slipped off and on the one next to that. I followed some youtube instructions first tweaking the high gear (outboard) delimiter screw to get things right out there. Then I switched to the low gear (inboard) delimiter screw and was unable to get a good position even with the screw backed way out. Then I remembered there is a cable barrel adjuster so I tightened the screw back to what I guessed was a reasonable position and was then able to get a decent adjustment with the barrel.

So, how do you figure out when to mess with the barrel and when to mess with the screws? Is there any "optimal" position for the screws or barrel to start in? Half way in/out on all three?

pastorbobnlnh 

Also, check your chain to see if it is stretched past its intended life. I have found on my 9-speed indexed bike, that when I experience unexpected jumping from the large to the second largest cogs on the cassette, the chain has stretched too much. A new chain solves the problem.

In my experience, the limit screws are designed to just keep the chain inside the cogs--- not too high and not too low--- just right. In other words, we don't want our chains skipping off onto the spokes or onto the chainstays.

The barrel adjusters, on the other hand, help to fine tune the indexed shifting. When shifting up or down, some gears might rattle a little. Sometimes small adjustments on the barrels can quiet the system out. The primary thing the barrels help to do, is to take any slack out of the cable at the high end of the shift. Lack of precise cable tension means the RD does not move the exact amount it needs in order to shift perfectly.

I hope this makes sense and is helpful. Best of luck.

Bud Bent

Also make sure the cassette isn't loose.

NOS88

I recommend a visit to the Park Tool site. https://www.parktool.com/repair/readhowto.asp?id=64

Allegheny Jet

One thing that may have happened is that during your travels the derailer hanger may have been bent. If your shifting was working correctly and then all of a sudden the issues began, it may be more than a microadjustment with the barrel or turning limit screws. Your LBS can tell you if the hanger is lined up correctly. I know about this due to my own failed attempts to adjust everything without any closure.

donheff

I have a new chain and cassette and things were fine before I drove to Florida so this may be the problem. I have it shifting smoothly now but if more problems crop up I will take it in to the pros to check alignment.

Edit: I just remembered that my wife was holding the bike while I was putting on a helmet or something and accidentally let the bike fall over - that may have been the genesis of my derailleur problem. If it wasn't such a nice bike I could use her mistake as my excuse to buy N+1

BluesDawg

You seem to be confusing the limit screws which adjust how far derailleurs are able to move with cable adjusters which control cable tension and (indirectly) derailleur location . These are very different functions.

donheff

I understand the concept and can picture it in my head. But the screw would not get the derailleur close to the final gear - it would have dropped to the pavement if I backed it off further. The barrel adjustment brought it right over. When turning the barrel I could watch the derailleur move - it was much more rapid than when I turned the screw. I'm guessing that the thing got whacked out of align and tension on the cable pulled the it over to the proper position. What I can't figure out is when to use which. Is it preferable to make minor adjustments to the cable tension first and then move to the screws or vice versus?

bikinfool

Once you set the limit screws during initial install of your derailleur you really shouldn't need to touch them. Use your barrel adjuster or reset the cable at the anchor bolt for cable tension. Does sound like your hanger got bent, though (and when something changes all of a sudden there's usually a reason like you've remembered now).

roadnsnh

The order of business is, assuming that the derailer hanger is not bent, or has been realigned, is limit screws, cable adjust, and b screew. See https://www.sheldonbrown.com/derailer-adjustment.html . This site and the Park Tool site will give you all the information you need.

Leland Yee

Retro Grouch 

We have a BINGO!

If it was ever right fooling with the limit screws isn't the answer.

bikinfool

Disagree in the case of an already installed/setup derailleur that has developed shifting trouble. During installation sure, limit screws and b tension screw, but once setup you shouldn't need to touch those. Changes in shifting performance afterwards are usually cable tension or bent hangers.

donheff

It sounds like we jammed it out of whack - mistake 1. I then fiddled with the delimeter screws - mistake 2. I then turned the low side screw back to about where I thought I found it and got it shifting smoothly with the barrel adjustment -- probably the only thing that was not a mistake.

If it continues to shift feels smoothly should I let well enough alone or am I better advised to visit the pros for a look see? I am hoping that a smoothly shifting rear end means I am not doing any further harm and can wait for problems to surface before pulling out the wallet.

bikinfool

If you can get the shifting smooth I wouldn't worry about it.

Retro Grouch 

How many rear cogs does your bike have?

Shift into a gear that makes your derailleur arm point straight down and look at it from the rear. If it looks like it's leaning in toward the rear wheel, you have a bent derailleur hanger. That's not uncommon and it causes exactly the kind of shifting problems that you originally described.

If you only have 7 rear cogs you can probably eyeball it back into line adequately. (DO NOT remove the rear derailleur before trying to bend it back or you may ovalize the derailleur mounting bolt hole.) Bikes with 9 or 10 rear cogs are much more picky about hanger alignment and need a gauge to get the alignment accurate.

wrk101

A guy on ebay sells a derailleur hanger tool for $12.50 plus shipping. I bought one from him, it works pretty good.

Just do a search on ebay for derailleur tool, you will find his, plus a number of more expensive options.

I also bought his headseat tool. I get enough use out of such tools as I have rehabbed about 40 bikes in the last year.

For a one time fix, I would just take it to your favorite LBS, IF it appears to be bent.

Ranger63

Didn't read everyones response so this may have already been covered.
Any chance you laid the bike down (regardless of how gentlly )on the derailleur side?
I did that last year with the shimano ultegra setup on the CF motobecane and darned if the hanger didn't bend 'just enough' to throw the shifting way off.