Why does my rear derailleur's indexing work for either the high or the low gears, but not both?

Question

I am adjusting my rear derailleur indexing. It seems impossible to find the correct tightness. When I tighten the tension with the tension barrel adjuster, I have one of two problems: either it is too tight and the derailleur jumps from the third largest to largest cog (skipping the second largest cog) or it is too loose and does not want to jump from the smallest to second smallest cog.

I used the smallest turns imaginable to adjust the barrel adjuster, but still no luck. I kept the front derailleur in the middle cog (of three). Also, adjusting the B screw does not seem to have any effect.

There are nine gears.

Answer 1

When adjusting the tightness of the derailleur cable, it should affect the shifting across the whole range of gears evenly. The way you phrased the question, it sounds like you are experiencing problems mainly in the high end when it's too loose and mainly in the low end when it's too tight. That would indicate that your derailleur hanger is out of alignment. You can check that by looking at your bike from behind and seeing if the two small pulley wheels on your derailleur are directly in line with the gears, and not bent at a slightly different angle. It's an easy fix but it requires a specific tool.

On the other hand, and as others have suggested, it is very common for grime to build up around the bend in the rear shift cable. It may be hesitant to shift into the highest gear because the remaining tension in the derailleur's spring (which will be at its lowest when the derailleur is most relaxed, or in the highest gear) is not enough to overcome the residual friction between the cable and housing. Sometimes you can solve it just by cleaning the cable. To check/clean it without any tools:

1. shift into the highest (most outward) gear in the rear
2. Hold the rear wheel off the ground (somehow)
3. Pedaling forward with your right hand, use your left hand to push the derailleur in (shift into the lower/inward gears). This will create slack in the shift cable.
4. Take out the housing coming from the rear derailleur from the frame stub holding it in. You can do this because the shift cable should not be under tension.
5. Slide the section of housing out of the way and wipe the cable down thoroughly with a cloth. If you have any, apply a drop of chain oil to the cable (oil your chain while you're at it).
6. Reposition the housing admire your newly reduced cable friction

Answer 2

Obvious thought is a mismatched set of shifter/deralieur/cassette mixing pull ratios and number of gears incorrectly.

If the components are correctly matched, especially on a bike that used to work fine with no changes to components, in my experience the most common cause of this kind of problem is the gear cable itself having too much friction - either from poor routing, a crimp or just being old. As gear cables are relatively cheap, the best solution is to replace the shifter cable, but if you don't have one around, a clean and lube of the existing cable can make a big difference.

Answer 3

at @mattnz says, shift cable and housing is the most likely problem. You can replace both the cable and housing relatively inexpensively.

You can at least check the shift cable run. Make sure the housing is sitting in the shifter, derailleur and frame bosses properly, there are no kinks or any damage. Make sure the cable is not frayed.

You should also check the derailleur hanger is not bent. Wedge the front wheel in something to hold is straight (or have someone hold the bars), lift the back of the bike up and look down the line of the chain. The derailleur cage should be parallel with the chainrings. If it isn't your local bike repair shop can straighten the hanger.