On one occasion in the past I paid attention to get the rear derailleur to be "just right". On eight gears on an MTB, I would get gears 1-2-3 and 5-6-7-8 to be perfect (minus the chain rub on one end or another, depending on the FD position), but I couldn't eliminate a nagging problem from gear 4. On occasion, the gear would shift back and forth between gears 4 and 5.
I'm now tuning a new bike, road this time, and would like to understand enough to never experience this again.
From "How to Adjust a Rear Derailleur – Limit Screws & Indexing" I pulled this image:
The image clearly shows that the distances moved by index shifting are not equal.
And so I'm now telling myself, perhaps my unsettled gear #4 was due to my having in the past pulled the cable too far in one direction or another. I was using the barrel adjuster to fine tune the derailleur under a cog, but the cable as a whole was misaligned. A picture will illustrate why this matters.
For some geometric reason, perhaps the misalignment didn't affect gears 1-3, 5-8, but affected gear 4.
Can this happen? I've never seen what's inside an index shifter. Will it happily shift beyond its range, stopped only by the limit screws?
As I no longer have that MTB, I don't really care at this time why gear #4 on that bike was sometimes unsettled. But I do want to understand:
Are rear derailleur cable intervals equal?
If the intervals are equal, then there is no issue. Shifting the cable cannot misalign the settings.
If the intervals are not equal, then something (the limit screws? the shifter? ...?) should stop the amateur mechanic from misaligning the entire cable.