Rear Shimano Acera derailleur skips chain ring after replacing cable

Question

I've had my Schwinn High Timber F5 mountain bike for probably 15 years and decided to replace all the cables. I've also already borrowed the rear wheel from my guest bike while I figure out how to properly true up the old one (another question will be coming on that).

After replacing the cable and wheel, I cannot get the shifting to work properly. It always skips a sprocket on the way up and down the cassette, never skipping the same sprocket in either direction. I've successfully indexed my gears before, but I cannot get them indexed with this new cable.

Things I've done to the bike so far:

• Straightened out a very bent derailleur hanger with the Park Tool DAG-2.2 Derailleur Hanger Alignment Gauge. I wonder if my derailleur had somehow adapted to the bent hanger or was adjusted accordingly at the bike shop when they installed it. Now that I've fixed the hanger, could it be causing indexing issues?
• As mentioned, replaced rear bike wheel. I've had this wheel on my bike prior to replacing the cable and it shifted no differently. The cogs are identical in size and spacing so far as I can tell
• Ensured that the limiter screws are properly adjusted

Thinking of just buying new derailleurs, but am hoping I am just doing something wrong.

Answer 1

An impact hard enough to bend the hanger is likely to have also distorted the derailleur.

For each index of the shifter pulling a consistent length of cable, the derailleur normally moves a certain distance along a path angled to be similar to the cone angle of the cassette, that distance corresponding to an axial (sideways) distance which matches the cassette spacing.

If the derailleur is deformed between the mounting bolt and the parallel plates, the angled path will be flatter, meaning the same movement along the hypotenuse will be a larger axial movement. There is a tolerance designed into the widths of the sprockets, chain and cassette gaps, so when you shift far enough from the point where you indexed the gears, the total excess movement from the flattened path will exceed the tolerance.

If the derailleur's inner plate is bent such that it's shorter than the outer plate, the distance moved along the angled path will be inconsistent, but the impact on shifting will be more extreme in the easier gears regardless of which gear you used when indexing.