Tango1

I have a Shimano Tiagra RD-4601 with the Tiagra 12-28 cassette on a used bike.

The rear derailleur was knocking the spokes ever so slightly when I hopped on it and in the largest cog. I figured I'd just readjust the L limit screw and be good to go - not so much. I've gone through the ParkTools adjustment video, made sure everything is shifting nicely on the stand but when I ride the bike I run into issues.

I can shift up the cogs without issue, however when down shifting large to smallest I can't seem to get it to shift into the second smallest cog. Going up is no problem, no excess noise, shifting is relatively crisp.

If I adjust the barrel adjustment to get the downshifting working when going small to large isn't crisp and doesn't shift well.

Again in the stand it works great, riding on the road is where I have issues. That said, at least my original issue of the RD knocking the spokes is fixed.

For reference I'm using a full Tiagra 4600 groupset and shifters.

Barry2 

If the RD was touching spokes, the hanger is almost certainly bent.
you’re going to need the hanger realigned.
Be warned, the hanger may break during alignment. Having a spare is a good idea.

Barry

Tango1

Wish I had a seperate hanger but unfortunately it's all one piece on the dropout.

ARider2

I suspect your shifter cable is sticking. You can usually feel a little drag on it which indicates there is corrosion. If this is the problem then lubing the cable can help but best to replace the inner cable and housing.

wheelreason

Hanger and or cage is bent, and cable friction is a tad high, I'd look at the wear level on the drivetrain and the shifter action as well.

Barry2 

A steel hanger can be aligned.
Unlike aluminum hangers, steel bends well and can be welded if it should break.

Barry