Pulling on the cable of the rear derailleur one gets the feel the the return force - particularly for the smaller rear sprockets - is quite weak (weakest part of spring, according to Hook's law). That's were most of shifting troubles are (I know, that status of cables and housings are most important) So does that spring wear/gets weaker over time? (My question pertains in particular to DA7800 rear derailleur)
Thanks for comments, happy holidays!

Yes, any spring will get very slightly weaker over time, more so if it is under extreme tension most of the time, from metal fatigue, and they do occasionally break. But that's unlikely to be the cause of a shifting problem, so it's not really relevent to diagnosis until you have eliminated other more likely causes causes. If the derailleur is not readily shfiting to the small cogs friction is most often to blame. Try shifting the derailleur by pushing in on it while you pedal and then letting go. If it shifts well then the spring is not terribly weak. Now try wrapping a rag around your fingers and pulling up on the shift cable at the chain stay and then at the down tube and note any change in shifting to the small cog. If it slows down a lot then you've identified where the friction is. Could be rust or dirt inside the cable housing, kinking of the cable/housing, a bad ferrule, etc.

RD springs don't get weaker or tired over time under normal use. The range of motion is limited, and there are enough turns that they stay well within their normal working range. Also, while there is less tension in the slack position it isn't that much less. A bigger element in computing lower loop tension is the cage angle. When the cage is vertical it's pulling on the chain at right angles, but as the cage swings through the arc the geometry changes. It's analogous to the balance of pedal force to driving torque as the cranks rotate.

If you're having problems on the smaller end of your cassette I suggest you look elsewhere for the cause.

BTW- The springs are replaceable, and were available for top end derailleurs many years ago, but to my knowledge none of the majors offer them as spares these days. You might find one in the "dead derailleur" bin at a local shop.

Maybe my RD on my regular beater didnt get "normal" use. Cables are great with no friction but every so often it wont shift in the last two higher gears. I have cleaned all the components too, to make sure its not just dirt causing the trouble. My next step is putting on another derailleur when it annoys me enough to change the old one out.

-SP

The only thing the RD spring affects is the chain tension in the lower loop, so it wouldn't be responsible for your shifting issues. Since the issues are only on the smaller sprockets when the RD is farther away, I'd look for issues causing flex.

If your chain is somewhat worn it would have become more flexible sideways. This means it'll take more overshift to get it to move over when the jockey pulley isn't close. Likewise, a worn jockey wheel exerts less precise control over the chain.

I's start by removing the Jockey wheel, checking it for wear, then cleaning and oiling it before putting it back and see if that makes a difference. Also check your chain for wear. If it's borderline for replacement, you might try a new chain. Save the old one, you can always rotate it back into service when the new one wears to the same extent, then rotate the two as they wear.

Mavic was making a rebuildable Derailleur.. Campag used to have a counter top cabinet,
drawers with parts on the back, and a parts catalog on the front,
[got one of those catalogs at the demise of that system]

Shimano has, typically, been poor on the small spare parts thing...

I recall reading about people adding a second booster spring, across the parallelogram..
So think about how you can do something like that.

For a full diagnosis, remove the chain, and unbolt any cabling. With just a hand, push the RD body in and out. As long as the RD is free to move and returns back in line with the smallest cog and retains some mild resistance to inward pressure when lined up with the smallest cog, then it's less likely an RD body spring. Check limit screws, B-screw, cable housing, lube, and cable housing ends (remove the caps and see if the cable is cut cleanly and flat). Check the top pulley as well for tooth wear or bushing play. Note also where the pinch bolt secures the cable and if this is lined up with the cable pull direction.