> I don't know who sold you a new cassette when you got your chain changed, but they are nothing but a salesman.
My most recent experience with new-chain/old-cluster was that the chain skipped a tooth three times per pedal revolution! Replaced gears, problem solved.
To go buy the new gears, I first had to temporarily put back the old chain, to make the bike ridable. :)
Adjusting the derailleur can help; in particular the "B tension" adjustment to get the jockey wheel to track the gear cluster more closely for more wrap. However, if it's that bad, it won't help.
I have decent experience adjusting derailleurs. Even taking apart the non-adjustable ones and drilling holes in new places to change spring tensions.
> 8-20 of them
You're not going to get 8 teeth of contact on an 11 tooth cog, nor 20 teeth of contact on a 26 tooth ring.
My most recent experience with new-chain/old-cluster was that the chain skipped a tooth three times per pedal revolution! Replaced gears, problem solved.
To go buy the new gears, I first had to temporarily put back the old chain, to make the bike ridable. :)
Adjusting the derailleur can help; in particular the "B tension" adjustment to get the jockey wheel to track the gear cluster more closely for more wrap. However, if it's that bad, it won't help.
I have decent experience adjusting derailleurs. Even taking apart the non-adjustable ones and drilling holes in new places to change spring tensions.
> 8-20 of them
You're not going to get 8 teeth of contact on an 11 tooth cog, nor 20 teeth of contact on a 26 tooth ring.