Originally Posted by
pexio
Hmmm, I'm having a little trouble understanding this. You're talking about the B screw adjustment, right? You fixed the RD jockey wheel rubbing on the biggest cog of the cassette by changing the granny ring? I seem to be missing something (likely).
The RD can only take so many teeth before it runs into problems. What was happening on 26f and 28r was that the upper RD jockey gear was running on the 28t on the cassette. Not enough movement on the RD to handle the number of teeth on the front plus the rear and the total was 43 which greatly exceeds the stated capacity of the RD. By shortening the chain I could pull that 26/28 but could not get anywhere near the 52/28 without locking up the drive chain. That 26 granny just gave too many teeth to change but by going to a 28t granny it was just about acceptable. You can adjust tension on the RD to pull the mechanism away from the cassette and this was all I had to do on the Tiagra set up when I went to the 12/30 from a 12/25.
This is the reason that anyone trying to run the MTB cassettes of 11/32or 34 will normally have to change the RD to a long reach version to be able to cope with the teeth change.