Can anyone help diagnose a poor shifting rear set up? Its a Suntour Ultra 6 13-28 with Suntour ARX changer and Powershift (non-index) shifters. Upshifts are very clean and quick, but downshifts are noisy and reluctant and slow. I have fully dismantled and relubed the derailleur, and the cable and shifter are all free and smooth moving.
Last night I borrowed a different RD, a Suntour VX GT, and its no better with that.
I could do with some pointers to help narrow down the cause, rather than replacing bits at random to see if it helps. Could this be caused by a worn chain?
Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
When shifting woes don't respond to normal tuneing, the first thing that I do is to check the derailleur hanger alignment. It's not uncommon for the hanger to get bent inward and that will definitely degrade shifting performance.
Shift your bike into a gear combination that makes the derailleur hanger point downward. They prop your bike vertically against something and examine it from the back. If the derailleur arm doesn't look like it's pointing straight downward, clamp a large adjustible wrench around the derailleur where it attaches it and gently bend the hanger on the frame it back into line.
If you try to bend the arm without the derailleur attached you risk ovalizing the bolt hole where the derailleur attaches. That's a very bad thing so don't let it happen.