I have updated a Cannondale tandem with a Bafang BBS02. Using a lekkie 42 tooth front chain ring allows for a perfect chainline as far as I can see. That is, with the removal of the triple crank, the chainline is right where the middle chainring was. I have a new Deore M592 derailleur (with jagwire index adjuster), new Sunrace 11-40 cassette, new KMC 9 speed ebike chain and a Wolftooth roadlink. I have a 9 speed 105 brifter.

The shift up from the 4th (18t) to the 5th cog (21t) almost never happens, it just wants to hang. I somewhat suspect the derailleur hanger though it looks pretty straight as I look at it and the derailleur line. I can increase the tension and that will make things happen at the expense of almost everything else. I am flummoxed, I would have thought shifting to the center chainline would be the easy part.