Had rear section of Calfee modded for Gates drive (ring clearance), now trying to get that set up and having some problems.

First problem that frame mod was not adequate for stoker timing ring clearance. But can fix that with about 3mm of spacers.

Second problem is that I can't get belt to stay on the stoker ring, it wants to walk off towards the frame. Have tried to fix by shifting front timing ring in and out (figuring out would be the way to go, but tried both directions in case, have gone +2 and -3mm relative to stoker ring), but this does not seem to help.

Any ideas?

Side note, from Gates site see that rings set up such that shoulder is on opposite sides for front and rear rings, for the tandem rings shoulder is on the same side. Wonder why the difference?