Interference
That chain is turning out to be problematic.
Adding the second idler took up the slack and stopped the chain falling off. As a side effect it eliminated all the pedal bob. So far, so good. I set the suspension to compress about 25% under static load – the sag – and that pushes the pivot down a little. This brings the frame closer to the chain line, and when the chain is pulled tight it touches the frame:
Notice the small groove the chain has worn in the swingarm. On the smaller cogs there’s no problem, but riding in 1-3 causes some nasty rubbing.
Tightening the suspension so there’s no sag is a non-starter. That defeats the purpose of having suspension in the first place, and it won’t work: the shock isn’t long enough to prevent the rubbing. Pushing the idler and chain horizontally away from the frame won’t work either: the deviation needed to avoid that hefty swingarm is too great. I have a nasty feeling that some cutting and welding is going to be necessary, but first I have get my head around the geometry…