I have a 1975 R90/6, just a bare bike, no fairing or bags, small gas tank, with a new Avon 100/90-18 rear tire, which has a tube in it because of the spoked rim. My BMW owners manual specifies 400 H-18 tires.
It feels very squirrely above 60 mph. It is marginally convergent at ~ 4 Hz in what feels like a weave mode. I assume this is from poor lateral stability in the rear swing arm/tire system, but the tire pressure is the recommended 30 psi and there doesn't seem to be any clearance in the swing arm bearings.
Somehow the rear tire has a smaller profile than I recall. What tire should I have?