Easy one old bean, a Tuber is a Buell with a tubular steel frame, the old original design before the introduction of the new modern XB alloy beam fuel tank frame bikes.

Yeah the new ones that're no longer available, easily damaged and impossible to repair/make......insurance companies write them off no matter what the damage is
At least the REAL Buells can be repaired, even if you have to make a whole new frame because it's just a load of bent tubes welded together

xb the one with the real buell engine.

To what does "Tuber" refer and what's the origin?

Were you thinking it was something to do with potatoes

I'd be keeping quiet about that if I were you

Now I am confused. The tube framed, Harley-Davidson Sportster engine bikes were built by Erik Buell.
The engine that Erik designed and installed in a chassis that he also designed and then assembled in Troy, not in Milwalkie along with the other Harleys, is actually a rebadged Harley. How come?
Folk lore has it that Harley were embarrassed by the performance and handling of XB range and then angry when the Rotax engine was introduced. I have even heard it said that during the brief period of ownership of M V that a Buell badged M V was created.