Tom- Yes. you have the gist of what Bill B called castor angle, a stability index of sorts. I haven't done the math but I wonder how it relates to Jan Heine's wheel flop factor. Back in the day when I was far more motivated to follow the math I came to understand that much of this effort is deciding which language you want to use to describe the same thing. Is it trail/steering angle or Castor or wheel flop or...?

One aspect not reflected by mere math on paper is the rotation inertia and how it changes due to wheel diameter (and maybe tire width). I alluded to this when I described that the smaller wheel seemed to shift the responsive feel from similar geometry (as defined by one's "language") with a larger wheel. I used to warn my potential Terry customers that the Classic (the Terry touring bike for years) handled more like a racing bike, with a 700C wheel, that they had before.

I wish we had hooked up may 2017 when I was in Portland to start my sort of cross country bike tour. Andy