Andy:

I woke up this AM with the answer to my opening question. It amazes me how often i'm able to finally "see" an answer when i'm fast asleep that baffles me when i'm (allegedly) awake. I stared at the geometry diagram that you kindly posted until I thought I understood it. I was even able to calculate "castor angle" figures for my bike which I quoted to you. But it wasn't until this AM that the "duh" moment really arrived. The Boston-Terry Castor Angle (which as you pointed out is not really the castor angle of the bike) is a Stability Index figure that's the result of the relationship between the radius of the wheel and the trail figure. Period. So as wheel diameter varies then, necessarily, the trail figure must vary by the same proportion for a constant BTCA (Boston-Terry Castor Angle).

If my opening question had been: "Does the "correct" trail figure vary with wheel size or is it a fixed constant?" then I probably could have answered it myself. IE: it takes less force to turn a canoe than an ocean liner. But this BTCA is a very useful tool if you deal with different wheel diameters - and, in fact, it's exactly what i've been looking for the last month. IE: I know what the desired or "normal" trail "should" be for a bike with 622 x 28 tires (700c). Therefore that same figure on a bike with 451 x 28 tires (20") is:

(451 x 28 radius / 622 x 28 radius) X "desired trail for 622 x 28 wheels"
or
(253.5 / 339) X 58 = 43.3mm

(you can get wheel diameters here )

(to solve for BCTA with radius & trail I use this app on my android phone)

So this BCTA formula told me that the folding bike that I bought doesn't have anywhere enough trail AND that the folding bike that i'm considering replacing it with has exactly the amount of trail that should work for me. So for that i'm in your debt. Need someone to cut your grass or take out the trash? Call me.

Tom