Your absolutely correct Bob - lots of variables there but I think we are on the right path... Even with my 12" wheels, it takes a pretty good tug on my smooth cement garage floor to move them...
Well, to much down force on cart wheels will result in loss of traction on bike's rear drive wheel... Too little down force and the bike will tend to wallow from side to side... It's really all in the feel and how firm you want the ride... And of course, tire pressure plays a part too, as does rear suspension settings on bike... You'll have lots of playing around to do this summer... That's why it took me so long to get everything dialed in...
I am one of those guys that is very attuned to my cars, truck, and bike... I know every little noise and idiosyncrasy on all of them, so if something changes, I can usually track it down...
I run my rear suspension at somewhere between 15-18 on the Wing's display and my TP cart tires are at 32 PSI (as opposed to Tow Pac's recommended 50 PSI)... The rear frame of my TP cart is 11" off the ground, and rear bike tire (which is a Yokohama Avid Envigor ZPS run flat car tire) is at 28 PSI... My front tire, a Bridgestone Battlax is at 41 PSI and I am actually running what they call double darkside because that front tire is actually a GL1500 rear tire...
I run darkside because tire mileage was so poor running the trike kit with normal bike tires and all the scrubbing action that takes place on the tires with these kits... My last set of Bridgestone G704/709 only yielded 8K miles and the front tire was cupped so badly, the handle bars oscillated until I got above 35 MPH...
My last Yoko yielded 17K miles and had about another 5K left and my Battlax front tire was at 20K with about another 5K left... Both tires were changed because I was riding our to the west coast in September with friends and wanted new tires on the bike for that trip...
I don't know of any HD guys running car tires but I don't see why it wouldn't work for you as well with the Harley... The bike handles equally well (actually better) with the car tire on the rear... Better ride 2 up and better handling in the rain - - AND - - - it is a run flat, so there is that!!!
I have TPMS on the bike so I can monitor tire pressure and temperature so that all works out pretty well...
Now - if it would just get warmer so I can ride!!! Santa brought me a set of LED headlights I have to install too, but gonna wait for a bit warmer weather before I get out in the garage... I could fire up the space heater, but it takes a lot of kerosene to heat the 30'X36' man cave with 11' ceilings - actually taller than that too because there is no ceiling in the garage it is a 8/12 pitch so the roof line is pretty close to 25' at the peak...
Les