Well, all is right with the world.....it seems Bean is correct. It's not intuitive or obvious at first, but I had to put pen to paper to convince myself or loose my sanity (I mean honestly, Bean can't be right, can he??).
90 kg person+bike (~200 lbs). I weight about 165lbs and my bike is roughly 35lbs (heavy I know, but it's old and I'd rather spend my money on ski gear).
Since Newton was the pimp-daddy of math and calculus: F=ma; so 90kg*9.8m/s^2. = 882N.
P=F/A, rearrange to solve for area; A=F/P. I measured the pressure in my rear tire at 18 psi (124,105 Pa).
Solving for area; A = 882N/124,105N/m^2 = .0071 m^2 (~11 in^2).
I'll admit, when I first ran the calc, 11 square inches seemed absurd especially since the width of my tire is roughly 2" (meaning the contact length would need to be about 5.5 in). I know tires deform, but that much seems ridiculous, right??
I sat on my bike with as much weight on the back tire as possible (the calc above assumes 1 tire is carrying all the weight) and had my wife mark the contact length on the garage floor (slid a straight edge under the front and back of the rear tire until it touched the tire, then marked the location with a sharpie).
Measuring the distance between the 2 marks yielded 5.75 in. - means good enough for government work.
Now that we've got that solved, we can go back to ridiculing people for not staying on the trail........or bean can keep posting topless photos of himself (I'd wouldn't post that kind of stuff on the interwebz if I were you, but that's your deal).