Seriously, tires change size to do different jobs and handle different levels of stress. The contact patch area really (really, really) is a function of the weight on that tire and the pressure inside it.
One exception would be grooves because the tire carcass is tough enough to resist deforming across tread cuts. So from the inside of the tire, it would have about the same contact patch size regardless of the tread so yes, a slick would put some additional rubber in contact with the ground (additional contact area) where a treaded tire had the tread cut out.
But really, when you increase a tire's size for a given vehicle, you have to reduce the pressure that inflates that tire to maintain correct tire geometry; it is the lowering of the pressure that increases the contact patch area, not the size of the tire itself.
And I didn't say that all tires could do any other tire's job or function, just that the contact patch area is a function of pressure and weight (force). There are some photos out and about on the 'Net to show this very thing (same tire, different inflation and correspondingly varying contact areas).
As far as the bicycle tires on an auto (any reasonable auto), no that would not work simply because the tire does not have enough area to satisfy the requirements of the vehicle load. In other words, there isn't enough A available in the tire to support the F no matter what the P.
But because bicycle tires will not work on an auto does not mean the physics has failed, only that another problem has become dominant. The same thing happens when a tire goes flat- just because pressure goes to zero does not mean that the area goes to infinity, it simply means that having a flat tire is the major problem and the car is now resting on the wheel edges through the flat tire carcass.
So changing a motorcycle tire to another applicable size won't change the contact patch area if the pressures used are the same. I am not trying to convince you here, just discussing the matter.
Brian
Brian, I know you are a really smart guy, but I still don't believe it. Why don't I just run bicycle sized tires on my Z06? On street cars, the tire pressures are no different from one tire to the next on cars like the Corvette - different models of the Corvette have three different sized tires. Can you honestly tell me that you believe that if I run a bicycle tire on my Corvette at 30lbs that it will have the same amount of rubber on the ground as my 325/30-19's with 30lbs in them? How do you account for tires with tread patterns versus slicks?
I'm not smart enough to explain it, but I really believe that the theory behind your contention is taken out of context when it comes to tires. I'll go to my grave not believing it that tread width and diameter of the tire don't make any difference if the weight on the tire and the pressures are the same.
