Originally posted by tboner:
I thought a lower profile tire has a thicker sidewall because there is less air to support the weight of the car.
Let's take this physics a bit further to the tire.
At rest, all forces balance. So the force the mass of the car exerts on the tire must be equal to the force the tire exerts on the car.
If I am not mistaken, this force, or at least some of it, is measured as the air pressure in the tire.
If we have a tire with less interior volume (our lower profile 18 in this case) I think it would have to operate at a higher pressure to support the same car mass.
So I'm thinking the sidewalls must be thicker in a lower profile car.
But I could be wrong here 
TBI believe a tire's main function, when speaking in terms of supporting the weight of the vehicle, is to act as a container for the air pressure, not to use the rubber/belts to support the mass itself. For example, an unmounted tire can hardly keep its circular shape without any weight applied to it. One attribute a lower profile tire has is a smaller amount of surface to keep the air contained within itself (which leads to a higher pressure distribution on the rubber). The only aspect I'm not sure about is whether the ability to contain the air pressure or the ability to support the longitudinal/lateral forces created at the contact patch is the limiting factor when determining the sidewall thickness.
But as for needing a higher pressure to support the mass of the car, this also depends. Forgive me for using another example
Let's say we have a vehicle that weighs 3000 lbs. Dividing this weight by 4 (750 lbs) will give us the amount of static load each tire must support. Let's assume the contact patch at each tire is 4"x6" (24 sq in.). This means the road is creating a pressure of 31.25 psi on the tire (and vice versa). Now if your tire presssure decreases to 20 psi, the contact patch must grow in area (to 37.5 sq. in) in order to support the 750 lb load from the vehicle weight. Obviously there is a limit to how much it can deflate before the wheel starts supporting the weight, and the lower profile tire has a higher minimum pressure for this due to the smaller volume of air it contains.
