Weight per square inch:

[pat32rf]

So if you run 25psi (for example) in your tires when on the vehicle, then you remove said tire, or just jack it up, the tire pressure will drop to zero, as you have no ground pressure any more????
The psi in the tire just gives it shape. The whole unit supports the vehicle but depends on the air within to maintain its shape and support, weither radia, bias ply or belted.
A large soft 32x10x15 tire will have a different ground effect than a 12ply 20" truck tire, even though they may have the same ground contact area.

 

[LD1]

I think everyone is trying to think too much into it. I think the op is trying to compair which tires/tractor combination may or may not make big ruts in soft conditions.

My manual states that for the R-1 I have to inflate them to 14 psi. The optional R-4 size for my tractor says 30 psi.

Does this mean that the wider r4 has over double the ground pressure??

I know if I were to drive over soft lawn (not spinning at all) the R-1's would cut much deeper ruts, because they are a lot skinnier.

I don't doubt the ballon analogy and contact pressure equalling psi inside but those that say sidewall stiffness only effects it a little bit are wrong. There is a whole lot more to it than inflation psi.

A properly inflated tire will distort very little under it's load range. The proper pressure is to maintain it's shape. Over inflating a tire by double does not double the ground pressure, but it will marginally increase it as the tire deforms a little less. As does decreasing the pressure to half of rated, does not double the contact area. I don't know all the science behind it but it logically doesn't make sense that contact pressure = PSI.

A couple more questions, the tires on my saturn are at 40 psi. Wheel and tire together only weigh about 40lbs. Does this mean that if I take the tire off the car, and set it on the floor, that there is only 1 sq in of contact??? What if I lower it to 20 psi (which isn't much difference with a tire off the car), will the contact double??

And lastly, what if I lowered the pressure on my truck to 2 of 3psi. Would I have 10-15 times more contact.?? For that matter, I'll just let all the air out to 0psi, that way I can fly

 

[FRIZ]

... It would have helped if you had said what type of tire you plan on using...

R3 turf

 

[FrogLick]

Minor quibble... It should be pounds of weight divided by square inches of contact area to get the avg contact pressure in PSI. The method quoted results in the inverse value, square inches per pound.

The value obtained won't be the same as the inflation pressure in the tire because of influences from the tire structure. If the tires were a thin walled flexible balloon, the two values could be quite close, but with a semi-rigid tire, the values can be significantly different.

Yes sir you are correct. I didn't catch my formula inversion until this morning when I opened the thread and started reading or I would have edited my original post.

 

[pat32rf]

While my R4s have a max pressure rating upwards of 25psi (and came from the dealer at that pressure) I reduced it to around 12psi after I found I was bouncing so much that the seat switch would cut out. 4000hrs later I have perfect wear accross the face of the tire without any "center balding" like I would have had at 25psi. Very little of my hours are on pavement with only the occasional (4-5times/yr) run of 6-8miles and most of the time is spent in "B" range at 1500-2000rpm on gravel driveway or bush trails (rocks, roots and mud)