Weight per square inch:

[LD1]

Let me start by saying that there are way too many variables to figure it to a tee without a lot of hassle. Ground condition, tire type, inflation pressure, F/R weight split, etc. It would have helped if you had said what type of tire you plan on using, R-1, R-3, or R-4's. That said, I have come up with a formula that works reasonabally well for comparitive purposes in average conditions.

I have found that in average conditions for most tires that the contact patch length is about 3/4 of the rim diameter and the width is about 1" less than the tire measurment.

So for the B3030 with AG's and MMM weighs about 2734 lbs and the tires are
F 7-12
R 12.4-16

Front (.75 x 12)= 9 multiplied by width (7-1)=6......9 x 6= 54 sq in
Rear (.75 x 16) x (12.4-1) = 136.8 sq in
(54 x 2) + (136.8 x 2) = 381.6sq in
2734/381.6 = 7.16 psi ground pressure

The JD weighs 4070 lbs (assuming about 500lbs for MMM as I could not find a spec) and it has 7-14 fronts and 11.2-24 rears

(.75 x 14) x (7-1)= 63 sq in front
(.75 x 24) x (11.2-1) = 183.6 sq in rear
(63 x 2) + (183.6 x 2) = 493.2 sq in
4070/493.2= 8.25psi ground pressure.

Like I said, there are a lot of variables, but this is a good way to compair tire sizes, and can be applied to r-4's and turfs. But keep in mind that the ag psi's that I calculated above assume that you are burrying the bar tread in the ground so that the body of the tire is contacting the ground.

 

[pat32rf]

DUH- What did he actually want to know again? And why?

 

[archerm3]

If you mean the air pressure in the tire, then no. You can inflate your trailer tires to 30psi (small utility trailer)while they are off the trailer, install them and they will still read about 30psi. Run the rider up onto the trailer and it will still read about 30psi.
Then jack up the wheel and slip something under it that will give you a tire imprint, both length and width. You can then calculate the area of the imprint. Next, let the air pressure down to about 12-15psi and check the imprint again.
When I used to run small 4x4s we would run 32x10x15 tires. Each tire had a load rating of over 3000lbs at 45psi (approx) but at 45psi there was only a small patch of rubber in the center of the tire touching the pavement. Rode like we had india rubber tires and would go center bald in a short time.
For highway use we would air down until the full width of the tire was touching the pavement. Maybe 18psi.
For off road use we would air down to about 10psi or less. This would give us the largest footprint and the lowest tire pressure per square inch on the ground. It would also allow the tire to conform to the surface of the trail so a sharp stick or rock would deflect the tire inwards, rather than piercing it.

Your statement just proves yes to the ballpark-th degree.

The PSI in the tire will equal the PSI of ground surface contact, in a laboratory with a tire of no tread and no stiffness (imagine a balloon). There will be spikes in ground pressure due to the stiffness of the tread and the tread block edges, and the stiffness of the side wall. But the average over the entire contact patch will be the same as the tire pressure adjusted for sidewall stiffness.
If you're rim touches the ground when there is no air in the tire, then sidewall stiffness is negligible.

 

[SPYDERLK]

Like several have said, the ground pressure, regardless of weight, is just the internal tire pressure with a small correction for carcass stiffness. The tire just keeps flattening [spreading the weight over larger area] until the flat area times the internal pressure holds the weight that is on the tire. Tread lugs have little effect if were talking average psi of the contact patch.
larry

 

[LD1]

Like several have said, the ground pressure, regardless of weight, is just the internal tire pressure with a small correction for carcass stiffness. The tire just keeps flattening [spreading the weight over larger area] until the flat area times the internal pressure holds the weight that is on the tire. Tread lugs have little effect if were talking average psi of the contact patch.
larry

I disagree with this. Lower pressure does let the tire flatten out more but to say that the ground pressure is the same as the internal pressure doesn't make any sense to me.

My rears at 10psi may widen by 1" due to the weight of the tractor. If I pump them up to 20 psi what you are saying is that the ground pressure doubled. That would require the tire to have half of the contact as before and that just isnt the case.

My P/U truck is the same way. I normally run about 30psi in the tires. If I know I am going to be hauling something heavy (sand, gravel, firewood, etc) I air them up to about 50psi and it dosent make much difference on ground contact. Certainally not 5/3rds less contact. And I have another truck with 35 x 16 super swamper boggers. They are uch wider and much taller. I run them at 30psi. Are you saying that it has the same ground pressure (ie contact area) as the smaller 265/75 r-16's on my other truck??

My 4-wheeler I run at 8psi when I go mudding. If I am going down the road, or back fencerows, I run 16psi, and again, the contact isn't half of what it was before.

Back in the day of the big 2wd farm tractors, extra wide rears were the thing because they reduced ground pressure as they were about twice as wide as a standard tire. But you are saying that a extra wide tire @ 10psi has the same ground pressure as a standard tire with 10psi???

I can go on and on with examples as to why it does not make any sence, so if I am missing something, i am all ears.

 

[ccsial]

This thread may become another "Will It Fly"

 

[Redbug]

Ohhh Lordy...Don't resurrect THAT one!

 

[Tom_Veatch]

...Devide the number of square inches by the pounds of weight on that tire and you have the ground pressure in pounds per square inches...

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.

 

[archerm3]

I disagree with this. Lower pressure does let the tire flatten out more but to say that the ground pressure is the same as the internal pressure doesn't make any sense to me.

My rears at 10psi may widen by 1" due to the weight of the tractor. If I pump them up to 20 psi what you are saying is that the ground pressure doubled. That would require the tire to have half of the contact as before and that just isnt the case. Keep in mind that the contact patch is getting longer front to back as well as wider, it doesn't take many hundreth's of an inch to get the area increased rapidly

My P/U truck is the same way. I normally run about 30psi in the tires. If I know I am going to be hauling something heavy (sand, gravel, firewood, etc) I air them up to about 50psi and it dosent make much difference on ground contact. Certainally not 5/3rds less contact. And I have another truck with 35 x 16 super swamper boggers. They are uch wider and much taller. I run them at 30psi. Are you saying that it has the same ground pressure (ie contact area) as the smaller 265/75 r-16's on my other truck??

My 4-wheeler I run at 8psi when I go mudding. If I am going down the road, or back fencerows, I run 16psi, and again, the contact isn't half of what it was before.

Back in the day of the big 2wd farm tractors, extra wide rears were the thing because they reduced ground pressure as they were about twice as wide as a standard tire. But you are saying that a extra wide tire @ 10psi has the same ground pressure as a standard tire with 10psi??? YES problem then becomes that the single tire doesn't have the traction in loose soil that the double tire will. Also, the narrow tire "caves" in further, causing more ground disturbance from the tread blocks collapsing, as opposed to the larger tire(s) which don't need to collapse as far to get the contact area to spread out to where 10 PSI holds up the weight.

I can go on and on with examples as to why it does not make any sence, so if I am missing something, i am all ears.

I think what you are missing is the inability to accurately measure the contact patch in your real world examples. Also, some tractor tires have significant sidewall stiffness which would let the ground pressure be higher than the tire pressure as compared to a truck or atv tire.

Edit: one more thing, when the soil is not firm, the pressure distribution (map) becomes non parallel with gravity, which turf tires or anything with a squarish section build work better.

 

[SPYDERLK]

I disagree with this. Lower pressure does let the tire flatten out more but to say that the ground pressure is the same as the internal pressure doesn't make any sense to me.

My rears at 10psi may widen by 1" due to the weight of the tractor. If I pump them up to 20 psi what you are saying is that the ground pressure doubled. That would require the tire to have half of the contact as before and that just isnt the case.
[SNIIP]
I can go on and on with examples as to why it does not make any sence, so if I am missing something, i am all ears.

It makes sense to me that it is the captive air pushing on the ground that supports the weight. How are you making sense of a situation where this is not so? I think if you were scrupulous in checking it empirically yould find a closer correlation than you have perceived between weight and psi X area. Why would it not be exact? ..... It has to be a function of the tire carcass.... Heres a thot on your 50vs30 truck analogy. When the tire is hard less tread shares the load. This tread is highly loaded and deforms more than if the tire were soft. This greater deformation causes a higher amt of tread to be in contact than would be ideally predicted. So, instead of 3/5 of the soft contact area you would observe maybe 4/6 in real conditions. Also the carcass stiffness enters more as the tire deflects so a low aired tire deflects less than yould think because the carcass starts contributing some support. As you go really soft the inside rubber over the tread can actually be higher than the thin sidewall that is nearly touching the ground. This too will prevent the actual contact patch from growing as much asyou would expect.