5425 vs 4320 vs 3720 Surface Pressure

[jenkinsph]

The point that I am trying to make is that THIS



Simply isnt true.

There may be a SINGULAR point at which PSI = Ground pressure. But any pressure or load change changes all of that.

My front tires @ 30PSI have LESS ground pressure with the loader OFF than they do with the loader on. As VERY evident by driving across a lawn both ways.

Same thing with the rears and the bushhog on vs bushhog off.

I think at this point, we have all said our piece and well just have to agree to disagree on this one.

It is true, simply put but I will agree to disagree.:laughing:

 

[couchsachraga]

Looking at my manual I'll have to go back to actual ground pressure, not tire pressure as what matters (assuming properly inflated tires...)

Why? The recommended tire pressure for R1 and R3 tires is very close - yet I think we can all agree that the contact area on an R3 is much larger and they sink in less - the GROUND pressure is less.

 

[LD1]

Why? The recommended tire pressure for R1 and R3 tires is very close - yet I think we can all agree that the contact area on an R3 is much larger and they sink in less - the GROUND pressure is less.

I agree, and is the point that I have been trying to make:thumbsup:

 

[jenkinsph]

The pressure per square inch is the same the surface area of the four patches for the 10,000 lb tractor would be equal to 2.5 times the surface area of the 4000 lb tractor.

This assumes smooth tires such as R3 tires, the cleats on R1 tires can increase the surface pressure on the reduced bearing surface. If you compare R1 tires of the same tread design it still works the same.

I mentioned this on page one to clarify. Post 7

 

[jenkinsph]

Looking at my manual I'll have to go back to actual ground pressure, not tire pressure as what matters (assuming properly inflated tires...)

Why? The recommended tire pressure for R1 and R3 tires is very close - yet I think we can all agree that the contact area on an R3 is much larger and they sink in less - the GROUND pressure is less.

The OP stipulated R3 tires when he posed the question. What is the ground pressure?

 

[couchsachraga]

Ground pressure is the pressure the tractor (via tires) exerts on the ground, in my world (snow) in PSI (generally lbs per square inch). For a given weight more contact area (wider tires, or aired down tires if you really want to get tricky to a certain point) = less ground pressure (same weight / more area = less pressure (less weight per square inch).). Assuming the same size contact patch for a given tire regardless of tire pressure (pressure of air in the tire) the actual tire air pressure reading doesn't matter (again, until the size of the contact patch changes - which it would with no air, or less air than would support the tractor (inflated tire in other words)).

And I completely agree R1's due to tread design (vs R3's) will tend to exert more pressure due not only to different width in general but the tread design (again, smaller contact patch, at least in "firm" ground).

As mentioned back in my first post, it's not hard to calculate ground pressure IF you know the contact area of the tires you are comparing - if someone can provide the contact area on average for an R3 of that size you've got the difficult work done already - put it in square inches, and divide the total tractor weight (working weight) you expect by that amount - and there is the pressure the tractor is exerting on the ground. Divide it by the R1 contact patch (smaller) and you'll get that pressure exerted on the ground - which will be higher (less area, same weight).

I do agree that IN GENERAL the manufacturer recommended tire air pressure is the same / slightly higher for a fully loaded tractor (max FEL for instance) than the ground pressure - which makes sense as otherwise the tire would get squished quite a bit and would be more prone to losing the bead seat.

 

[jenkinsph]

Ground pressure is the pressure the tractor (via tires) exerts on the ground, in my world (snow) in PSI (generally lbs per square inch). For a given weight more contact area (wider tires, or aired down tires if you really want to get tricky to a certain point) = less ground pressure (same weight / more area = less pressure (less weight per square inch).). Assuming the same size contact patch for a given tire regardless of tire pressure (pressure of air in the tire) the actual tire air pressure reading doesn't matter (again, until the size of the contact patch changes - which it would with no air, or less air than would support the tractor (inflated tire in other words)).

And I completely agree R1's due to tread design (vs R3's) will tend to exert more pressure due not only to different width in general but the tread design (again, smaller contact patch, at least in "firm" ground).

As mentioned back in my first post, it's not hard to calculate ground pressure IF you know the contact area of the tires you are comparing - if someone can provide the contact area on average for an R3 of that size you've got the difficult work done already - put it in square inches, and divide the total tractor weight (working weight) you expect by that amount - and there is the pressure the tractor is exerting on the ground. Divide it by the R1 contact patch (smaller) and you'll get that pressure exerted on the ground - which will be higher (less area, same weight).

I do agree that IN GENERAL the manufacturer recommended tire air pressure is the same / slightly higher for a fully loaded tractor (max FEL for instance) than the ground pressure - which makes sense as otherwise the tire would get squished quite a bit and would be more prone to losing the bead seat.

Thanks for clarifying, your ground pressure is the same as surface pressure. Lots of variables to consider, the belted tire body is cleated (or ribbed) on the outside to increase the surface tension of the tire which enables it to develop traction. The inside of the tire surface is smooth and it will average out to make the contact patch expressed in square inches X the tire air pressure = to the weight on the wheel. I would agree that the R1 tires will increase the surface pressure but if you measured the forces just a few inches below the surface it wouldn't matter as it would average out. That few inches may be equivalent to a rut though.

FWIW, when I was a kid and buying records (vinyl) I read that a good stylus with 3/4 gram force would exert about 75,000 to 80.000 psi on the surface. I soon switched from cheap Shure cartridges to the Empire EDR.9 cartridges that would work at .1 to .2 grams. So much for useless information.:laughing: I mentioned this because you can step on a sharp shovel and develop a very high ground pressure or surface pressure, something to think about.:confused2:

 

[couchsachraga]

I agree completely... including the sharp shovel comment!

What has been "bugging" me is the thought that tire pressure = ground pressure (surface pressure in your terms, and perhaps more fitting). As you point out with the stylus and shovel analog, "contact patch" as I keep calling it and weight matter! Perhaps I'm just misunderstanding what others are trying to articulate - different terminology for the same thing.