Weight per square inch:
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FrogLick
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The hardest part of finding the answer to this question will be to determine the ground contact area of the tires. Determine that then it's a simple matter of dividing by the weight.
tractorworks
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I'm probably missing someting here but isn't tire pressure in PSI equal to weight per square inch.
newbury
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Yes
BUTT
It sounds like OP wants to know how much downward force will be exerted on the surface underneath.
This will be dependent on the tire contact area.
Thus narrow tires would exert more.
Pumped up high pressure R1's would have less ground contact than under inflated R3's
A simple math problem exacerbated by the failures of the American education system.
/edit - other info needed -
Fueled full?
Operator weight?
How accurate?
Last edited:
Redneck in training
Elite Member
Assuming the correct tire pressure maximum pressure/sq inch will be applied on firm surface such pavement or concrete. On softer surface it will be somewhat less depending on how much the tire sinks. There are probably several ways to determine the contact area. I would jack one rear wheel up, wet the tire on the top, turn the wet spot down, insert a sheet of paper and set the wheel on it. Lift it back up, remove the paper and determine contact area. Repeat for front wheel. Add it for all wheels and divide the mass of the tractor by the number. Needles to say that you will get an average pressure. To get exact value for particular axle you will need to know how much weight it carries. In example front wheel carry less weight but the specific ground pressure might be higher than the rear wheel due to lesser contact area.
You can make pretty good guess if you know the mass and estimate the area just by looking at the tires.
You can make pretty good guess if you know the mass and estimate the area just by looking at the tires.
Redneck in training
Elite Member
No. It has nothing to do with what he is asking.
RickB
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Need the following info to answer:
Front tire contact patch.
Rear tire contact patch.
Front axle static weight.
Rear axle static weight.
Do you want separate values for front and rear tire PSI on contact patch, or an average of F/R? Yes, it is a simple math exercise, but only after the legwork is done. If the OP knows the various tire sizes and has the time and talent to search the internet for tire and tractor specs, an average value can be calculated without knowing the F/R weight split. It just takes a little ambition.
What you really want to know is the ground pressure measurment. This is dependent upon several things; weight per wheel, footprint size (partially determined by tire pressure) and surface/soil condition.
To determine it on a hard surface at a given tire pressure jack the vehicle up and ink a portion of the tire and lower it onto a piece of poster board. Jack the tractor back up and remove the board and then determine the number of square inches of the individual elements of the print. There are some scanner programs that will do this for you but I doubt you would have access to them. 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. This measurement is only valid for the hard surface at one tire pressure. That is why military vehicles have adjustable tire pressures. The military usually measures this at three different tire pressure settings; highway (highest pressure), crosscountry, and mud sand and snow (the lowest). You can see, using this method, that a large agresive tread pattern (like AG tires) will have a much higher ground pressure on a hard surface then highway tires that have much more rubber in contact with the road.
Determining it on different soil types is much more involved as the tread is less involved when it sinks into the soil.
To determine it on a hard surface at a given tire pressure jack the vehicle up and ink a portion of the tire and lower it onto a piece of poster board. Jack the tractor back up and remove the board and then determine the number of square inches of the individual elements of the print. There are some scanner programs that will do this for you but I doubt you would have access to them. 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. This measurement is only valid for the hard surface at one tire pressure. That is why military vehicles have adjustable tire pressures. The military usually measures this at three different tire pressure settings; highway (highest pressure), crosscountry, and mud sand and snow (the lowest). You can see, using this method, that a large agresive tread pattern (like AG tires) will have a much higher ground pressure on a hard surface then highway tires that have much more rubber in contact with the road.
Determining it on different soil types is much more involved as the tread is less involved when it sinks into the soil.
SPYDERLK
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Yes - no but. Neglecting carcass stiffnes, the tire deflects until the contact area times the inflation psi supports the weight. Weight is the downforce applied by the tire. The pressure is just the internal pressure. More weight more contact - ~same pressure [tire internal pressure only goes up very slightly].
I trust the OP is talking average pressure over the contact patch. The tread lugs cause very high peaks that could be a concern - depending on what he is actually wanting the info for.
larry
pat32rf
Veteran Member
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.
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.