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Hey, I was wondering if any of ya'll knew how to figure out wieght displacement on tires? Why I am asking, is that we have a 4x4 drill truck. It wieghs in at 22,000lbs. And the tires that are going to be put on it will be about 33-35"es wide..... So we were wondering if any of you guys could figure out the wieght displacement going to the ground. I guess it would be per square inch of the tire. We have a job coming up out on the tundra... And they "have" to know:rolleyes: Thanks alot, and any help would be appreciated....
 

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Just figure up the area of the contact patch of the tire, x4 for all the tires. Then divide the weight of the thing by the contact area of the tires. (this is assuming that weight is equally displaced on the rig, front to back, side to side)
 

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Get some 4x4 squares of plywood, paint white. Apply black paint to tires and let tack slightly. Check your truck to make sure that the inflation pressure and loaded weight is going to be close to the same as you are going to use on the muskeg. Jack truck up and set on each chunk of plywood. Remove and measure contact patch. Take truck to weigh scale and try to manouver it so you just get one tire on at each time. Divide the weight on that tire by the contact patch previously measured. Should give you an accurate ground pressure measurement for solid ground which would be a worst case scenario. In muskeg the tires are going to sink more and increase your contact patch which lessens your ground pressure.
 

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The size of the tire does not make any difference. The pressure that the tire exerts on the ground is the same as the inflation pressure of the tire. If you have 3000 pounds on a tire that is inflated to 80 psi, the tire will have a contact patch of 3000lb divided by 80 lb/sq. in., or 37.5 square inches. If you use a wider tire, it will still be 37.5 square inches, it will just be wider and shorter. The only things that can reduce the psi loading on the ground is to air down (making a larger contact patch), or add more tires. If the customer wants to know the area of the contact with the ground, you will have to take the truck to a scale and get the weight on each axle. If they only want to know the psi loading on the ground, it is only limited by how low you feel comfortable running your tire pressure.
 

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I'm not sure I buy that Black Dog.

Let's say my rig weighs 1000lbs even at each corner. Are you saying that if I air down to say 2psi, that the contact patch of each tire is going to be 1000lbs/2psi = 500 sq inches?

That would mean, if the contact patch were 10" wide, it would have to be 50" long??????

Doesn't seem quite right.
 

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BillaVista said:
I'm not sure I buy that Black Dog.
Let's say my rig weighs 1000lbs even at each corner. Are you saying that if I air down to say 2psi, that the contact patch of each tire is going to be 1000lbs/2psi = 500 sq inches?
That would mean, if the contact patch were 10" wide, it would have to be 50" long??????
To push it a little more towards the absurd... if I have a 4k pound truck, on 44" Boggers... I can run them with zero psi. At least, back when I had Boggers, I could, and the truck hasn't gotten any heavier... so is my contact pressure zero?

The theory behind it is appropriate, but it falls apart as sidewall stiffness starts taking over as the primary load carrying item, vs. air pressure. I'm sure there's a range through which it works, though.

Assuming it's acceptable practice to allow the vehicle to sink in to where the tire tread is into the soil, and the carcass is also supporting the weight of the truck, it might be more accurate to calc it from the load on each tire, divided by the gross size of the contact patch of the tire. Again, chances are, the tire pressure calc noted by Black Dog, will probably be accurate for pressures above 15psi--below that, and I think it'll fall apart.

As for figuring the gross contact patch area... the white plywood trick seems like a slick one to me. But driving across it won't generate an accurate number. However, jacking up the truck, then lowering it onto the plywood, and measuring just what is marked, would be more accurate... except that you really can't calc well for how much the tread is sunk into the ground--plywood is flat, tundra is squishy. The tundra will allow a bigger contact patch than the plywood, assuming identical conditions otherwise, because the truck sinks into the tundra, and doesn't sink into the plywood.
 

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black dog is correct to a point.
the inflation pressure is the same as the contact pressure, but at extremely low pressure with extremely stiff tires, the side wall will start to carry more of the load.

I just did somethign with this at work for axle loads of more than

50,000lbs

of course, at 2 psi, your tire rim is pro'ly carrying all the load.:flipoff2:
 

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To push it a little more towards the absurd... if I have a 4k pound truck, on 44" Boggers... I can run them with zero psi. At least, back when I had Boggers, I could, and the truck hasn't gotten any heavier... so is my contact pressure zero?

The theory behind it is appropriate, but it falls apart as sidewall stiffness starts taking over as the primary load carrying item, vs. air pressure. I'm sure there's a range through which it works, though.

Assuming it's acceptable practice to allow the vehicle to sink in to where the tire tread is into the soil, and the carcass is also supporting the weight of the truck, it might be more accurate to calc it from the load on each tire, divided by the gross size of the contact patch of the tire. Again, chances are, the tire pressure calc noted by Black Dog, will probably be accurate for pressures above 15psi--below that, and I think it'll fall apart.

As for figuring the gross contact patch area... the white plywood trick seems like a slick one to me. But driving across it won't generate an accurate number. However, jacking up the truck, then lowering it onto the plywood, and measuring just what is marked, would be more accurate... except that you really can't calc well for how much the tread is sunk into the ground--plywood is flat, tundra is squishy. The tundra will allow a bigger contact patch than the plywood, assuming identical conditions otherwise, because the truck sinks into the tundra, and doesn't sink into the plywood.
Black Dog - your theory will only work if the tread has the same contact area as the carcass area that holds the tread on. Chevron tread tractor tires inflated to 20 psi are going to have a much higher surface pressure than 20 psi on a hard surface to use an extreme example. I have had to do these measurements and your theory is not proved by the data.

Scott - if you had read my whole response both of these points as far as jacking and lowering the truck, as well as the difference between plywood and muskeg were covered. Any time I have needed these figures for a job the requesting agency was happy with hard surface numbers. Also in my experience the agency is going to give a max. depth that you are allowed to sink, and with this you can calculate your contact patch and surface pressure at this depth. Another point is brought to mind - if you are using chevron tread flotation tires you will need to estimate and draw a line on the plywood where you think the carcass will contact in soft surface but it won’t on the plywood.
 

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I had forget this until now, as it has been a while. We have also simply driven on top of sheets of a fabric material marked in a 1" grid pattern and cut off the pieces that protrude from under the tire when stopped. Much easier than the plywood method.
 

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take some pieces of paper and slide them under the tire until all 4 sides of the contact patch have paper around them then measure the opening of the paper then divide the weight on the tire by the area of the contact patch
 

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jasonmt, you are right about the tread on the tire making a difference, I forgot about that. Others are also correct that the structure of the tire will carry more and more of the load at very low pressures. This is obviously true on light rigs that can run at zero psi. I think for what you are trying to do, though, that this calculation is what they are looking for.
 
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