923 ground pressure on 1400's

[eviioiive]

The psi is not the same. A spare sitting on the ground at 0 psi has almost the same footprint as when it's at 70... There is more to it than just the air pressure in the tire

 

[Bighurt]

The psi is not the same. A spare sitting on the ground at 0 psi has almost the same footprint as when it's at 70... There is more to it than just the air pressure in the tire

Like I said there is no one equation for all situations and tires.

However the report identifies on average the vertical PSI of the tire to ground is equal to the PSI. That should be good enough for the intent of the OP. Any other equation is used to develop the tire itself. Given DOT standards of bridge weights manufactures know what vertical PSI they need to work around and can design a tire for the load, terrain and use that doesn't exceed those limits.

BTW for aircraft tires there are defined equation's because there shape remains nearly the same across all air frames. That footprint is a 1.6 ellipse. A Boeing 777-300 for example max take off of 662000 lbs. Based on weight tables that is 52317 for each wheel of the main gear and 243.3 sq inches of contact area as determined by formulas in US Engineers S-77-1 Report. This equals 215.03 PSI of vertical PSI. As recommended by Boeing the main landing gear are to be filled to 215 PSI. The report actually uses weight per tire divided by tire PSI in it's formula for contact area.

 

[Julian]

The psi is not the same. A spare sitting on the ground at 0 psi has almost the same footprint as when it's at 70... There is more to it than just the air pressure in the tire

On a hard surface and with significant load the tire psi is generally equal to psi on the road. When the load is light relative to tire stiffness as in a spare sitting on the ground, the tire stiffness plays a greater roll in contact patch area. One can tell whether or not a gasoline tanker is full or empty by looking at the length of the tire contact on the road. A long contact area means full. The tire has the same pressure full or empty. The tire will flatten out until the load supported by the tire divided by the contact patch area equals internal tire pressure. That is the reason the design air pressure for a tire is set by the "static loaded radius"- the proper height of the center of the axle above the ground regardless of the load.
For a every different load on a tire there there is a corresponding air pressure that will result in the design static loaded radius... But that is a whole different topic.