# Automotive/Truck Tire Pressure Vs. Load

## Main Question or Discussion Point

Hi, I'm new to the forum, so please be kind.
The question is: What is the relationship between a truck's tire pressure unladen and laden.
Parameters:
11R22.5 Truck tire
@ 90F
@ ??? PSi laden with a total (including nominal) load of 5800 lbs.
Any help is appreciated.

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mgb_phys
Homework Helper
Homework question or real world?

If it's just a simple HW question then the load is just the tire pressure * the contact area

In the real world, especially on heavier trucks, the tire deforms at high load to put more contact patch on the road and so the pressure doesn't increase as much with load.

Real world. Are you describing the pressure exerted to the road surface? I'm trying to determine if there is a measurable pressure difference in the tire.

mgb_phys
Homework Helper
Depends on the tire, you should check the manufactures site - they normally have specs for this.

Basically it all depends on how the tire changes shape.
The load on the wheels has got to be the same as the force on the road.
The force on the road (in lbs) is the tire pressure (lbs/sq in) * the area of contact at the bottom of the tire (sq in).

If you had a very large soft balloon tire, like you might have for logging machines in a soft ground then the pressure inside the tire might not change at all - it would just flatten out so that there were more sq in in contact with the ground.

On a very stiff tire, like a low profile car tire, the sides of the tire remain rigid, the contact area on the road doesn't change at all and so the pressure in the tire must go up: load (lbs) = pressure (psi) * contact_area (sq in)

For a typical tire it's somewhere in between - the spec for the tire will give you the maximum load pressure.

Thanks, the information you supplied was very helpful. I'll run some numbers to make estimations.

mgb_phys