# Automotive/Truck Tire Pressure Vs. Load

• mikesr53
In summary, the load on the tires has got to be the same as the force on the road for the pressure to stay the same.f

#### mikesr53

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.

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.

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.

You can easily measure the pressure in the tire with a gauge
You can also estimate the load by measuring the contact patch, length of flattened bit on the road * width and multiplying by the PSI.

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).
Those same stiff sidewalls that reduce the tire deformation do so by exerting more force when there's more load on the tire. The equation for contact area is not the simple case you mention, since the sidewalls and tread stiffness also contribute to the force generated by the tread. This is why "run flat" tires still function just fine with 0 psi pressure.