Automotive/Truck Tire Pressure Vs. Load

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    Load Pressure Tire
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Discussion Overview

The discussion centers on the relationship between tire pressure and load for truck tires, specifically focusing on how tire pressure changes when a truck is unladen versus laden. Participants explore the implications of tire deformation under load, the mechanics of pressure exerted on the road surface, and the variability in tire behavior based on design and specifications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant inquires about the relationship between tire pressure and load for a specific truck tire model under different loading conditions.
  • Another participant suggests that in real-world scenarios, tire deformation affects the relationship between load and pressure, indicating that pressure may not increase proportionally with load.
  • A participant clarifies that they are interested in the measurable pressure difference within the tire itself, rather than the pressure exerted on the road surface.
  • It is noted that the behavior of tire pressure under load depends on the tire's design, with some tires maintaining pressure while others may change shape significantly.
  • A participant provides an example of how different tire types, such as soft balloon tires versus stiff low-profile tires, react differently to loading conditions.
  • Another participant mentions the possibility of measuring tire pressure directly and estimating load based on the contact patch dimensions and pressure.
  • A later reply emphasizes that the contact area equation is more complex than initially presented, as factors like sidewall stiffness also influence the force generated by the tire tread.

Areas of Agreement / Disagreement

Participants express varying views on how tire pressure and load interact, with no consensus reached on a definitive relationship. The discussion reflects multiple competing perspectives on the mechanics involved.

Contextual Notes

Limitations include potential missing assumptions regarding tire specifications and the effects of different tire designs on pressure and load relationships. The discussion does not resolve the complexities of tire behavior under varying conditions.

mikesr53
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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
@125 PSi unladen with a nominal load of 2500 lbs.
@ ? PSi laden with a total (including nominal) load of 5800 lbs.
Any help is appreciated.
 
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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.
 
mgb_phys said:
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.
 

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