Comparing pressure of air in a tire installed in a car & a free tire

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Discussion Overview

The discussion revolves around the comparison of air pressure in a tire when it is installed on a car versus when it is freely lying on the floor. Participants explore the implications of the weight of the car on tire pressure and how this can be analyzed using the ideal gas law.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that the pressure of the air in the tire when installed on the car (P_{c}) is greater than when the tire is free (P_{f}) due to the weight of the car compressing the tire and reducing its volume.
  • Another participant agrees with the initial claim but notes that the difference in pressure is minimal and likely undetectable on a tire gauge.
  • Further elaboration includes a hypothetical scenario where the weight distribution on the tires is analyzed, indicating that the deformation of the tire due to the car's weight results in only a slight change in volume and pressure.
  • A later reply questions whether one should inflate the tire to 30 psi while it is free, suggesting that this pressure would be appropriate for when it is installed, as the increase in pressure upon installation is negligible.
  • One participant confirms this suggestion, indicating agreement with the reasoning presented.

Areas of Agreement / Disagreement

Participants generally agree that the pressure difference between P_{c} and P_{f} exists but is very small. However, the discussion does not reach a consensus on the practical implications of this difference in terms of tire inflation practices.

Contextual Notes

The discussion relies on assumptions regarding tire deformation, weight distribution, and the ideal gas law, which may not be fully explored or defined within the conversation.

Who May Find This Useful

This discussion may be of interest to automotive enthusiasts, physics students, and individuals concerned with tire maintenance and pressure management.

scoutfai
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Imagine an ordinary car wheel with rubber tire, installed in a car axle. So the weight of the car (some of the weight of course, because there are 4 wheels) is pressing on it. Let's denote this pressure of the air in the tire as [itex]P_{c}[/itex].

Now the same wheel (with its rim and tire) remove from the car axle, so it is now freely lying on the floor. Hence no car weight is pressing on it. Let's denote the pressure of the air in the tire as [itex]P_{f}[/itex].

Question: Is [itex]P_{c}[/itex] different than [itex]P_{f}[/itex] ? How to show it by using ideal gas law equation? You may assume the weight of the rim as negligible if needed.

My personal guess: [itex]P_{c}[/itex] is more than [itex]P_{f}[/itex] because with the weight of the car pressing on the tire, volume of tire (hence volume of gas) reduces, thus pressure increases. In other words, if a tire pressure is 30psi while installed on car, once take it out from the car axle, its pressure will drop.
 
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You are correct, but the difference is so small that you will not notice it on a tire gauge.

Suppose your car is 4000 pounds with 1000 pounds on each wheel. Also suppose your tires are 10 inches wide. At 30 psi your contact patch for each wheel will be about 33 square inches. Since the tires are 10 inches wide that means your contact patch is only 3.3 inches long. Deforming the tire with a typical 6.5+ foot circumfrence so that there is a 3.3 inch flat spot will change it's volume (and pressure) only very slightly.
 
mrspeedybob said:
You are correct, but the difference is so small that you will not notice it on a tire gauge.

Suppose your car is 4000 pounds with 1000 pounds on each wheel. Also suppose your tires are 10 inches wide. At 30 psi your contact patch for each wheel will be about 33 square inches. Since the tires are 10 inches wide that means your contact patch is only 3.3 inches long. Deforming the tire with a typical 6.5+ foot circumfrence so that there is a 3.3 inch flat spot will change it's volume (and pressure) only very slightly.

Hence, if I was to pump air to a tire but I can only carry a free wheel instead of driving the car to the air pump, says I wish to have 30psi in my tire when it is installed on axle, then I should just directly pump in 30psi instead of a slightly lower pressure (for instance, 29psi) because the increase in pressure after the tire is installed is just so insignificant?
 
Correct.
 

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