# Tire pressure question

1. May 22, 2006

### gibroni1

Hi All,

This is my first time posting here - I hope you can help!

I'm working on a vehicle shock simulation, and my question below will help me understand some dynamics (really statics) involved in my problem.

How is it possible to change the pressure in your car's tires? If one were to hook up a pressure guage to a vehicle tire, wouldn't the pressure always have to equal

p = (m*g)/a

where m is a quarter (approximate) of the car's mass, and A is the surface area the gas is acting on.

If the pressure is more or less than this, wouldn't the car not be static?

I understand the volume of the tire is changing when air is added or subtracted, but it seems to me that the pressure should always be the same, jus the mass of air contained in the tire is changing.

I hope my question is clear. I appreciate all help!

2. May 22, 2006

### Staff: Mentor

As you increase the tire pressure, the tire hardens up more and more, so the area of the contact patch with the ground gets a little smaller. But I don't think that's the main effect. You can also put high pressure into your spare tire, which isn't supporting the car at all, right? So a lot of the pressure goes into creating tension in the rubber of the tire itself.

3. May 22, 2006

### gibroni1

thanks for the reply, berkeman. I'll have to think about that one.