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# Tire Footprint and Pressure Dynamics

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## Tire Footprint and Pressure Dynamics

(OP)
Hello,

After reading about tires in Race Car Vehicle Dynamics by Milliken I have a few questions.  According to the book the coefficient of friction of the tire decreases with an increase of load.

I am wondering what happens to the footprint and pressure when a tire undergoes an increase in load.  I suspect that there is a slight increase in both footprint area and pressure.  Is this assumption correct, and if so does anyone know what the relationship is between the two.

Can anyone explain in more detail what causes the decrease in Cf, and/or suggest a book on tires that is more comprehensive than the chapter in RCVD.

TIA

### RE: Tire Footprint and Pressure Dynamics

The tyre pressure does increase a bit, but not much.

In fact you could do an experiment (this is interesting). Fill your spare tyre. Leave overnight. Measure the pressure. Fit it to the car. remeasure the pressure before driving it. TELL US THE DIFFERENCE. Oh, here's an easier way - measure the pressure in a tyre on the car. Jack the tyre off the ground. Remeasure the pressure.

Most of the extra load is taken in the sidewall, which buckles the contact patch up in the middle, so making it less effective.

Cheers

Greg Locock

### RE: Tire Footprint and Pressure Dynamics

(OP)
GregLocock,

I took your advise and ran a little experiment this evening on my Ford E-150 conversion van.  I measured the driver side front at 33PSI on the ground.  Then I jacked up the tire, and measured 32PSI.  Lastly I lowered the tire back to the ground, and measured 32PSI.

I really expected a bigger change in PSI, so it looks like you were right on the money.

Thanks.

### RE: Tire Footprint and Pressure Dynamics

>>Can anyone explain in more detail what causes the decrease in Cf, and/or suggest a book on tires that is more comprehensive than the chapter in RCVD.<<

I would suggest "The Racing and High Performance Tire" by Paul Haney.  He goes into great detail about the three ways a trie develops traction: mechanical keying (deformation), adhesion, and abrasion.  The book is an excellent look into a complex subject that is not sufficiently covered elsewhere.

Chuck

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