Morgan Conrad

Not saying you are wrong about how tires work, but...

Estimating the volume of my Spec Miata tires, (I guessed that the sidewalls were 3" tall, sound about right?) I got about 5000 cubic inches. In order for them to measurably change in pressure, say from 30PSI to 31PSI, due to being lowered off the jacks, the volume of the tire would have to change by about 1/30, or 3%, which is 150 cubic inches. Since the width of the tire is about 8", this means that the area on the side of the tire would have to change by about 20 square inches. Which seems way way higher than what it actually changes by. (Maybe a couple of square inches???)

I don't think that when you sit on one of those big puffy yoga ***** their pressure goes up by much either.


I'm sure the sidewalls do some of the supporting. But clearly not all. The simple question is how much does contact area go up when you lower PSI? It's not simple physics. But what is it?

John/TSR

...your statement: "I'm sure the sidewalls do some of the supporting. But clearly not all."

Huh ? Please explain. What's the clearly part ?

Morgan Conrad

then the tire clearly crunches down. If the sidewalls alone were completely supporting the tire, it wouldn't crunch down at all. If air pressure alone were supporting the tire, the contact patch would triple. Reality is somewhere in between.

I just have no idea where that is. :-)

Mark Dalen

It depends on the tire (and of course, weight of car). On my race car, you can not tell the difference visually between 30psi and 10psi. Many low profile street tires, you can see a slight difference but it is not all that obvious due to very stiff sidewalls.

I am not sure what the point is of this discussion. If one were to go out and measure, as I suggested, one would see that there is not a very strong relationship between contact patch size and tire pressure. The original premise was flawed as it ignored very signficant mechanical aspects of a tire.

Mark Dalen

In using these gas equations, temps and pressures are absolute. You can not use psig (what you measure on a tire guage).

John/TSR

...as the whole discussion ignores the mechanics of the tire carcass.

I'll stand by my statement that the sidewall bears the entire load of the tire. Air pressure inside the tire defines the shape and characteristic of how that weight is being supported, but nothing other than the sidewall support any weight.

I'd guess that an experienced tire design engineer could confirm what the role of air pressure is in defining the contact patch of the tire, but I'm going to go out on a limb here and speculate that his initial response would be 'Who cares ?'

Why would he respond that way ? Because air pressure defines the shape of the sidewall, which controls its deflection as it rolls and tries to change direction, which is a much greater mechanical consideration that the size and shape of the contact patch. Air pressure also sets the mechanical spring rate of the tire itself, which I have never seen analyzed.

If that last line has you scratching your head, think about this - 1 lb. of air in the tires feels like 10-20lbs. of spring in the suspension. Why ? What is it in the complex mechanical system of the tire makes this true, and does tire 'compliance' (that's a real phrase, not something that I just whipped up) mean anything ? I think it does...a hell of a lot more than contact patch.

I spent the last 2 years fiddling with a race car that I switched from radial tires to bias ply tires...and it taught me a lot...although I haven't figured out just 'a lot' actually is.

Mark has even more experience (and success) in setting up sophisticated suspensions, and has both radial and bias ply knowledge. He's probably scratching his head too, because he knows that it's a whole lot more complicated than 'tire pressure/contact patch'.

An interesting discussion has been triggered, but it has nothing to do with the original poster's premise.

Morgan Conrad

How is it different that PSI?

Hmm, now that I think of it, a tire gauge that's just sitting around reads 0 PSI, not 15. (which is PSI at sea level on earth) So I guess when it reads 30PSI it's really 45?

Your's and John's point that the effect of pressure on sidewall shape and hence turn-in, and on spring rates, totally overwhelms any small effect on contact patch, sounds absolutely right to me.

Mark Dalen

PSIG is Guage pressure or pressure above atmospheric. PSIA is PSI absolute. Add atmospheric to do the math you were doing.

The point is that a tire is far more a structure than a perfectly elastic container.

Tomasz@Startup

I agree with many of the things you are saying. But the point of the post is:

Same car, same tire type, going wider will change the contact patch shape, making it wider, but will not give you a larger contact patch. Can we agree on that?

(Only way to get a larger contact patch is to throw more weight on that tire - these pictures I have.)

Tomasz@Startup

<center><img src="http://www.startupracing.com/Other/Pictures/40PSI.jpg"></center><p>
OK, again runflats will behave differently. But racing slicks are more rubber and air than steel. So here you go. And since my HTML skills are low, 2nd picture is in the following link.

40 PSI looks like this.