wanna911

I got into it recently with a bmw guy on this issue, and I'd like to know what the racers take on it is. And it's not nearly as simple as you might think. But before I post results I'd like to see what people think. All else the same (diameter of wheel and tire, PSI, Tire Compound, car weight)

Keep this in mind.

Pascals principle...Archimedes may apply also...

assume the car is supported by the air pressure, ie, ignore the sidewalls or assume they are the same for both tires, assume it's 50 psi inflation...

each wheel supports 1000 lbs...

what is the force on the ground? 1000 lbs.

what is it's area?

1000 lbs / 50 lbs/in^2 = 20 in^2 (the lbs cancel)
tire shape does not enter into it...it will take what ever shape the tire configuration will allow, but it will be 20 sq in, regardless...

if the tire is very, very narrow it will be a 10 L x 2 W patch
if the tire is very wide it will be a 2 L x 10 W patch

it has to: it has to support the car...and the force on the ground has to equal the wt of the car and air presure (force) supporting it, exactly equal so they cancel, the definition of static system...

Glen

The Mark Donohue principle still holds true is all I would say....

tkerrmd

a "wider tire" on a "wider wheel" answers your question.

wanna911

Well appearantly this has been a heated debate because accoring to physics, the contact patch should be the same, with the tire adjusting to the weight of the car, no matter what the size of the tires, at least that's what some people have been arguing, it makes sense to an extent though. A smaller tire gets compressed more, making it flatten out and put more % rubber on the ground and certain PSI than would that same weight on a wider tire.

JWERION

My 2 cents
A wider tire will have a larger contact patch if all other factors are equal (brand, model, air pressure, aspect ration, and if mounted on a rim of the correct width, etc. The weight distribution or lbs per sq. inch in the contact patch will decrease on the wider tire due to the wider contact patch.

Mark in Baltimore

What if the tire were 100 yards wide? Would it have the same contact patch/area where the rubber meets the road as a tire that is only six inches wide, diameter of wheel and tire, PSI, tire compound and car weight all being the same?

I don't think so.

Joe Weinstein

A tire is clearly not the theoretically perfectly flexible bladder that elementary
physics requires for a car to be supported only by pressure. A tire is somewhere
between the perfectly flexible balloon and a solid block. Elementary physics
would say that a solid block's contact patch would grow linearly with an increase
in block size. Particularly for current stiff-walled tires, the contact patch can be
significantly the same in size for a range of pressures, and can certainly be bigger
with a bigger tire.

Bull

Given the usual "all other things being equal" part of this argument inducing question, which all other things never are, and given the restrictions of the possible tire sizes that any particular car can utilize, the reality is that the differences in contact patch are very small, if there is any. The important issues that come into play with different size tires are the differences in the shape of the contact patch, as well as those many variables that are existent with different tire sizes....sidewall height, etc.

Paul 996

of course the contact patch gets larger for a wider tire/wheel. It's the pressure across the contact patch that decreases with a wider tire/rim since the weight of the vehicle is supported by a larger area you get a lower lbs/sqr inch.

Bull

Are you suggesting that the fore-aft size of the contact patch remains the same while the width of the patch grows with a wider tire, all other things being equal?

John Welch

I would agree that the contact patch gets WIDER with a wider tire, but I'm not so sure the total area will be much different...

Veloce Raptor

...

BostonDMD

I am no physicist, but this is a no brainer.....

otherwise one size would fit all.......

Gary R.

I'm with you Mark, and the laws of physics hold true regardless of scale.

deep_uv

Geez, with a wider tire on a wider wheel, of course it's bigger. The real question is what about a wider tire on the same wheel?

Steve