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Re: Going to look [now tire contact patch]

To: "wizardz" <>
Subject: Re: Going to look [now tire contact patch]
From: Max Heim <>
Date: Mon, 20 Dec 1999 16:40:57 -0800
I am also getting tired of this discussion, but one more time...

wizardz had this to say:

>In the analogy (below) given... you used two different
>balls (round example) then you compared patch shape change using a football.
>Unless you have a WIDER football to compare (like the round balls)
>your analysis is void and incomplete.

No, I did not. I attempted to demonstrate using a spherical ball that the 
only factors that affect contact patch area are 1) weight (hand force); 
and 2) air pressure. I brought in the American football to demonstrate 
that the shape of the ball has no effect (round or oval). I meant this to 
be analgous to the width of the tire (round ball: "length" of patch = 
"width" of patch; oval ball: "length" of patch different from "width" of 
patch, choose your direction). What the heck you mean by a "wider" 
football I cannot imagine. But volleyball, basketball, beachball, it 
makes no difference. Size does not matter. 
>A wider tire gives a larger contact patch as compared to a tire
>of similar circumference (not OD...circumference) that is narrower... period!
Wrong. Period. (BTW diameter is directly proportional to circumference so 
I don't understand your distinction, not that I ever mentioned diameter)

Let's resort to mathematics, then, if that's what it takes.

It is convenient that the weight distribution of the car on its contact 
patches can be expressed as pounds per square inch, the same term we use 
for tire pressure. This is not a coincidence. To keep things simple, 
let's use 1/4 the weight of the car and 1 tire as a stand-in for the 
whole setup. For our 2000 lb MGB, that gives us 500 lbs.

For a tire pressure of 25psi (a little low, but convenient), the formula 
for the size of the contact patch is as follows: 25 lbs/1 sq. in. = 500 
lbs/"X" sq. in., or 20 sq. in. With a typical  MGB tire, that gives you a 
patch about 5 inches wide by 4 inches "long", possibly a little flat 
looking but the pressure *is* a little low. With 30 psi you get a 16.6 
sq. in. patch, which is perhaps more typical. Since the tire is still 
about 5 inches wide, the patch "length" is now only 3.3 inches. I think 
everyone is familiar with this phenomenon, that an underinflated tire has 
a "long" flat spot, and an overinflated tire has a smaller-looking flat 
spot where it meets the road, when viewed from the side. Conversely, if 
you leave the inflation pressure the same, but add 300 lbs of passengers 
or sandbags, you will observe a similar flattening effect, lengthening 
the contact patch and thus increasing the contact patch area.

Note that this formula includes NO variables for tire width, 
circumference, or aspect ratio. That is because they DO NOT MATTER. The 
size of the contact patch is entirely determined by vehicle weight and 
tire inflation pressure. If our MGB in the example above were to mount 7 
in. wide racing slicks, also inflated to 25 psi, the contact patch would 
STILL be 20 sq. in. -- it would just be 7 inches wide and 2.8 inches 

Can you grasp this? The inflation pressure of the tires "resists" the 
flattening effect of the weight of the car. If the car weighed nothing, 
the tire would be perfectly circular and the contact patch would be an 
infinitely thin line the width of the tread. If the tire were completely 
devoid of air pressure, the entire lower surface of the tire would be 
perfectly flat (obviously), and the weight of the car would be resting on 
the wheel rim. Within the tire's design parameters, the tire pressure 
balances the weight of the car by supporting the wheel on a cushion of 
air, so to speak. Lbs/sq. in. (tire pressure) = lbs/sq. in. (weight of 
car/surface in contact with ground).

It is the other effects of low aspect ratio construction, NOT an increase 
in contact patch area, that provide the handling benefits of modern wide 
tires. Note that I am speaking strictly of a static situation -- one of 
the benefits of low aspect ratio tires is the maintenance of the size and 
shape of the contact patch under lateral loads.

>Now,.... as to which gives better traction in snow! Neither can claim that 
>title specifically
OK, let's not get into that, then...
>(ROTFLMAO over all this guesstimating being spewed as facts.)

Sorry, I am not "guesstimating" in the slightest. I cited authoritative 
sources (Road & Track, The MGB Survival Guide), and I am sure any vehicle 
dynamics textbook would back them up. At any rate, the math speaks for 
itself. I apologize if my analogies were unclear. 


Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.

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