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## Re: Going to look [now tire contact patch]

 To: "wizardz" Re: Going to look [now tire contact patch] Max Heim 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 "long". 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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