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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