Jon;
You're math sounds okay, but try a simple experiment to test it with a
vehicle and tire of your choice. Jack it up on one corner, paint a section
of the tire with a sticky white paint, very slowly lower it to the ground
for a minute, jack up the vehicle again, then measure the area of that ugly
spot on your garage floor. Repeat for different tire pressures and make big
polka dots all over your garage floor.
You'll get slightly different results depending on the construction of the
tire even with the same tire aspect ratio but the above experiment is
probably to crude to measure any such difference. Your analysis assumes that
a tire is like a special kind of balloon that stretches in only one
dimension.
It also implicitly assumes that a bigger contact patch means better
traction. Can't help remembering that a series 70 tire will start
hydroplaning at a MPH = 9.8 x the square root of the inflation pressure. A
series 60 tire inflated to the SAME pressure will start hydroplaning at a
lower MPH (even though the contact patch has a larger area) because the psi
between the road and the tire patch is lower. Based on what some of the guys
say about the salt, I think that accelerating on the salt must be more akin
to a wet road than a sticky asphalt surface, so a bigger contact patch by
itself does not necessarily mean more traction for a fixed vehicle weight.
In any case, a static analysis to define the contact patch with the vehicle
standing still is way different than the area of the contact patch and the
contact psi you'll get at speed with high RPM centrifugal force acting on
the OD of the tire.
Lance
----- Original Message -----
From: "Jon Wennerberg" <jonwennerberg@nancyandjon.org>
To: "autox List" <land-speed@autox.team.net>
Sent: Monday, November 17, 2008 8:45 AM
Subject: [Land-speed] Contact patch size
>I posted this on the landracing.com forum -- and I'll try it here, too, to
>see if there's a good answer to the quandary.
>
> "I've got this nagging thought about the size of the contact patch -
> maybe someone will tell me if it's right or wrong.
>
> That is, the contact patch -- the number of square inches of tire that's
> contacting the ground -- would be a function of tire pressure and the
> weight that's on those square inches. As an example: 40 psi and 1000
> pounds would need 25 square inches of contact patch. A five inch wide
> tire would need to have five inches on the surface -- a ten inch wide
> tire would need 2 1/2 inches. The weight/pressure would dictate the
> amount of rubber in contact with the ground, not the width of the tire.
> Want more contact patch? Run lower tire pressures, not different tire
> sizes.
>
> Okay -- that's the math. If it's not the way it really works -- why
> not?"
>
>
>
> Jon Wennerberg
> Tall guy with moustache
> and a pair of 2 Club hats
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