[Land-speed] Contact patch size

[joseph lance]

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 at nancyandjon.org>
To: "autox List" <land-speed at 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