[Land-speed] Contact patch size

[Jon Wennerberg]

On Nov 17, 2008, at 7:48 PM, Benn wrote:

Awright, my 2 cents:
If your tire were a 1-mil-thick balloon, then the approach strictly  
using
tire pressure might be appropriate.  But tires aren't balloons--e.g.  
"run
flat" tires can have zero pressure and still have close to the  
original tire
patch.  Obviously stiffer (and/or shorter) sidewalls are required for
run-flats (I don't think they make any 70-series run-flats), so the  
sidewall
stiffness has a LOT to do with it.  And said sidewall stiffness will
therefore influence the contact patch....I'd be v. surprised if you  
found a
tire that had anything approaching a linear relationship between  
pressure
and contact area, unless you're looking at a v. small range.
Benn
opinion worth what you paid for it, and two cents ain't much




Okay, Benn-- you got me there.  I have been ignoring the amount of  
load that is supported by the sidewalls.  I had assumed it was  
negligible (and, for that matter, I thought run-flat tires have a  
solid filling rather than using just the strength inherent in the  
sidewall construction to support the load.

And yes, I admit that the 1-mil balloon is close to an ideal tire for  
the sake of this discussion.  But -- compared to a fullly-inflated  
tire -- how much load will an empty tire support?  Is it enough to be  
a real factor in this?  If the tire can support 1,000 pounds at proper  
inflation and 50 pounds when at atmospheric (hey, just a wild-a**  
number thrown in for the sake of the discussion) -- you're talking a  
five percent error in the contct patch/tire pressure equation.   
Significant -- but not all that much in the grand scheme of things.

       Jon Wennerberg
Tall guy with moustache
and a pair of 2 Club hats