At 12:58 PM -0400 4/6/08, LouDeeter@aol.com wrote:
The writer said that tire patch as measured in
>area is weight of the vehicle divided by pressure of the tire. I'm not sure
>whether you have to take into account the number of tires in that equation.
Not for the total contact patch, but of course, with more tires, that
patch will be distributed over more tires. That is why semi-trucks
have more wheels than cars.
> I
>found it interesting in that intuitively, if I'm riding a 700x28 at 100 pounds
>I would think that I have more tire patch than if riding a 700x20 with 100
>pounds.
You don't - the contact patch is the same. If you weigh 200 lbs., and pump up your tires to 100 psi (pounds per square inch), then your contact patch will be 2 square inches (200 divided by 100), or 1 square inch per tire. No matter the width of the tire. (Assuming a 50/50 weight distribution and a flexible tire.)
The shape of the contact patch can vary. That is the reason why in theory, wider tires have lower resistance: The size of the contact patch is the same, but the shape is rounder with wider tires. Check the article on "The Physics of Rolling Resistance" in Bicycle Quarterly Vol. 5 No. 1 for more on this issue.
Jan Heine
Editor
Bicycle Quarterly
140 Lakeside Ave #C
Seattle WA 98122
http://www.bikequarterly.com