Re: [Phys-l] Another tire question

[Brian Whatcott <betwys1@sbcglobal.net>]

At 11:28 AM 11/6/2007, you wrote:
> Looked through the archives and Googled this one, but didn't find a
> definitive answer.
>
> How does a tire actually hold up a car?
>
> At the Way Things Work site they talk about the footprint area of the tires
> times the pressure equalling the weight of the car.  However, if I look at
> the forces:  The road pushes up on the tires, but the tires must push up on
> the wheels--with a force equal to the weight of the car (more or
> less--depending on if you count the weight of the tires themselves).   The
> two factors I see as possible are 1)  A pressure difference between the top
> of the wheel and the bottom of the wheel--giving a net upwards force.  Can
> the pressure difference be enough?  The effective area of 4-wheels is under
> 500 square inches--so you'd need a 4 lb/in^2 differential to support a 1 ton
> car.  The other--and I think this is the answer but need confirmation--is
> that most (if not all) of the weight is supported by the sidewalls of the
> tires which are stiffened by the air pressure.
>
> Still the air-pressure times footprint area plays a roll--partially deflate
> the tires and the footprint increases--but what about the sidewall forces
> then?
>
> A bit confused on this.....
>
> Rick
>
> ***************************
> Richard W. Tarara


Let's wave arms. An inflated torus is intended to enclose a maximal
volume of pressurized air. The torus seals on a metal rim.
 When the torus is pushed from one direction, it pushes back while
locally deflecting.  In this situation, the pressure exerted by the
 tire beads on the wheel rims   varies around the bead circumference,
less on the opposite rims, more on the adjacent rims.   Though the total
internal pressure  rises only by the inverse ratio of the small reduction
in volume that the external force footprint on it secures.
  The tire stiffened pneumatically acts rather like a  solid rubber torus
The force is transmitted over a large portion of the tire's surface.


Brian Whatcott    Altus OK    Eureka!