Could it be the difference in the vertical forces exerted by the sidewalls? I'm thinking of a bicycle and the same question: what holds it up? The spokes are all under tension so they are all pulling on the hub. It must be that the upper spokes are pulling harder than the lower spokes, with the difference equalling the weight of the bike frame and rider.
-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Rick Tarara
Sent: Tuesday, November 06, 2007 12:29 PM
To: phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] Another tire question
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
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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN
rtarara@saintmarys.edu
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