Huw Powell audi at mediaone.net
Wed Jan 31 13:23:32 EST 2001

Dan Masi wrote:
> > wider tires = wider but shorter contact patch.
> >
> > until you are riding on the rims, your "weight per square
> > inch" is what
> > you measure with your tire pressure gauge...
> A simple thought experiment might convince you that it's not
> quite that simple.
> Drive your car over a lift, put gauges on the tires, and
> observe the pressure.  Now operate the lift enough that it
> takes just some of the car's weight.  What did the pressure
> do?  Ok... now take even more of the car's weight on the
> lift.  What's the pressure doing?  Ok... now take enough
> weight on the lift so that the tires are just brushing the
> pavement.  Zero psi???  'course not. 

dum de dum... that's because some of the weight is being held up by the

*when* the tires are on the ground (and inflated) their total contact
patch area is determined by the weight of the car divided by the PSI of
the tires.

> Here's another thought.  My car has, say, 800 lbs. supported
> by each tire.  Using the simple load x area = pressure
> assumption,

that's not an assumption, that's a definition

> that'd be a contact patch of 20 sq. in. at 40 psi.
> For 8" wide tires, the rectangular length of the patch would
> be 2.5".  Ok, sounds fine.  Now reduce the pressure to 10 psi.
> The expected length would now be 10".  Not likely.  But, ok,
> reduce the pressure to 5psi. I guarantee you won't see a 20"
> long contact patch!

the formula works until you are either riding on the rims or, slightly
before that, the crumpled up sidewalls.

> The problem with the simple load x area assumption is that
> it implies that the tire doesn't exist.  The air is *not*
> supporting the car.  The *tire* is supporting the car, and
> the air is giving the tire it's shape.

the air is supporting the car.

Huw Powell



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