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