hydroplaning

[Dan Masi]

> 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.  In fact, probably little
difference from the first measurement.

Here's another thought.  My car has, say, 800 lbs. supported
by each tire.  Using the simple load x area = pressure
assumption, 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 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 sidewall at
6 o'clock is supporting the wheel from underneath, the sidewall
at 12 o'clock is pulling the wheel upwards.  The air inside the
tire is supporting the shape of the tire.

Dan Masi