hydroplaning

[Dan Masi]

> > 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
> lift?!

Oh.  So, at what point does the tire pressure suddenly stop
being related to contact patch area and load, then?  If you 
don't get this, consider it another way; build your car on
the wheels, then measure the tire pressure.  Now take the
car apart and build it again on the same wheels/tires, but
out of lighter material.  Keep doing this until the material
becomes infinitely light....

> > 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

I meant the *assumption* that the tire pressure was related
*only* to the contact patch area and the load on the tire, not
the definition of pressure as being a force per unit area.

> > 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.

Oh, okay.  Thanks.  How about this, though:

The tire is supporting the car.

Here's another thought that may help.  Imagine taking a tire 
and cutting it into, oh, quarters.  Take one of the quarter
sections and rest a book on the top edge of the sidewalls.
Is it supported?  Sure!  Ok, how about a 30-lb. iron block?
Sure.  Ok, now ask a 300 lb. guy to balance on one foot on
the sidewalls.  They may buckle;  but you could keep them
from buckling, by applying a side force to those sidwalls
using your fingers.  How much force would you need to apply?
Depends on the construction of the sidewall, right?  Think
about it.

-dan