Re: Hydroplaning (long)

[Don Hoefer <hoefer@eng.pko.dec.com>]

Chris,

You have the answer right here:

> I'm not convinced that tire pressure is directly related to downward force per
> unti area.
> 
> But:  tire pressure does play a part in contact patch size, thus, it does
> indirectly relate to downward force per unit area.   However, DFPUA is 
directly
> related to contact patch size, no?

The DFPUA at a tire is the result of the load (weight) on the tire
divided by the contact patch.  What determines the area of the contact
patch?  When loaded the tire will collapse until the force that it
produces equals the load.  This will be produced by the air inside and
whatever resilience the tire has by itself.  The tire itself is probably
good for 100 lbs or so (sit on one and see if you can flatten it when it
is empty of air).  A car is very heavy so most of the force is given by
the air inside, seeing as the tire will be out and out flat if not
inflated.  If the tire is inflated with air, to say 30 PSI, that is the
maximum force per unit area available that it can produce.  The volume
of the tire never changes so (ignoring temperature) the pressure never
changes.  If the tire is loaded, with say 900 lbs., it will collapse until
the contact patch is 30 square inches, or about 5.5 by 5.5 inches (if
square).  The FPUA produced by the tire cannot exceed the internal air
pressure and it will not be less given that the tire is flexible.

If the tire were a solid piece of material then the contact patch would
be constant and the DFPUA would be proportional to the weight of the car.
Because the tire is flexible the contact patch will change as the load is
changed.  The contact patch is proportional to the air pressure inside.

While this is not a scientific proof, I hope that it helps to explain
that the DFPUA is determined by the tire's internal air pressure.  See
any comprehensive automotive text to read more about tires and their
dynamics.  As a further point of thought, consider a tractor trailer.
It can weight more than 20 times what a car can, yet it has fewer than
80 tires.  It's tires are bigger and are probably inflated to at least
60 PSI.  Twice the pressure with twice the contact patch gives four times
the load capacity per tire.  Now that truck will only need five times
as many tires instead of twenty.  In reality they have 18 tires, so the
actual contact patches are a little bigger.

Don Hoefer
'82 Coupe

Massachusetts, USA