Date:         Mon, 28 Jul 2003 14:21:55 -0700
Reply-To:     Pensioner <al_knoll@PACBELL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Pensioner <al_knoll@PACBELL.NET>
Subject:      Tire profiles and pressures
In-Reply-To:  <200307281919.h6SJJ9iI023272@mtaw3.prodigy.net>
Content-Type: text/plain; charset="iso-8859-1"

I went through the tire/tyre thing a few years ago.

Try this.  Measure the loaded weight of your van front/rear.  You will now
know the "per axle" load.
Using this data on a per wheel basis multiply by 1.1 or 110% ( pensioner's
tolerance factor).
This will give you a "normal load" per wheel.  See how close to the maximum
cold pressure rating shown on the tire this "normal load" is.  EG, If the
rear per wheel normal load is 1500 lbs and
the tire is rated for 1750 lbs at maximum cold inflation pressure then you
have a margin of 250 lbs before you overload the tire at max cold pressure.
If the tire is rated at 1450 lbs at MCIP then the tire is insufficient for
the load at any pressure or max pressure.

The inflation pressure is optimum when the tire flex meets your comfort
requirements, handling requirements and tire heating parameters.  The
heating of the tire during use is a primary cause of "ply separation".  The
"stiffer" the tire the less it will flex and hence for a given tire
structure the less it will heat under use.  A larger structure tire with a
larger footprint will require less inflation pressure to support a given
"normal load" and allow a greater margin than a similarly rated tire with a
smaller footprint.  Inflation PSI times the footprint area in square inches
will always equal the normal load.  So if you know the load and the
inflation pressure you can calculate the footprint/ground pressure.  To
increase the footprint you can lower the pressure for mud, sand, lagoons of
swine manure or whatever you must travel upon.  To reduce the tire flex you
can raise the pressure until either it's stiff enough for BenT Tuna or you
approach the MCIP.

I run 215/75-15 (1765 Max Load at 55 MCIP) at 45psi for a fully loaded
syncro westie with an additional 200 lbs of engine weight.  This works for
me.  Derek, dabbling through the mud with similar tires could run 30psi for
traction via low ground pressure and larger footprint.

Remember, vanagons are light trucks and should be treated as such, IMNSHO.

p