Re: [GZG] GZG] AFV ground pressure ( was Re: New to the list, and 2

On Tue, Jan 13, 2009 at 11:23 AM, Tom B <kaladorn@gmail.com> wrote:
> 1) I know tire pressure varies with temperature. As does the ability
of the
> tire to retain air usually. I live in Canada and get seasonal
demonstrations
> of this.
>
> 2) My argument was that my car has had two differing types of the same
size
> tires (same rolling diameter), both of which were Z-rated performance
> radials, and the difference in recommended pressure was 28 psi to 35
psi.
> That suggests about a 20% change in contact patch. I'm skeptical that
this
> much of a change is made by that extra inflation. Tread pattern may
well
> impact this - one of the tires was a directional asymetric tire, the
other a
> non-directional asymetric tire.

It is actually less than twenty percent, as that is guage pressure,
not absolute pressure (need to add in ambient atmospheric pressure
[about 14.6 psi absolute]).   The reason for the range of tire
pressure recommendations is the trade-off of fuel economy and comfort.
 The size of the contact patch is the vehicle weight divided by the
ground pressure.  If you are on slicks (balloon tires with no tread),
the ground pressure is the tire pressure.  If the contact patch
multiplied by the ground pressure does not equal the vehicle weight,
the vehicle is accelerating in the vertical.  In deference to
Professor Phil Eastman, author of the Sir Isaac Newton Physics
Competition, I apologise for not starting this paragraph with "Draw a
free body diagram!".  Remember that the whole point of putting tracks
on a vehicle is to give it a positively enormous contact patch.  A
tank may have a fifth of your car's ground pressure, despite having
sixty times the mass, merely by having three hundred times the contact
patch.

Notes:	1)  the ground pressure is an average!	Even if the scorpion
has less ground pressure than a man, when a roadwheel passes over your
toes, they will likely be crushed.

2)  Now that I have thought about it for a few moments, the road
pressure of a tracked vehicle is significantly higher than its ground
pressure.  Roads do not flex to conform to the track, so the vehicle
weight is only born by the roadwheels-- driving a tank on black ice
must by a nightmare.  As a lot less track is in firm contact with the
road, they are harder to steer and brake, without exceeding their
traction limits.  They are probably even worse on cobblestone roads
and might hydroplane easier than anything else we are likely to
drive(unless you have driven indoor forklifts in a warehouse with a
leaky roof).  Tracked vehicles are not designed for roads.  I welcome
someone who actually knows telling me I am wrong.

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