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

At 1:23 PM -0500 1/13/09, Tom B 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.

Ahh, But, temperature of tires ALSO varies with pressure. In racing or
high speed applications, you'll get more or less stick based on the
pressure. Lower pressure, higher temperature (more flexing of the tire
casing/sidewall) and the compound get softer.

>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.

Hopefully this is front to rear and not side to side? Differnet tires of
EXACTLY The same type may have different shapes. I've seen 1100x20 inch
NDTs on a dual setup on an M35 that rubbed (the dual tires on the same
axle rubbed their sidewalls which is BAD!). The same spec of tire by a
different manufacturer rubbed. Airpressure wasn't the issue.

Cord/Sidewall/Casing/belting construction of different makes of tire
will vary, even for the same function.

>3) I have driven big trucks - 24' ban body 6-wheeler with a diesel.
They come with adequate mirrors and huge windscreens with wide
visibility a lot of the time. I suspect most armoured vehicles have far
more limited viewpoint, which is why large armoured vehicles trying to
manouver through tight urban spaces = damage to urban spaces. A Weasel
or some of the smaller vehicles are a horse of a different colour.

Trucks are easier because you ARE up high. Tanks, not so much, you're
way out front.

Drive a carrier some time. you're down inside, and you can see forwards.
The radiator is over your left shoulder so you can't hear much either.

Jim had FUN with this....
http://picasaweb.google.com/renactr2/BrenCarrierLoading#5112134855710143
330

This event was fun too.
http://picasaweb.google.com/renactr2/15RecceCarriersInAction#51124577067
19939810

>And to all you 'higher pressure is good' folks: That's only one side of
the story. Car tires  have optimal ranges based on traction requirements
and tread pattern and they also account for composition of the tire. If
you over inflate for better gas mileage more than a few psi, you can end
up creating a safety risk. Certainly, I never ran my Z-rated radials at
overpressure - I bought Z-rated radials because I wanted the maximum
stick that non-slick drag tires could provide. When I was cornering at
180 kph plus, I wanted every bit of stick to the track I could get. I
never hit 240 kph, but I had an (unconfirmed) feeling that a Z-rated
radial had more traction at all speeds than a T-rated, for instance.
That's why I bought Z.

You also probably have softer compound tires and they'll probably wear
faster. I run my low rolling resistance tires on my Honda Insight at a
higher than standard pressure. Less rolling resistance and I don't drive
it like it's stolen. So far I've not had any adhesion issues. It DOES
help that the car weighs all of under a ton wet. I'm still in the range
for the tire manufacturer's specs.

>
>Wear numbers like 220 suck for most people for replacement cycle (25K
km was enough on some camaros and corvettes), but I had 260 and never
had a problem with my tired - I'd get 65K-70K km on them. Mostly I
didn't drive like I was on the way to a fire, just once and a while and
then I wanted sticky. I wonder what the wear number equivalent would be
on tank rubber pads? I'm guessing they're pretty hard, although less
hard than asphalt and probably ablate thus saving the asphalt.

Hard acceleration and braking will wear them faster.

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