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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.
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.
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.
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.
Running your tires under or overinflated beyond a few PSI is a bad idea and
not terribly safe nor good for the tire. Buying fuel economy at the cost of
ending up in a ditch during a snowstorm or wet road conditions isn't smart.
> At 1:23 PM -0500 1/13/09, Tom B wrote:
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
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
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
Hard acceleration and braking will wear them faster.
> On Tue, Jan 13, 2009 at 11:23 AM, Tom B <kaladorn@gmail.com> wrote:
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