"James Wiehe" <j.wiehe@sympatico.ca> wrote:
> lets say your truck weighs in at 24,000 lbs., it rides on 10 tires
> therefore each tire supports 2,400 lbs. If your truck has 6 tires
> than each tire supports 4,000 lbs. [...] Each tire where it meets
> the ground, ( footprint) ? uses X number of square inches. So lets
> say your 10 wheeler @ 2,400 lbs. per tire with a footprint of 10
> square inches now has a footprint of 240 p.s .i.
That would be true if the tires had a contact area of 10 square inches
and were rigid, so that the contact patch area would remain constant
regardless of the load. However, the tires are pneumatic, and the
upward pressure exerted by the ground (2,400 pounds) must equal the
pressure exerted by the air on the inside of the portion of the tread
that's in contact with the ground. Thus, the tire will flatten out
against the ground until the contact area is 2,400/50 = 48 square
inches. If you increase the load, the tire will flatten more, and if
you decrease the load, the tire will un-flatten and become more
rounded. Changing the air pressure will have the same effect. As a
sanity check, an 8" by 6" rectangle would have an area of 48 square
inches, and that seems like a believable contact patch for a deuce
tire to me.
Now, that's an approximation, because an uninflated tire will support
its own weight without flattening down to the rim because its
sidewalls are somewhat rigid. However, I think that would be a fairly
minor effect, because once it's on the truck the load will be much more
than enough to flatten the tire down to the rims. I also assume that
the tires aren't one of those wacky runflat tires that support a
vehicle's weight without going flat, and without a runflat assembly
like the HMMWV uses. I remember a picture of a tire with a big segment
cut out of it in Popular Science many, many years ago... my theory
wouldn't apply to that kind of tire. I think. :-)
Another way of looking at it is to figure that if the contact area was
really 10 square inches, and the width of the tread in contact with
the ground was about 5 inches (a guess that seems reasonable for a
deuce tire), then the length of the flat spot would be about 2 inches.
My deuce's tires are a lot flatter than that!
> p.s.i. Another way of looking at would be to take a M-48 Battle Tank
> with its large track sitting on the ground only uses 11.87 p.s.i. of ground
> pressure. If anyone can explain it better please do.
A tank is a different beast... it doesn't have pneumatic tires, and
the contact area remains constant regardless of load. I've been told
by a tanker that the ground pressure ratings for tanks are computed
under the assumption that the contact area is the width of the tread
times the length of the flat portion in contact with the road. The
actual contact area is much, much smaller because the load is
supported by the cleats, and thus the ground pressure is much, much
higher on hard ground. Plus, that neglects the lower pressure that
will be asserted by the links between the road wheels, compared to the
links directly under the road wheels. That's why that "12 PSI" M48
will sink right into asphalt, while a 50 PSI deuce (or even an 80-100
PSI semi) usually won't... the tank's actual ground pressure on firm
ground is much higher than the roughly 50 PSI exerted by that deuce.
With a pneumatic tire, I believe that the tire will flatten until the
area of the rubber actually in contact with the road times the tire
pressure equals the load on that tire.
Is anybody sufficiently motivated to put it to the test? If somebody
has the means to measure the actual axle weight of one axle of their
vehicle (it could be the front axle to keep things simple), and then
they jacked up one wheel, put some paint or ink on it, lowered it onto
some cardboard, and then jacked it back up to remove the cardboard,
they could then measure the actual contact area and compute the actual
wheel load. I think the computed actual ground pressure will be pretty
close (on the high side) to the measured tire pressure in that tire.
-- Mark J. Blair, KE6MYK <mblair1@home.net> PGP 2.6.2 public key available from http://www.keyserver.net/ Web page: http://www.qsl.net/ke6myk/ DO NOT SEND ANY UNSOLICITED COMMERCIAL EMAIL TO THIS SITE
This archive was generated by hypermail 2b29 : Sun Dec 03 2000 - 20:29:52 PST