on 5/16/2009 4:19 PM (ET) Pat wrote the following:
On May 16, 9:31 am, RicodJour wrote:

On May 16, 7:37 am, wrote:


On May 15, 12:26 pm, RicodJour wrote:

Technical jargon? The only term Pat used that you didn't is unladen.

I for one don't understand what Pat was saying either. Aside from
dragging tire pressure into the picture, I don't see how tire pressure
actually got factored into contact area of the tires on the ground to
give any more realistic an estimate of the effects. Pat just made
another guess as to the exact area. Like Steve, I can tell you a
truck with 5 tons of load on a wet lawn isn't a good idea.

Let's face it - no one understands what Pat is saying! Sorry,
Pat.

I was merely referring to the "technical jargon" comment. Assuming
that the truck's tires are within the normal range, there's not going
to be a big effect, and it certainly won't have nearly the effect of
the difference between a wet and dry lawn.

R


Hey, hey. Just cuz I'm a hick from the sticks ...

Tire pressure has EVERYTHING to do contact area.

Take your car, for example. We'll use your car instead of my because
mine is sort of messy right now.

Let's say your car weighs 6000 lbs (just a random number).

More unrealistic than random. A 3 ton car? The military Humvee is only
5500 lbs.

It
probably has 4 tires inflated to 30 lbs of pressure (okay, it should
be 32 but you've been slacking off and haven't checked your tires
lately). For ease, let's say the weight is distributed evenly so each
tire holds 1,500 lbs. Got it so far? Rico, I know you do.

Each square inch of tire is literally holding 30 lbs. That's why your
tires are at 30 lbs per square inch. So if you take the 1,500 lbs
at 30 lbs per square inch, then you have to have 50 square inches of
contact space with the road. If your tire is 10" wide, then you have
5 inches on contact, front-to-back.

Oh, but you didn't notice that one tire has a leak. The pressure has
dropped to 20 lbs. How did you notice the leak? It looks "flat" and
has more front-to-back contact with the road. So now the 1,500 lbs is
supported at 20 lbs per square inch. Now it needs 75 inches of
contact with the road. At 10 inches wide, you now have a 7.5" contact
area. The flat look is just spreading the weight over a large area
due to lower PSI.

This also explains why your wife's smaller car has smaller tires. If
her car weighs only 2500 lbs, then she needs only 625 lbs per tire.
Therefore she needs only 20.8" (~ 20") of contact with the road. If
she had the same 10" wide tire, it would have only 2" of contact,
front-to-back. It would look funny, so the tire is made narrower, to
say 6". This gives 3.33 inches of contact, front-to-back.

Some people believe that if you want more traction in snow, you should
let out some air from your tires. I don't think it really works, but
it will give you more contact with the road because of the lower PSI.

Finally, this also explains why your little, tiny donut spare has 50
psi in it. That's how they get it small.

Trucks carry heavier loads. Therefore they use higher inflation in
their tires. Otherwise the tires would have to be huge (probably
really wide).

Here's a corollary to think about. If you know (or can measure) how
big the contact area between your car/truck is, and if you know what
the tires are inflated to, then you can simply calculate the weight of
the truck (without using a scale).

How this help explain the relationship of tire pressure to contact
space and therefore the amount or ruts in the lawn.



--

Bill
In Hamptonburgh, NY
In the original Orange County. Est. 1683
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