Rich Grise wrote:

Saturday morning TV is mostly cartoons, yes, but there's one show that's
not a cartoon, but it's still fun:
http://www.beakmansworldtv.com/

It's like Stealth Educational TV - like Mr. Wizard for the 21st century
kid. And they do Real Science.


Hey if someone is getting those little heads of mush thinking abit good for
them.

I'm almost tempted to check the air pressure on my car tires and then jack
it up and check again. It is a no brainer that pressure in tire increases
when a load is put on it since the bottom compresses, the air compartment is
restrained and the car is hanging from the rim.

Wes

Wes wrote:
Rich Grise wrote:


Saturday morning TV is mostly cartoons, yes, but there's one show that's
not a cartoon, but it's still fun:
http://www.beakmansworldtv.com/

It's like Stealth Educational TV - like Mr. Wizard for the 21st century
kid. And they do Real Science.



Hey if someone is getting those little heads of mush thinking abit good for
them.

I'm almost tempted to check the air pressure on my car tires and then jack
it up and check again. It is a no brainer that pressure in tire increases
when a load is put on it since the bottom compresses, the air compartment is
restrained and the car is hanging from the rim.
The change in tire pressure when the tire is loaded is quite
small. maybe a couple PSI at the most. The change in footprint
is large! Take the spare tire and set it on the ground.
generally, it will be sitting on a point smaller than a dime,
assuming a standard radial auto tire with 30+ PSI in it. Then,
when you put the car's weight on it, the footprint becomes a
huge rectangle. The only way I know to accurately measure the
footprint is with pressure sensitive films. They make this
stuff for checking the fit of flanges, cylinder heads and stuff,
it changes color depending on the PSI load on that spot. There
may be a low-tech way to do the same thing, like two peices of
plastic with butter or something between them. The weight of
the car will squish out the filler and make the two plastic
sheets touch. You'd have to jack the car up, put the sensor
film under it, lower the car, then raise it again to remove the
film for measurement.

Jon

Jon Elson wrote:

I'm almost tempted to check the air pressure on my car tires and then jack
it up and check again. It is a no brainer that pressure in tire increases
when a load is put on it since the bottom compresses, the air compartment is
restrained and the car is hanging from the rim.
The change in tire pressure when the tire is loaded is quite
small. maybe a couple PSI at the most.

What if one lowered initial inflation pressure? Seems like variatio from
loaded to unloaded would increase.

Wes

Jack your car up and put a bathroom scale under a wheel. Deflate the tire
and then lower it onto the bathroom scale enough to make the sides bulge out
the same amount as when it's holding up the car. That's how much you have
to add to the pressure x area product. My guess is it's going to be a very
small correction.

To measure the footprint, I would dust a piece of dark colored paper with
flour, and slip it under the jacked-up wheel. Lower it and raise it, and
then take the paper inside to the desk where you keep your planimeter.

Or you could cut out the footprint with scissors and weight it on a delicate
scale.

On Sun, 23 Sep 2007 02:24:14 GMT, Leo Lichtman wrote:
Jack your car up and put a bathroom scale under a wheel. Deflate the tire
and then lower it onto the bathroom scale enough to make the sides bulge out
the same amount as when it's holding up the car. That's how much you have
to add to the pressure x area product. My guess is it's going to be a very
small correction.

Holy crap, Leo, how heavy does your scale go up to?

Leo Lichtman wrote:
Jack your car up and put a bathroom scale under a wheel. Deflate the tire
and then lower it onto the bathroom scale enough to make the sides bulge out
the same amount as when it's holding up the car. That's how much you have
to add to the pressure x area product. My guess is it's going to be a very
small correction.

To measure the footprint, I would dust a piece of dark colored paper with
flour, and slip it under the jacked-up wheel. Lower it and raise it, and
then take the paper inside to the desk where you keep your planimeter.

Or you could cut out the footprint with scissors and weight it on a delicate
scale.


But you have to eliminate the parts of the pattern that the tread
dosent touch. Use a bald tire with no tread OK. :-)
...lew...

On Sep 23, 1:00 am, Wes wrote:
I'm almost tempted to check the air pressure on my car tires and then jack
it up and check again. It is a no brainer that pressure in tire increases
when a load is put on it since the bottom compresses, the air compartment is
restrained and the car is hanging from the rim.

Wes

I doubt if you can detect the difference in pressure on a regular tire
pressure gauge.
The volume changes very little. The fabric in the tire keeps the
surface area very close to a constant and while the tire squishes in
on the bottom, it also bulges out on the sidewall.

Have you ever fixed a flat or mounted a new tire? If you inflate the
tire before mounting and then check it after it is on the car and
loaded, it is still about the same.

Dan

According to Dave Hinz :
On Sun, 23 Sep 2007 02:24:14 GMT, Leo Lichtman wrote:
Jack your car up and put a bathroom scale under a wheel. Deflate the tire
and then lower it onto the bathroom scale enough to make the sides bulge out
the same amount as when it's holding up the car. That's how much you have
to add to the pressure x area product. My guess is it's going to be a very
small correction.

Holy crap, Leo, how heavy does your scale go up to?

He said to *deflate* the tire (presumably with the valve stem
removed), so when you lower the car just enough to duplicate the
sidewall bulge, you are measuring only the force needed to bulge the
sidewalls -- which you need to subtract from the calculated weight on
that wheel.

You don't put the *full* weight on the bathroom scale -- at
least not more than once. :-)

Enjoy,
DoN.
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