tire pressure puzzler

ckissick · 05-11-2009, 04:56 PM

I was talking to a friend about systems to pump up your tires on a 4X4 after four-wheeling. We've seen small air compressors that attach to the car battery, but they fill the tires very slowly.

Someone told my friend if you get the car's weight off the tires, they will pump up much faster. True? Discuss.

kaisen · 05-11-2009, 05:01 PM

Quote:

Originally Posted by ckissick

We've seen small air compressors that attach to the car battery, but they fill the tires very slowly.

100-120psi reserve tank so compressor can work slowly to refill it, but tire-fills will be fast

Zeke · 05-11-2009, 05:29 PM

If that were true, the tire pressures would go up when you set the vehicle back down on the ground.

Who tells your friend

this stuff?

ckissick · 05-11-2009, 06:52 PM

Quote:

Originally Posted by milt

If that were true, the tire pressures would go up when you set the vehicle back down on the ground.

Who tells your friend

this stuff?

Exactly what I was thinking. I have my doubts.

jyl · jyl's Garage

Just make sure to change the air in those tires when it goes stale.

dad911 · 05-12-2009, 12:07 AM

If you want to fill fast, use a nitrogen tank with a regulator.

legion · 05-12-2009, 04:55 AM

Yes, it will fill faster off the ground as the compressor does not have to work against the vehicle weight pressing down on the tires. But, it has already been mentioned that the tire pressure will go up once the vehicle is set down...

Zeke · 05-12-2009, 06:41 AM

Quote:

Originally Posted by legion

Yes, it will fill faster off the ground as the compressor does not have to work against the vehicle weight pressing down on the tires. But, it has already been mentioned that the tire pressure will go up once the vehicle is set down...

zing.... high and wide.

71T Targa · 05-12-2009, 06:48 AM

Quote:

Originally Posted by legion

Yes, it will fill faster off the ground as the compressor does not have to work against the vehicle weight pressing down on the tires. But, it has already been mentioned that the tire pressure will go up once the vehicle is set down...

What???

So, what pressure should I keep my spare so that it's 35 pounds when I need to use it???

legion · 05-12-2009, 06:59 AM

Try this:

Blow up an air mattress with your mouth. Then deflate the air mattress and have your friend lay on it, then try to blow it up again. Is it harder the second time?

Similarly measure the air pressure of an air mattress. Then have your friend lay on it and measure the air pressure again.

T77911S · 05-12-2009, 07:39 AM

with this crowd, i know someone is out there jacking their car up as i type to check the tire pressure to see if it changes.

71T Targa · 05-12-2009, 07:44 AM

Quote:

Originally Posted by T77911S

with this crowd, i know someone is out there jacking their car up as i type to check the tire pressure to see if it changes.

If I were home I would be.

ckissick · 05-12-2009, 07:48 AM

I did a little research and found out some interesting things.

First, you can measure the weight of your car based on the psi in each tire. You measure the "contact area" (in square inches), or the area of the tire that is in contact with the ground. You multiply this by psi to get pounds for each tire, then you add up all tires. This confirms a direct link between tire pressure and weight from the car.

But, I also read that the tire will pump up faster when not weighted, although the difference is negligible. The contact area compared to the total surface area of the tire is too small to make much difference.

It would be interesting to jack up the car and see what the difference in psi is.

71T Targa · 05-12-2009, 08:22 AM

Quote:

Originally Posted by ckissick

First, you can measure the weight of your car based on the psi in each tire. You measure the "contact area" (in square inches), or the area of the tire that is in contact with the ground. You multiply this by psi to get pounds for each tire, then you add up all tires. This confirms a direct link between tire pressure and weight from the car.

I'm having a hard time getting my arms around this one.
So if I'm running tires that are 10" wide @35psi my car weighs more than if I'm running tires that are 6" wide? The tire diameter stays the same, so the front to back contact patch is the same. The only change is the width of the contact patch.

Lets break it down. Assume for a minute that the front to back distance of the contact patch is 4". As the tire diameter doesn't change this will be a constant.

So with my 10"x4" contact patch of my 10" wide tire, I get a contact patch of 40". This times 35psi = 1400 pounds per corner. Or a 5600lb car.

Now I change to my 6" wide tire, and get a new contact patch of 24" (6" * 4"). A 24" contact patch @35psi = 840lbs per corner or a 3360lb car.

gtc · gtc's Garage

I inflate my tires whenever I'm up in the mountains. The air is thinner at higher altitude, so my wheels weigh less which in turn improves handling becuase of lower unsprung mass.

Hugh R · 05-12-2009, 08:35 AM

71T

No, regardless of tire width, at a given PSI, the contact patch is always the same # of square inches. Just a different shape. If you have a 6" wide tire or a 10" wide tire, at 35 PSI with say 700 pounds of load on the tire, you'll have 20 sq. inches in either case.

If you put 35 PSI in a tire off the car, then put the tire/wheel on the car, the PSI of the tire remains the same, the contact patch changes from say a loading of 35 pounds of the weight of the wheel/tire and 1 sq. inch of contact to 20 sq. inches with a 700 pound load.

Zeke · 05-12-2009, 08:39 AM

The tire deflects when on the ground, both out to the sides at the bottom and I suppose a little deformity elsewhere. On the old bias ply design, the overall diameter may change a fraction, I don't know. The air does not compress that much more and therefore the pressure does not go up by an amount that would make airing the tire more difficult. What I do know is that I don't see any racers making sure there tires are on the ground before adjusting pressures. Taking temps, yes.

To fill a tire faster, you need air volume. If you had a 1" opening between your charge vessel and the tire, 2 pounds difference would equalize almost immediately. If you are going to use the valve stem, it is extremely constricted with the valve inserted. That's why tires shops initially air up a tire with the valve out of the stem. They need the volume to air up more quickly and seat the bead.

If you have a large pressure difference between your filler air and the tire, more air is forced by the valve. You can see how this works if you start with a vessel charged to 40 lbs. and use it to air up a tire to 32. As they become closer to being equal, the volume becomes low. Increase the opening and, as I said, the air will move between the 2 faster. Or increase the pressure from the air source.

Having the car on the ground has nothing to do with it in practical form. I'm not saying that scientifically one can't measure the weight of an object by taking pressure differentials laden and unladen, but I am saying one won't notice any difference in the speed in which air is transferred until the volume of air transfer is allowed to increase.

Conclusion: higher air pressure differential and/or higher volume are the only 2 ways to air up a tire faster.

Zeke · 05-12-2009, 08:41 AM

Quote:

Originally Posted by gtc

I inflate my tires whenever I'm up in the mountains. The air is thinner at higher altitude, so my wheels weigh less which in turn improves handling becuase of lower unsprung mass.

fixed it for you.

71T Targa · 05-12-2009, 09:52 AM

Quote:

Originally Posted by Hugh R

71T

No, regardless of tire width, at a given PSI, the contact patch is always the same # of square inches. Just a different shape. If you have a 6" wide tire or a 10" wide tire, at 35 PSI with say 700 pounds of load on the tire, you'll have 20 sq. inches in either case.

If you put 35 PSI in a tire off the car, then put the tire/wheel on the car, the PSI of the tire remains the same, the contact patch changes from say a loading of 35 pounds of the weight of the wheel/tire and 1 sq. inch of contact to 20 sq. inches with a 700 pound load.

Thanks Hugh, that makes more sense...