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Neil924 · Joined: 18 Mar 2003 Posts: 4225 Location: Canada

When putting air into a tire does it matter if it is on the car? ie: tire says 44 psi, on the car it has 44 psi. Take that wheel off, does it still read 44 psi?
I think it is the same.

Smoothie · Posted: Thu May 08, 2003 10:41 am Post subject:

I'm thinkin' it should read less off the car.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

78porsche924 · Posted: Thu May 08, 2003 10:47 am Post subject:

Yeah definantly it would. You don't have all that extra weight on top of it. Think of it when u push on a plastic bottles side, you will get more a lot more pressure in there.
_________________
90 944 S2
78 924 NA <---now sold and killed by new owner
snailshell trans
Bae turbo kit
to check out my 944 S2 http://www.cardomain.com/member_pages/view_page.pl?page_id=388139

924RACR · Posted: Thu May 08, 2003 10:53 am Post subject:

It's the same.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Neil924 · Joined: 18 Mar 2003 Posts: 4225 Location: Canada

Mes is confusd. Smoothie took a guess, 78porsche924 said I made a mistake and 924RACR said I'm good. Is this the shell game? Under which cup is the right answer!?

78porsche924 · Posted: Thu May 08, 2003 11:03 am Post subject:

You serious vaughn??? I would have guessed it would be way different.
_________________
90 944 S2
78 924 NA <---now sold and killed by new owner
snailshell trans
Bae turbo kit
to check out my 944 S2 http://www.cardomain.com/member_pages/view_page.pl?page_id=388139

Neil924 · Joined: 18 Mar 2003 Posts: 4225 Location: Canada

Please tell me you are as smart as I think you are Vaughan.

Smoothie · Posted: Thu May 08, 2003 11:14 am Post subject:

I wasn't guessing actually - pretty sure you'd have more pressure with tires on the car and the weight of the car on them. Common sense physics, right?
Ouch

my brain hurts.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Neil924 · Joined: 18 Mar 2003 Posts: 4225 Location: Canada

Damn it! I took geography,geology and biology instead of physics and chemistry. I think I will wait for Vaughan's reasoning {If he will give it} before giving Smoothie and 78porsche924 first prize.

Smoothie · Posted: Thu May 08, 2003 11:23 am Post subject:

Vaughan has you on a technicality either way. You didn't say if the 44 psi measured while on the car was with it jacked up, or not jacked.

_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

1982Porsche924 · Joined: 14 Nov 2002 Posts: 679 Location: Cupertino, CA

when a tire manufacturer tells you to inflate a tire to 35 psi, so it when the tire is on the car, the car on the ground. It'd be pretty stupid to tell people to lift the car before inflating a tire, how inconvenient would that be? Anyways, it shouldn't make that much of a difference, because the volume doesn't decrease that much when the car is on the ground. If you look at the tire, it's still fairly round. Pressure doesn't increase under load, it only increases in loss of volume.
_________________
1979 Porsche 924 "S"
Engine: Euro pistons, Light Flywheel, Weber TB, Bursch Header, Cam Wheel
Suspension: Front coil-overs, adj sways, Solid bush. Thicker tors. bars, 5-bolt, BBS rims
Body: Fiberglass widebody kit, 931 Nose

78porsche924 · Posted: Thu May 08, 2003 11:54 am Post subject:

They deform a lot more then it looks. I take physics but never did fluid mechanics. Would have rather done that then E and M. I hate that crap.
_________________
90 944 S2
78 924 NA <---now sold and killed by new owner
snailshell trans
Bae turbo kit
to check out my 944 S2 http://www.cardomain.com/member_pages/view_page.pl?page_id=388139

Neil924 · Joined: 18 Mar 2003 Posts: 4225 Location: Canada

Smoothie the wheel was in my hands, off the car. I was checking the summer set of tires before putting them on this weekend.

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

Not an engineer here, but I think of the psi being the volume in relation to the capacity of the tire. As an example, a bicycle tire needs a lot less air to get the same psi. When you fill the tire to a set psi then add the weight of the car, the capacity should decrease due to the tire flattening on the bottom, which would increase the psi. BUT the tire is flexible and expands in relation to the volume of air, so the same capacity/psi is realized.

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

john h · Posted: Thu May 08, 2003 4:00 pm Post subject:

Why are you running the tire at 44 pounds - thats much too high for everyday street use.

Even under racing I only have the tire pumped up to about 34 pounds.

Or do you guys (Americans) use a different pressure scale than the rest of the world?
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