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  #1  
Old 01-05-2009, 09:52 PM
Northern Piper Northern Piper is online now
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Why do my tires lose pressure in really cold weather?

& why don't the friggin' air pumps at the gas stations work??!!!

As some of you may know, Saskatchewan has been going through a period of extended low temperatures (-30 C) for the past month or so.

Since that started, I've been having trouble with low tire pressure on both the Piper vehicles, with completely different types of tires. Two of them have gone really low, and one was absolutely flat this evening when I left work, necessitating a hike home.

Is it something to do with Boyle's law? really low temperatures lower the air pressure? perhaps enough to break the seal between the tire and the rim, so that they go flat?

And why don't the air pumps at the gas stations work??? They're all "out of order" when ever it gets really cold, just when I need them!!!

I ended up getting a little compressor from, where else, Canadian Tire (the first time I've, like, bought anything tire-related at Canadian Tire). It runs off the cigarette lighter in the vehicle. Is there any chance that if I use it too much, I'll end up with nicely inflated tires and a dead battery? Cause that would be

Or I could just call in sick until oh, May.
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  #2  
Old 01-05-2009, 10:00 PM
kenobi 65 kenobi 65 is offline
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Quote:
Originally Posted by Northern Piper View Post
Is it something to do with Boyle's law? really low temperatures lower the air pressure? perhaps enough to break the seal between the tire and the rim, so that they go flat?
That'd be my guess, but I'm no physicist, nor am I an auto repairman. I'd note that they also advise you to check your tire pressure on "cold" tires (that is, not after an extended drive), because the heat from friction, braking, etc. will raise the tire pressure, and you won't get an accurate reading.

Quote:
Originally Posted by Northern Piper
I ended up getting a little compressor from, where else, Canadian Tire (the first time I've, like, bought anything tire-related at Canadian Tire). It runs off the cigarette lighter in the vehicle. Is there any chance that if I use it too much, I'll end up with nicely inflated tires and a dead battery? Cause that would be
As long as you're not using the compressor, then letting the car sit without driving it for a while, you should be OK. Running the car lets the alternator re-charge the battery.

Last edited by kenobi 65; 01-05-2009 at 10:00 PM..
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  #3  
Old 01-05-2009, 10:20 PM
Northern Piper Northern Piper is online now
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Originally Posted by kenobi 65 View Post
I'd note that they also advise you to check your tire pressure on "cold" tires
trust me, them tires is cold!!! we've warmed up to a balmy -17 C today.
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  #4  
Old 01-05-2009, 10:22 PM
Stan Shmenge Stan Shmenge is offline
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P * V = nRT

Yah, I'm a wiseass.
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  #5  
Old 01-05-2009, 10:26 PM
Speaker for the Dead Speaker for the Dead is offline
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Do I EVER know. But if it were actually the cold alone, then everyone around here would be having the same problem. And although one of my tires kept going flat, it was because it had a rug nail in it.
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  #6  
Old 01-05-2009, 10:31 PM
Northern Piper Northern Piper is online now
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that's what I thought when this happened last year, but once I filled them up and the warm weather came, they stayed inflated. I asked them to check the tires when I took the car in for routine service, and they couldn't find a leak.
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Old 01-05-2009, 10:35 PM
kenobi 65 kenobi 65 is offline
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Originally Posted by Northern Piper View Post
trust me, them tires is cold!!! we've warmed up to a balmy -17 C today.
Yeah, that's brisk, all right. I grew up in Green Bay; it doesn't seem to be quite as cold, for as long, here in Chicago. I miss going to Packer games in December and feeling the inside of my nose freeze up.

Last edited by kenobi 65; 01-05-2009 at 10:38 PM..
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Old 01-05-2009, 10:37 PM
kenobi 65 kenobi 65 is offline
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Originally Posted by Speaker for the Dead View Post
But if it were actually the cold alone, then everyone around here would be having the same problem.
I suspect that Piper's got a couple of imperfect seals on his tires (due to dented wheels, old seals, whatever), and the combo of that with the cold is enough to get them to deflate.
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Old 01-05-2009, 10:40 PM
Northern Piper Northern Piper is online now
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that could be it - the Blazer is a 1988, with the original rims.
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Old 01-05-2009, 10:59 PM
kenobi 65 kenobi 65 is offline
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Originally Posted by Northern Piper View Post
that could be it - the Blazer is a 1988, with the original rims.
Two years ago, I wound up having to get the wheels on my PT Cruiser replaced. Having hit enough potholes, the right front wheel had several dents, and wouldn't hold a good seal anymore (particularly in colder weather). As it turned out, buying an entire set of 4 new aftermarket wheels was less expensive than buying a single original-equipment replacement wheel.
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  #11  
Old 01-05-2009, 11:13 PM
Critical Mass Critical Mass is offline
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Way back when, I was a tow truck driver in Winnipeg. I saw lots of vehicles disabled from extreme cold. Changed many tires at -35

I don't think the cold reduces the air pressure. Rather, it causes some component of the wheel / tire to leak air. Valve stems or cores or some minor imperfection with the tire seal against the rim are likely the cause. Aluminum rims are more susceptible than steel to air loss in extremely cold conditions.

If it persists, a trip to the tire store to have the tires removed from and re-installed on the rims will likely solve the problem. While the tire is off, any rust or other gunk on the rim will be ground down and new valve stems will be installed.

Last edited by Critical Mass; 01-05-2009 at 11:17 PM..
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  #12  
Old 01-05-2009, 11:20 PM
ZipperJJ ZipperJJ is online now
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Quote:
Originally Posted by Speaker for the Dead View Post
Do I EVER know. But if it were actually the cold alone, then everyone around here would be having the same problem. And although one of my tires kept going flat, it was because it had a rug nail in it.
So did carpeting the garage end up making the tires stay warm?
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  #13  
Old 01-05-2009, 11:27 PM
K364 K364 is offline
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As others have indicated, the extreme cold jeopardizes the valve and/or the seal between rubber and rim... mainly (I suspect) due to the rubber losing flexibility - when you hit a bump the seal fails due to the rubber acting like steel instead of rubber.

The tire pumps are a victim of "The Law of Adiabatic Expansion" - an expanding gas will cool. That means the tire pump mechanisms in cold weather will be EXTREMELY cold... the valves/connections/etc will freeze and fail.

It's a bad combo that recalls a certain '71 Cutlass and it's tires that chose the coldest days to spontaneously deflate.

Maybe someone could figure out the difference in pressure when the temp goes from 20 C. to -20 C. Maybe about 18%??? An underinflated tire will lose it's rim seal more easily.

Last edited by K364; 01-05-2009 at 11:30 PM..
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  #14  
Old 01-05-2009, 11:31 PM
Critical Mass Critical Mass is offline
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Quote:
Originally Posted by K364 View Post
As others have indicated, the extreme cold jeopardizes the valve and/or the seal between rubber and rim... mainly (I suspect) due to the rubber losing flexibility
I have also theorized that the extreme cold causes the metal rim to contract just enough to cause the seal to leak.
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  #15  
Old 01-05-2009, 11:43 PM
usedtobe usedtobe is online now
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Tire stores (I've heard) hate it, but I swear by fix-a-flat tire repair in an aerosol can. I don't do cold anymore, but I'd sure as hell get the car (and the can) nice and toasty, then give it a shot LOOONG before I'd even think of walking in that temp.
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Old 01-06-2009, 12:04 AM
Fubaya Fubaya is offline
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Quote:
Originally Posted by Northern Piper View Post
I ended up getting a little compressor from, where else, Canadian Tire (the first time I've, like, bought anything tire-related at Canadian Tire). It runs off the cigarette lighter in the vehicle. Is there any chance that if I use it too much, I'll end up with nicely inflated tires and a dead battery? Cause that would be
To be safe, leave the car running while using it, and for a few minutes afterward.
Quote:
Originally Posted by K364 View Post
Maybe someone could figure out the difference in pressure when the temp goes from 20 C. to -20 C. Maybe about 18%??? An underinflated tire will lose it's rim seal more easily.
A tire loses 1 psi for every 10 degree Fahrenheit drop in temperature. If my math is right, that's just 7psi in 20 C. to -20 C weather.
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  #17  
Old 01-06-2009, 04:45 AM
Jinx Jinx is offline
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Ok, so at what temp do you read your tire pressure? STP? If not, should one choose to inflate cold tires, they may very well be overinflated once you start driving on them. So, what is the industry standard temp? (And, please define STP as each professional association has their own definition of the "T" in STP!)
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  #18  
Old 01-06-2009, 05:01 AM
Bill Door Bill Door is offline
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OK, let's say a car tire has a volume of 30 liters, or 1.06 cubic feet. I'm doing this in English numbers, because all my spreadsheets are set up that way.

The English gas constant, R, is 10.73 psia-ft3/lb mole-degree R

When we drop the temperature from 20 deg C to - 20 deg C the pressure of 30 liters originally at 2 bar gauge pressure (29 psig) will drop to 1.6 bar (23 psig).
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Old 01-06-2009, 06:45 AM
zut zut is online now
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Originally Posted by qwest View Post
I have also theorized that the extreme cold causes the metal rim to contract just enough to cause the seal to leak.
That would be extremely unlikely, as

a) The coefficient of thermal expansion of steel is quite low: around 12X10-6/oC. That means a 40oC temperature swing changes a 14" wheel diameter by only about seven mils (0.007"). I'd be surprised if manufacturing tolerances were that close. (Aluminum has a coefficient of thermal expansion roughly 2 times higher than steel).

b) Rubber, on the other hand, has a coefficient of thermal expansion an order of magnitude higher than steel. So the contraction of the tire is far more significant than the contraction of the wheel.
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Old 01-06-2009, 09:03 AM
Quercus Quercus is offline
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Originally Posted by Bill Door View Post
OK, let's say a car tire has a volume of 30 liters, or 1.06 cubic feet. I'm doing this in English numbers, because all my spreadsheets are set up that way.

The English gas constant, R, is 10.73 psia-ft3/lb mole-degree R

When we drop the temperature from 20 deg C to - 20 deg C the pressure of 30 liters originally at 2 bar gauge pressure (29 psig) will drop to 1.6 bar (23 psig).
I get a (slightly) different answer.
P=nRT/V with T in an absolute scale, since nR/V doesn't change, a given % drop in T gives the same % drop in P.
From +20C to -20C is 293Kelvin to 253Kelvin, a change of (40/293 = 13.6%).

So going from +20C to -20C should give a pressure drop of about 14% for an ideal gas. (which is probably a decent approximation for air in tires). So from 2 bar to 1.72 bar, or starting from a slightly higher 32 psi results in going down to 27 1/2 psi.
At any rate, enough to affect gas mileage, but not completely flat.

By the way, I though you were supposed to measure air pressure when the tires were warm, after driving a bit. Am I mistaken?
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Old 01-06-2009, 09:36 AM
Santo Rugger Santo Rugger is offline
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Quote:
Originally Posted by Jinx View Post
Ok, so at what temp do you read your tire pressure? STP? If not, should one choose to inflate cold tires, they may very well be overinflated once you start driving on them. So, what is the industry standard temp? (And, please define STP as each professional association has their own definition of the "T" in STP!)
During the day when the tires are cold; before you leave work is perfect (assuming you work 8-5). I'm guessing a most mechanics think STP is an engine cleaner, and don't associate it with standard temperature and pressure. There is no industry standard temp, because the "standard" is whatever temperature the tires are operating at. As long as you're checking your tires once a month, it shouldn't be a problem.
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Originally Posted by Quercus View Post
By the way, I though you were supposed to measure air pressure when the tires were warm, after driving a bit. Am I mistaken?
Yes.
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  #22  
Old 01-06-2009, 11:06 AM
Bill Door Bill Door is offline
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Quote:
Originally Posted by Quercus View Post
I get a (slightly) different answer.
P=nRT/V with T in an absolute scale, since nR/V doesn't change, a given % drop in T gives the same % drop in P.
From +20C to -20C is 293Kelvin to 253Kelvin, a change of (40/293 = 13.6%).

So going from +20C to -20C should give a pressure drop of about 14% for an ideal gas. (which is probably a decent approximation for air in tires). So from 2 bar to 1.72 bar, or starting from a slightly higher 32 psi results in going down to 27 1/2 psi.
At any rate, enough to affect gas mileage, but not completely flat.

By the way, I though you were supposed to measure air pressure when the tires were warm, after driving a bit. Am I mistaken?
The reason you got a slightly different answer was that you forgot that the pressure is absolute, and not gauge. 32 psig is 46.7 psia. A pressure drop of 13.6% gives you a pressure of 40.3 psia. 40.3 psia minus 14.7 psia atmospheric pressure is 25.6 psig.
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  #23  
Old 01-06-2009, 11:36 AM
Philster Philster is offline
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Overinflated tires are safe right up to the max PSI rating. Ride comfort might suffer slightly, but will not be a big deal with the variances discussed here.

Under-inflated tires can be quite dangerous.

When filling your tires, and concerned temps might plummet, go ahead and add a few more PSI. If your recommended tire pressure is 32, going up to 40 is no big deal, and helps ensure you won't drive on under-inflated tires.

YMMV. Literally.

Last edited by Philster; 01-06-2009 at 11:37 AM..
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  #24  
Old 01-06-2009, 12:28 PM
Una Persson Una Persson is offline
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Quote:
Originally Posted by Quercus View Post
By the way, I though you were supposed to measure air pressure when the tires were warm, after driving a bit. Am I mistaken?
Generally you are supposed to measure tire temperature when cold, or at least after the vehicle has rested for several hours.
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  #25  
Old 01-06-2009, 12:34 PM
Telemark Telemark is offline
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Originally Posted by Philster View Post
Overinflated tires are safe right up to the max PSI rating. Ride comfort might suffer slightly, but will not be a big deal with the variances discussed here. .
I don't think this is completely true. Over-inflated tires can interact with your suspension, increasing stopping distances. You may end up bouncing your tires.
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  #26  
Old 01-06-2009, 01:44 PM
Mr. Slant Mr.  Slant is offline
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I don't think this is completely true. Over-inflated tires can interact with your suspension, increasing stopping distances. You may end up bouncing your tires.
On certain hypermiling message boards, posters can be found that routinely run 175% of routine suggested PSI.
Some posters have reported that their car's handling gets worse. One driver with a small FWD car noted that his car has a serious tendency towards oversteer once he went past 75 PSI.
Another noted that his vehicle's ability to climb snowy hills decreased dramatically when he goes from 32 PSI to 44 PSI, and then again precipitously as he moves to 55 PSI and 65 PSI.

There's a poster who has driven over 110K miles with his 55 PSI tires running at 110 PSI, and has apparently neither been killed by it, nor experience excessive tire wear, as he's still on the factory tires.

[1] Hypermiling defined by Wikipedia:
Hypermiling is a term used in North America that refers to a set of techniques used to maximize fuel economy. Those who practice the techniques are referred to as "hypermilers." The term was originally coined by Wayne Gerdes, who is considered by the media to be one of the top hypermilers in the world,[1] and is known to hold the record for gas mileage in some common vehicles, including 30 miles per gallon (mpg) in an Acura MDX and 59 mpg in a Honda Accord.[2] In 2008, the word hypermiling was selected as the best new word of the year by New Oxford American Dictionary.[3]

Last edited by Mr. Slant; 01-06-2009 at 01:45 PM..
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