** & 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!!! :smack:
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 :smack::smack:
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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!)
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).
a) The coefficient of thermal expansion of steel is quite low: around 12X10[sup]-6[/sup]/[sup]o[/sup]C. That means a 40[sup]o[/sup]C 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.
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?
