A Weird thing with my tire yesterday.

As I drove home from work yesterda,y I realized about 12 miles from home that my tire pressure was very low (20psi) in my rear tire. Once I made it home, I pumped the tire up to see if I could find the leak. I couldn't find a nail, scar or anything on the tread nor on the outside side wall. BUT I did notice there were weird scratches on the rim. Anyway, I let the pump run to 42 psi matching the other tire. This morning the tire pressure is normal and all seems to be fine. What do you think happened?

yesterday was a cold day compared to the previous that reached up to 65 + degrees. hot air expands and cold contracts.

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That wouldn't explain why the other rear tire wasn't low, too. I've never seen the temperature affect the pressure by 22 PSI.

I will be more specific. The combination of the rim bead, tire rubber and temperature had something to do with it. Perhaps a bad valve stem too.

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+1 Maybe the needle was slightly off seat slowly leaking and you seated it properly when you refilled with air.

Tippy's explanation seems most likely.

I agree it could be the valve stem or even the valve needle as Chao and Tippy mentioned. You also may have experienced an attack from the 'Tyre-Deflator'. This person, or persons, wanders parking lots, streets, or wherever cars are easy targets. With nothing to do but armed with a valve needle remover, they go to work. They will normally remove all 4 valve needles or loosen them enough to where a slight leak is created. But removing all 4 needles is best since knowone carries 4 spares, and when you think you can just pump the tyres back-up, nope you cannot unless you get some needles. So I would go around and tighten the needles, maybe even replace them when you get a chance if you suspect that is a problem (get a set as spares just in case though and keep them in the car).

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I park in a secured Fed Govt building. I only hope no needle robber can just walk in. lol! But as you guys mention, it just may be the stem was knocked out of wack, I did hit two major pot holes yesterday, one in the morning and one that evening. That could also explain the weird scratches on the rim.

If the temps have been cold, performance tires do not like that. Cold temps will cause a dramatic pressure loss in performance tires. C.

i don't think temps have anything to do with it... nothing to that extreme. very likely valve stem or some other human being getting back at ya.

You mentioned that you hit two potholes. Was it with the tire that lost air? If the hit was bad enough, you might have bent the wheel causing an air escape. Check your wheel flange for any bend.

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Wheel flange? Can you guide me to the flange's whereabouts? I sprayed water all around the lip area looking for a leak but I saw no bubbles yesterday evening.

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... or was it... I hit the same pot hole yesterday, once in the morning and once that evening .

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About a pound per ten degrees, give or take, regardless of the type of tire. Doesn't explain this by a long shot, IMHO. (You are right that performance tires get nasty in the cold, but this has to do with the compound).

Wheel flange = lip. This happened to me with an old BoxsterS about 5 years ago. I hit a lagre pot hole and noticed a major pressure reduction at that wheel when I got home. Closer inspection revealed a slight bent in the wheel. Low profile tires/wheels are more prone to this problem. I guessed that the impact caused a momentary seperation at the tire bead.

I'm thinking if it's the same tire that hit the potholes, that split second when the rubber "flexed" hard, it could have been hit hard enough to separate from the seal against the rim, long enough for some air pressure to escape in a very quick short burst. Are there any scrapes on the tire wall? For more consistent tire pressure as far as temp is concerned, nitrogen would be your best option.

Well, I'm home and the tire pressure is the same, normal. I'm hoping the rim isn't bent. I wont be able to drive the car again until Friday or so depending on the weather. Thanks.

Whelp, turns out there is a half an inch crack on the inside lip of my rim, so sign me up for a WTB 18" turbo twist OEM set.. Does anyone know what the dealer charges for 1? I've search all over Ebay but cannot find a non chromed set.

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No, don't go propagating the nitrogen myth again! All gasses obey the ideal gas law: PV=nRT. The only advantage that nitrogen has - in some cases - is that it's very dry. Water does not obey the ideal gas law (it's not a gas - duh!) and expands quite a bit upon heating. You can achieve the same thing by using dry air.

Jorgen is that why guys playing paintball using Nitrogen, rather than Co2, maintain more consistent pressures in varying temperatures? WDCPorsche, I'd suggest a set of 17's (I know they don't look nice) for in-town use. You'll have a less attractive wheel but you'll have more sidewall protection for the wheel. As far as replacing the single wheel, try www.wheelenhancement.com . They might even take your cracked wheel.

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What's funny is both potholes are in the suburbs of Maryland.

[QUOTE=viper501;5136360]Jorgen is that why guys playing paintball using Nitrogen, rather than Co2, maintain more consistent pressures in varying temperatures? Not sure that paintball guys should be considered experts on the physicis of gasses. There's really nothing to argue about - it's basic physics. Gasses obey the ideal gas law. Yes, there are minor variations from ideality, which is why the Ferrari F1 team tried using CO2 in the tires - for more consistent pressures. What does that say about the paintball guys? Do you think Ferrari spends more on research than paintball guys? Anyway - you're way ahead of the game if you use DRY air. Don't waste your money on nitrogen fills.

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water vapor does not obey the ideal gas law? One thing about bent rims is that a vibration can be felt which gets worse as you go faster. I bent three or four rims on a Volvo 850 turbo on potholes, $100 to fix each time. Good luck with your wheel search.

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Water vapor does obey the ideal gas law (with some departure from ideality). However, the problem occurs if there is liquid water (from condensation) in the tire, and it boils due to heating of the tire. The phase change causes a major change in pressure. I was at a tire shop a while back that was pushing the dry nitrogen fill, but in the tire mounting process they were using a soap and water solution on the tire bead. So you put water inside my tire and they try to sell me an expensive dry nitorgen fill? That makes sense. That's why it's important to use DRY air - or nitrigen or helium or argon or any friggin gas you can afford.

I'm not a physicist like some folks here, but here's what I know: Racers use nitrogen. The recent article in Excellence in which they compared the GT3 RS to the RS-R, for example, the road car's tires gained 10psi during lapping, eventually making it hard to evaluate the handling. The Flying Lizards' RS-R was running nitrogen and ran consistently. If it's a myth, you might want to mention it to them. And then get all the physicists on this board to explain why the race car's tires' pressures don't fluctuate that way.

FWIW: Why don't they use normal air in race car tires? Many race car teams use nitrogen instead of air in their tires because nitrogen has a much more consistent rate of expansion and contraction compared to the usual air. Often, a half pound of pressure will radically affect traction and handling. With track and tire temperatures varying over the duration of a race, the consistency of nitrogen is needed. Nitrogen pressure is more consistent than normal air pressure, because air typically contains varying amounts of moisture due to changes in the relative humidity on race day. Water causes air to be inconsistent in its rate of expansion and contraction. So, a humid race in the southeast United States or a dry race in the desert western United States could make for unpredictable tire pressures if "dry" nitrogen were not used. Nitrogen is also used in the high-pressure tires on large and small aircraft.

Agreed with cdodkin and PJorgan. If water goes through a phase change in the tire, it is problematic. Nitrogen is the cheapest gas you can buy in a tank with reasonable purity (no water). Nitrogen will change pressure with temperature, but it can be predicted. Dry air works just as well, may just be easier to get a 3000 PSI tank of nitrogen.

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dresler and cdodkin get it - it's all about the water, or lack of it, for most of us guys with budgets smaller than F1 teams. All gasses expand when heated. However, the expansion of a pure, well-characterized gas is more predictable than ambient air, which can vary. So an F1 team will use a pure gas (nitrogen, CO2, even argon) and spend lots of time and money characterizing the performance of that gas under very specific conditions in their tires (or tyres). That doesn't really help most of us. So, like dresler says, use the cheapest, dryest gas you can get. Probably should avoid pure hydrogen, though the Germans did build the Hindenburg.