[craig66]

I know that this has been discussed before but I am still having a hard time conceptualizing and so perhaps someone can give me a little help/advice

My winter tires are according to Porsche to be set at 31 at 68 degrees.
My garage is 60 and so using the 1/ten degrees farenheit I set the tired so that in my garage they read 32.
I am in the NE where temps are typically 40 degrees +/-10.

I assume that after sitting out in the cold for hours they will read around 30 and then gradually heat up to where after driving they will settle around 33-35 depending on how spirited the driving.

Does this all make sense or do I have this backwards

Thanks

 

[Third]

If your question is will tire pressure decrease as the temperature drops and increase as it rises, the answer is yes.

 

[CaymanMatt]

I always try to set my tires so that the starting temperature is what's recommended. I kind of figure that the pressure delta from cold->warm is proportional to the temperature delta no matter the starting point. ie: -20F->+20F (delta=40F) is the same as +50F->90F (delta still = 40F), at least within reason. Note that I won't be driving our Cayman at -20F; that's what the Subaru is for!

 

[craig66]

If your question is will tire pressure decrease as the temperature drops and increase as it rises, the answer is yes.

No that much I figured I was just trying to get a sense of how to set my tires given that the 68 that is recommended is by definition not a temperature I will see in the winter if I have winter tires on.

I always try to set my tires so that the starting temperature is what's recommended. I kind of figure that the pressure delta from cold->warm is proportional to the temperature delta no matter the starting point. ie: -20F->+20F (delta=40F) is the same as +50F->90F (delta still = 40F), at least within reason. Note that I won't be driving our Cayman at -20F; that's what the Subaru is for!

That makes sense and so I guess I should just set them at 31 in my 60 degree garage understanding that when I park overnight outside I will find them to read around 28 in the morning but that by the time my engine is warm the tire pressures will have increased to around a level where handling and wear wont be compromised

 

[DriveInHouston]

The ratio of pressures in different temperatures is proportional to the ratio of temperatures!... In Kelvin!

 

[CaymanMatt]

The ratio of pressures in different temperatures is proportional to the ratio of temperatures!... In Kelvin!

Delta(K) = delta(C) =delta(F)/1.8. The pressures are all proportional to the temp but the actual proportionality constant is dependent on both scales (pressure and temp) you are using also, if I remember all this stuff correctly!?

 

[duegatti]

I've gotten into this with the BMW motorcycle riders, where the manufacturer has also specified a tire pressure at 68 degrees. This has led some people to conclude, in essence, that there should be a constant number of gas molecules in the tire, such that pressures will drop below 68, and rise above. But, hey - it's set for the specified pressure at 68, like they say, so these people feel that this is correct. They are expecting to only add gas molecules that are lost out of a tire, and to otherwise never adjust for ambient temperature.

I think this is some horrible, accidental misunderstanding, which may have arisen by bad translations of German specifications, or some fashion of manufacturers thinking that they are covering they're asses. I think it's a totally incorrect interpretation.

If Porsche says your tire pressure should be 31 at 68F, they mean it should be 31. If it's 40 F, you set your tire at 31 with your (non-temperature corrected) gage. If you want to attempt corrections for the difference between your garage temperature and the outdoor ambient temperature, and/or a temperature later in the day, by all means, do so. Because the idea is to have 31 pounds on an undriven, unwarmed tire at the temperature where it will be used. This is the way we've done it for a hundred years. I don't think we've gotten to the point where we want 31 pounds at 68F, 28 pounds at 38F, and 34 pounds at 98F. That doesn't make any sense at all.

If your garage is 60F, and your typical ambient is 40 +/- 10, and Porsche specs 31 (at 68F, as they say), I say you should set your pressure for 31 + 2 = 33. If, on the other hand, you decide you want to literally meet Porsche's specification of 31 AT 68F, then you would want to set it LOWER than 31 (in a 60 degree garage), not higher, such that you'd set it at 30 pounds, which would then give you 31 if the tire warmed approximately 10 degrees. So the magnitude of the correction that you shared is correct, but you came at it from the wrong direction.

I really don't know how this confusion took hold. But it makes no sense the way these specifications read in English. Perhaps there is a misguided regulation stating that pressure specifications must be associated with a temperature; I don't know. Perhaps it's a translation issue. I don't know.

 

[duegatti]

And, I think I can tell you why these companies spec at 68F. 68F is 20C; it's one of the temperatures which has integer values on both scales. Every 9 degrees F gives you 5 degrees C, and 68 is a more or less common ambient temperature.

 

[jimmuller]

And, I think I can tell you why these companies spec at 68F. 68F is 20C; it's one of the temperatures which has integer values on both scales. Every 9 degrees F gives you 5 degrees C, and 68 is a more or less common ambient temperature.

In science 20degC is nominal room temperature, and along with 760mm Hg is what scientists refer to as Standard Temperature and Pressure. It also happens to be an integer in both scales. It being a science standard may be why the manual states a pressure at 68F, but I wouldn't bet my farm on there being another reason too or instead.

But it makes no sense the way these specifications read in English. Perhaps there is a misguided regulation stating that pressure specifications must be associated with a temperature; I don't know. Perhaps it's a translation issue. I don't know.

Hmm, probably not a translation issue. The real puzzle is that it gives just that one figure instead of a range of operating conditions, especially 68degF for winter ties. Nobody re-adjusts their pressures to account for warmer or colder days (at least for street use), and 68 isn't even close to an average winter temperature for some of us.

Since we don't re-adjust pressures day by day yet the tires manage to survive anyway, this tells us that the tires can indeed accommodate a wide range. Which tells us that the nominal figure needn't be followed slavishly despite the presence of a digital temperature readout on the instrument cluster. We do however change pressures, and even the tires themselves, season by season.

I could set my tires to the spec by leaving the car in the garage or by calculating the temperature correction as described above, but I don't. I set them so that when I start my morning commute with temps in the 20's they read 29 to 30, lower than spec but at a lower temperature. As I drive they go up to 31 or 32. That's actually higher than specified because were I to set them to 31psi at 68degF they'd be down in the mid-20s on our really cold mornings. They haven't blown up yet.

 

[Third]

....
If Porsche says your tire pressure should be 31 at 68F, they mean it should be 31. If it's 40 F, you set your tire at 31 with your (non-temperature corrected) gage. If you want to attempt corrections for the difference between your garage temperature and the outdoor ambient temperature, and/or a temperature later in the day, by all means, do so. Because the idea is to have 31 pounds on an undriven, unwarmed tire at the temperature where it will be used.
....

I agree. I believe the published tire pressure is intended to refer to a tire that is resting at the local ambient temperature. That is, if the recommended pressure is 33psi, the pressure should be 33 psi when the car is "cold" regardless of the ambient temperature. (In fact, don't they refer to it as "Cold Tire Pressure"?) I believe the intent is to ensure adequate tire pressure for safe handling and optimal tire wear when the tire is cold, while allowing for the pressure to rise and remain safe to operate as the tire heats up in worst-case ambient driving conditions, e.g., black surface in the desert during the summer.

 

[johan]

With winters that goes from +10C down to -25C i check the tire pressure when it is around 0 and then i leave it.

 

[Third]

With winters that goes from +10C down to -25C i check the tire pressure when it is around 0 and then i leave it.

I might've set it while at the lowest expected temp for the winter, e.g, -25C, but in any case yours is the practical solution.

 

[duegatti]

Maybe it's the motorcyclist in me, but about the only time I don't check my tire pressures is within a day or two of my last check - if the temperature is the same. Otherwise, I'm checking them, and I'll adjust if they are a pound or more off. I'm less fastidious about my 2006 Murano beater, and I really shouldn't be, because that's the one that will deal with the snow here.

 

[johan]

I might've set it while at the lowest expected temp for the winter, e.g, -25C, but in any case yours is the practical solution.

Kind of, my winter tires never never gonna see spirited driving and winter tires have better grip in the snow if they are slightly under inflated.
So i adjust my driving after the weather.
Also it think it is a ghost chase, how many degrees up or down before they should be adjusted for optimal grip?

Imho

 

[Viffermike]

I feel that @duegatti 's points relative to motorcycling put the issue in perspective far better than any four-wheeler context does. Why? Because small adjustments in tire pressure matter far, far more than the same adjustments on a 'cage' (as we bikers tend to call four-wheeled vehicles). It can be the difference between staying upright and crashing.

Think about it. Sportbike tires typically hold only about 6-8lbs of pressure more than a sports car's: 35-38 PSI. On a bike, the front is almost always set at a lower pressure than the rear by 3-6 PSI because the front is where at least 75 percent of both turning and braking force is applied. It needs the extra grip that a lower PSI supplies, particularly since the front is usually significantly skinnier than the rear. The relationship between front and rear PSI is more symbiotic, too -- you don't want too much pressure in the rear so it doesn't grab, but you also don't want to little pressure in either tire because ride compliance becomes an immediate issue, and tire wear -- your typical sportbike tires last only 4-8K on the street, maybe more if they're a touring compound -- becomes a long-term one.

This is all independent of temperature, which affects a motorcycle tire more for the above reasons in concert with the volume held at pressure being far less -- roughly half of a typical cage tire, and more than half on the front. That 1 PSI-per-9-degree rule holds, but rubber is rubber, and motorcycle tires heat-cycle far harder than cage tires. I remember days when I'd start a ride at 6 a.m. in 55-degree weather, and by the time the 90s hit mid-afternoon, my tires were running 6 PSI higher then what I set them at. Yes, you read that right: SIX. That's massive on a bike.

I make all of these points because I honestly believe that cage PSI differences as caused by ambient temperature is not a huge issue for anyone. Porsche likely recommends 31 PSI at 68 F knowing that it will fall to 28-29 PSI at a UHP summer tire's operational limit, and rise to 35-36 PSI on a hot summer day. Can one feel that difference? Absolutely. Will it affect tire mileage and long-term efficacy? I believe just marginally. That's part of the reason I run my tires at lower PSIs as they wear down: 32F-35R when new, and 29F-32R when near the wear bars.

 

[jimmuller]

I feel that @duegatti 's points relative to motorcycling put the issue in perspective far better than any four-wheeler context does. ...

I make all of these points because I honestly believe that cage PSI differences as caused by ambient temperature is not a huge issue for anyone. Porsche likely recommends 31 PSI at 68 F knowing that it will fall to 28-29 PSI at a UHP summer tire's operational limit, and rise to 35-36 PSI on a hot summer day.

I don't ride a motorcycle but this sounds like a good explanation.

Still, it doesn't explain why they would list 68degF as the target for a winter tire, unless they expected tire pressures to be set in a warm garage. It suggests to me that it doesn't matter much so they just didn't bother to give a different spec.

 

[Viffermike]

I don't ride a motorcycle but this sounds like a good explanation.

Still, it doesn't explain why they would list 68degF as the target for a winter tire, unless they expected tire pressures to be set in a warm garage. It suggests to me that it doesn't matter much so they just didn't bother to give a different spec.

I suspect the warm-garage part is key to the reasoning. Winter-tire compounds and construction are far more stout than performance-tire ones, so I suspect PSI variability matters less, too -- kind of as if you were running a sorta-kinda RFT (run-flat tire).

 

[DriveInHouston]

There is no reasoning for 1bar/20C conditions other than that is the standard. It is called the Normal Condition in engineering and that is that. You can’t go around specifying other pressure/temperature combinations, unless they are the specific operating conditions for an engineering application.

Now you can take the cold tire pressure specified under Normal Conditions and convert it to the specific pressure/temperature conditions of your garage, to inflate the tires cold and then let them warm up wherever they may, or, as I do for Houston, set my target operating pressure at 35 psi (which is pretty much the pressure they would warm up to in a nice 68F day in Texas), guesstimating my cold tire pressure on a 90 degree day...