alexb76

I am neither mechanical or aerospace engineer, being a Computer engineer. However, logically and from personal experience/trial, I have to 100% agree with Fred.

I had to do this EXACTLY TODAY! I haven't changed my tire pressure this season since temps dropped, my usual 20 degrees 33/39, has been dropped to 29/35 cold (morning), and to 30-31/36-37 warm (after a long drive at high speeds), the car felt like CRAP, and handling was absolutely bismal. As I've seen this debated before, I tried the *chalk* experiment and drove around today and did some spirited driving, THE TIRES DEFINITELY ROLLED MORE than before and were right into the sidewal... I then adjusted the pressure to 33/39 COLD, and then drove around... NIGHT AND DAY DIFF! The car's definitely handles better, and I do NOT roll as much as I did before (with cold 33/39 at higher temps).

So, all that long explanation falls flat in real life experience at least in our climate where temps fluctuate more than say California. I just basically try to maintain cold 33/39 at all outside temps, and it's the ideal pressure for both handling and ride.

simsgw

Just down the road, we have the headquarters, publications center, and presidential hovel of the Flat Earth Society. The X-15 flights used to take off right across his ... uh, the organization's property. That's the nice thing: everyone is entitled to their own beliefs.

Gary

ADias

I assume you are running summer tires. Assuming that, I understand the symptoms you describe but you reach the wrong conclusion. If your rear tire pressure (39PSI at 20C/68F) dropped to 35PSI, means that the tire cold temp was -6.7C/20F. No summer tire functions at those temperatures (minimum temp is 40F). You are operating your tires grossly outside their operational envelope.

And one more thing... Tires filled at 33/39PSI at 20F must feel and act as hard rocks. OK, it's your car and your life.

ADias

alexb76

And that means what? And how related to the subject? Apart from being arrogant and elitist?

Whatever your expertise, the real life experience and trial contradicts everything you took the 3hrs to write!

alexb76

Well, I can't remember the exact temp I set my car previously to 33/39. That was back in september I believe and might have been around 15-18 celsius.

The next adjustment today was at 7 degrees. So that's the drop of 9-11 degrees and yes it does not match ur formula but it is, what it is.

I take my car's handling, feel, ride, actual roll over opinions of stubborn folks anytime a day! thank you very much.

Maybe you should go out there and try it for youself and *might* learn something?

alexb76

At the end of the day. That's what worked for me. None of this is exact science. Jenson Button and Hamilton run different tire pressure on their McLaren cars to suit their driving style, preference. As long as ur not overly under inflated or over-inflated a cpl of PSI here of there is a wash.

Peace!

simsgw

You'd be surprised how quickly professors write, but I was trying to keep this friendly. If you prefer a long answer:

You can tell Tony and I went to the same graduate school. The first thing that caught both our eyes was your running the same pressures while the temps dropped from a balmy 20 degrees down to ... well, more on that in a moment. Cold anyway. And if you kept the pressures, you clearly kept the tires wrapped around them. The air gets away otherwise. That isn't a good thing you know, tires appropriate at warm temps might as well be carved from marble once the temps get down toward single digits.

You didn't specify, but I will assume you're using those big fat degrees that Celsius invented. For consistency with the previous discussion, I'll translate everything to the Fahrenheit version that began this thread. That of course makes your "20 degree" settings into 68F and presumably you mean that you set them originally the way we described. That is, either you chose a perfect 68F morning, or you did the arithmetic to find the compensation.

Taking that as a point of departure, your tires showed only 29/35 this morning: That's a four-pound pressure drop and requires a temperature drop of 57F. That temperature drop means this morning was a crisp 11F for you. This is my longer version of Tony's "wrong tires" but we'll carry on a bit more. Being a pilot, I'm usually at least mildly conscious of weather a continent away. Monday morning must have been amazing at your house because the record low temp for the Vancouver Airport for this day (okay, this I had to look up) is 12F, or minus 11.1C to be precise. But never mind. Let's carry on.

Now you drove at high speeds and the tires rose to only (using the higher values) 31/37 or two psi above their pressure sitting in the garage. That much rise requires only 29F of temperature increase. They began at 11F and rose to 40F in miles of fast driving. Yeah, I'll just bet they felt like crap with those cryogenic coolers installed. Or we could suppose that your house was no colder than the rest of Vancouver, so they began at some temp within a few degrees of freezing. 5C perhaps or 40F, just for a wag. That puts them at 79F after "a long drive at high speeds". Not a cryogenic result, but ... unlikely, let us say.

Well, if this exchange were taking place in Transactions or the ACM my answer would be about like my comments on that 'demonstration' of cold fusion a while back, but since we're all friendly here, I'll just say I don't believe you. Nope. Sorry. Your data and your 'measurement' technique would be laughable if I believed your description. But I don't. I believe it was no colder this morning at your house than it was at Vancouver airport. Your tire pressures were under the Porsche spec before you began and if you drove at "high speeds", they would have gained at least 70 degrees (more likely 90) and been at least 120F by pyrometer. At that point, their pressures would have been 34/40 if we start from your under-pressurized assertion. If you actually had them at the spec before beginning, they would have risen to 35/41. You didn't mention the model of tires, but unless they are 185-R50 series, you didn't roll them onto the sidewall at those pressures, even allowing your 911 to 'plow' in a low speed corner.

Nope. I don't believe your data or your description. Sorry.

If forced to speculate, I'd suppose you are still running summer tires as Tony remarked, but their use is reasonable in November if you stay around Vancouver where it rarely gets below 40F until the middle of the night -- and not much below even then. (It's 35 degrees at the airport right now.) The tires were low before you began, and -- still speculating -- you don't have the experience level in racing to recognize the effect on handling of either a thirty-degree change in tire temperature or a four-pound pressure change that's equal at all four corners. And as for their working pressures being only two psi above their 40F 'garage' temp, I don't believe you left home in the middle of the night to run this test. So you began mid-morning at the earliest and more likely late afternoon if you have a job. That puts the ambient temp and the tire temps at around 70F already, so a rise of 2 psi would put them at 100F, which is still too low for a high performance tire after it reaches working temps by traveling at high speeds. Since I don't suspect you of lying about that, I presume you either don't have the same standard as most of us about 'high' or you didn't read the pressures correctly after that fast driving.

My 'opinion' comes with an explanation from Physics 101. I could ramp it up to EN603 level if need be. Would you care to attempt an explanation at any level of how your tires demonstrate a revised gas law that tire engineers never encountered? You probably should send a copy to Porsche so they can recall their owners manual and issue a new one for Canadian cars.

To clean up some details, you should know that temps in desert climates fluctuate more than coastal ones, as do our altitudes when driving in this particular desert. If you just mean 'colder', then you should consider that where Fred lives now is probably a lot colder than Vancouver ever manages. Duluth, where I lived before Dayton, has a summer/winter range of 180F at worst and I personally saw 150F swings from August to February. Duluth is a spa compared to the rest of the North American inland regions that stretch from about Nebraska up to the MacKenzie Mountain region where I described flying aircraft. And we do use pneumatic tires in aircraft, not wagon wheels. So much for "real life experience and trial." As for the science, it was while teaching flight cadets that this subject first arose, so I didn't conceive my explanation in an afternoon drive. Along with me, the FAA agree with Porsche, the damned elitist fools. We should send them a piece of chalk.

I think the version that asserted your right to your own beliefs was kinder than this, but since you insisted.

The bottom line is that it doesn't pay to attempt a refutation of science with loose numbers and crippled logic. Not if you hope for a respectful hearing. Do you suppose Porsche and I got these ideas about tires after a wild night in the bar?

I repeat: you are entitled to believe six impossible things before breakfast and scoff at science that was being learned three hundred years ago and has been confirmed by fifteen generations of scientists. It is your right to such beliefs, but you're not entitled to respect for them, however much you decry us as being arrogant.

Gary, who hasn't used "the chalk method" since they quit putting 60 series tires on sports cars and can provide a link to decent pyrometers if anyone cares

Zeus993

It appears for such a simple question on gas pressures that the temperature has risen as has the gas volume and pressure. In fact there seems (empirically observed yet unproven by objective data) that we've even created more gas that went in and may have actually violated one of the laws of thermodynamics. Eureka! Who knew, right?

ADias

It's not my formula. It's a proven physics law. If you say your data does not match it, well... your data is wrong. Actually... you say so, as you state that you do not know the temp you set your baseline 33/39. A 15-18C range is a huge range, but you do not seem to understand that.

Regarding you last cocky statement - "Maybe you should go out there and try it for youself and *might* learn something?" I am pretty sure I go out more than you do and my tire pressure experience matches what I described to a "T" and the on-board TPMS perfectly. I suggest you check your data more closely.

simsgw

Steady state theory revives! Hoyle would be so pleased. And I always thought he was just full of it. Gas, I mean.

Gary
P.S. I apologize to the forum for getting annoyed last night. Things have been a little fraught and my tolerance is not all it should be right now. Maybe I'll go perform some physics experiments ... on a golf ball.

Fred R. C4S

I once had a professor who told me, "The implicit variables are intrinsically meshed in such a away that no absolute solution can be found. In such cases we fall back on our empirical experience. In other words, we guess". Seems he wasn't far off the mark in many cases.

simsgw

No, no. Times have changed. With all the new technology, a modern engineer can do much more: a wild-assed guess.

Gary

Fred R. C4S

There's always the S.W.A.G. - Scientific Wild *** Guess. We usually saved those for when we really needed to impress someone.

alexb76

Amen to that.