alexb76

@Gary, no disrespect intended, and I am not debating the science nor your expertise. All I can go by is to apply the thoery and see what works and 33/39 worked for me, and no matter how you and Tony beg to differ, it doesn't change the facts on the ground and that's the bottom line! Also, since I am in Vancouver, my experience is MOST relevant to OP, Joel who I know locally!

Now, tell me this, if conditions were reversed and I set my cold pressure at 37/44 (for max load) at exactly 20 degrees, and then I went tracking and my tire pressure went up as high as 50 PSI in the rear, shouldn't I DROP the pressure as I am at max tire inflation limit?! I always have to adjust the pressure down significantly during tracking as I lose traction at anything above 46 PSI in the rear, so what I do is continually adjust pressure to keep it between 33/39 - 38/46 maxiumum during all sessions and that's when I got the best lap times, cornering speed and handling. I then HAVE TO ADD AIR after sessions are done to keep it at 33/39 for street driving. Logically I think the same adjustment needs to be applied in cold temps when your inflation drops and one needs to compensate. Again, that's how it's worked for me, as recently as yesterday.

Lastly, please don't assume it's only you and Tony who went to Graduate school, you both are around my parents age, so I do respect your opinions, but I don't appreciate sarcasm nor arrogance as I also went to Gradate school and I've learned to challenge theories when the real life experience is different. It's all about learning here, so I am all ears, love to learn the theories and see how it works for me, if that doesn't, I adjust accordingly!

Cheers!

simsgw

Oh damn. I thought that technique was classified after its development at NASA!

ADias

How do you know your father and I are the same age? Are you psychic?

alexb76

I know urs from the Age/Occupation thread... you're 3 yrs younger than him!

ADias

Not me. Not on that thread.

alexb76

Not the recent one, the older one that I even tallied the average age. It doesn't really matter... you definitely got more birthdays than me, and hence I do listen to you and Gary even if I disagree in some cases.

equ

As a one-time physicist and a tire pressure adjustment fanatic, I'm with Fred & Alex on this. That chart is simply pressure's relationship to temperature, nothing wrong with that. It's silly to say 'don't think about below 40F'... What about winter tires or any other car?

To put it simply, 32-33 front and 38-39 rear (real gauge measurement, my TPMS reports +1psi too high) are good pressures to begin your driving for the day... PROVIDED... your garage temperature is not too far off ambient temperature. If your garage is much warmer, you probably need to air up a bit to account for your already warm tires, conversely if your garage is much cooler, you'll find yourself on overly-inflated tires.

When street driven, all four tires seem to gain 2 to 4psi, this happens whatever pressure you start at (assuming you're not too far off manuf' specs so they are heating up uniformly). Perhaps 2psi on a shorter, cooler drive and ultimately gaining 4psi after a long drive (full warm up). So you've reached about 36-38/42-44 hot. Guess what? That is the pressure window dialed in by the manufacturer for street driving. That's the pressure at which the tire has the correct profile for optimal grip and wear and weight support. They don't say 'adjust to 37/44 hot' in the manual because a lot of people don't know how long it takes to get to 'hot' or don't know what 'hot' is. Apparently even 'cold pressure' is open to interpretation.

If we went by the alternative interpretation, we'd never have to adjust air up tires when ambient temperature dropped, as the drop "required" is the drop that happened. This is what one side is getting confused by.

alexb76

+1

Fred R. C4S

equ,

Please.......I tried to end this peacefully before I had to get my two best (Japanese bamboo and German aluminum alloy) sliderules out for a duel. Let's just all agree to disagree although I'm with you on this one.

ADias

Boulevard driving maybe a 4PSI increase. 6 PSI increase is what I routinely find. talk about empirical knowledge.

And re the 40F lower temp limit... that applies to summer tires. Winter/snow tires is a different ball game.

simsgw

Worse yet, of my two sliderules, one is circular and God knows where using that would take this discussion.

But Alex did ask politely about track work and I feel I owe him an answer, so before we drop this:

First, let's never mind what we do in race work at the front of the pack. You wouldn't believe how complex the problem is when you know all the factors involved. I'll just say we don't set pressures based on driver comfort level with the handling. And you need a contact pyrometer and a proper racing pressure gauge good to at least a quarter psi to use our procedures. Every time I started an even cursory explanation here, I ended up with a bunch of thousand word paragraphs that mean nothing in fun days at the track.

In simple terms, the reference pressure we set in the morning should work all day -- even when the day includes track work. The rising and falling gauge pressures reflect the temperatures of the tires in response to work, and as I said before you don't want to interfere with the feedback loop that protects the tires and keeps them within the working range of temperatures and clamping pressure for the tread compound.

I do suggest using different reference pressures on a track day, as do Michelin. My own reasons hark back to that technical discussion too complicated to attempt. My description of my own application of the racing procedure is around here somewhere and can be found with a search I'm sure. I arrived at 32/35 for the Michelin Pilot Super Sports. That may not be the optimum for other brands and models, but with all the complex factors we're ignoring here, it is the one that will give the correct gradient of temps across the tread of the MPSS on a Carrera 2S, and it results in a tread temp measured in the pits of 160F-170F, which we surmised is a good target for MPSS.

Michelin Cup tires are a different compound, and their target temp range for the compound is much higher, so they need a lower reference pressure to get the tire working at its best grip levels. The PS2 is another different compound and structure as well, and may have a working range a little lower than the MPSS. (Or maybe not.) But a PS2 may need a pound or two one way or the other. The front/rear differential is driven mostly by the car's suspension. I arrived at it with track testing, but it is no surprise it's the differential Porsche is recommending with the latest owners manual supplement. The point is that track work puts different demands. We want the highest grip levels the tires can deliver, so we want more steady-state heat to keep the tread up in its working range. Lower reference pressure=higher carcass heat.

Choosing those pressures, we didn't use lap times at all. At a track day, those are too influenced by subjective driver response to handling changes, and by traffic issues on course. We did use the equipment I mentioned. That of course means we were adjusting the pressures at trackside to reach the 'right' pressures and mostly we were bleeding pressure because I started on the high side. Adding air is more difficult than bleeding without taking along equipment I didn't want to carry. But I still don't recommend you do it. Not unless you're going whole hog and doing it the way we did as described in that other note. Here's why:

The range of pressure and temps in a combined use tire like we run is very broad. I am running one set of pressures at the track and another for road use solely because I want the best performance at the track but on the road I'm more concerned with tire wear than the last tenth g of grip. That trackside setting for the tires provided fine performance on the way home as well.

Now about pressures we saw at the track. Notice that working temp we used as a target: 160F-170F. That's a rise of 100F from the reference temp and produces a pressure rise of seven pounds. We were measuring (with a barometrically compensated racing gauge) pressures of 39/42 right after leaving the track. And the on-board TPMS was accurate as well, though obviously not as precise since it only reports to the nearest whole degree.

My point? Well, some people complain of their tire pressures rising as track days progress. They should not. They should be higher in the pit than they were at home, and they should be higher while you're on the track if you know how to infer that from the TPMS, but they should drop back to what we might call a 'warm' pressure once they sit in the pits awhile. Just for a number, let's say 36/39 if you start out with them using 32/35 as a reference pressure as I will be doing from now on.

When you observe the tires getting slippery at higher pressures, I doubt they are "going off" as we say it in racing. That really means [...well, never mind. There's a thousand words saved.) I would ascribe it to contact patch control. [Another 1000-word paragraph I just deleted.] 50 psi is just too high, or at least the limit of 'workable' and certainly not optimum when you care about lap times. Don't start out with the max-load pressures Porsche specifies. Not unless you're hoping to set those lap times with three big friends riding along.

If you start at a lower reference pressure, something like that 32/35 I'm using now, think how hot the tire would have to get to reach 50 psi. It would have to rise 215 degrees F in the rear. That's 215 above reference, so the measured temp of the carcass would be roughly 285F and it would be toast. Worse yet. If you lower the pressures, which have been protecting the tire by limiting flex, then the driving that led to such high heats will still be there and the tire will flex even more, so it will heat faster. Toast, as I say.

If you pick the correct reference pressure for track days, the pneumatic feedback loop will keep the tires in their optimum range. That doesn't mean you can't influence their grip with your driving style. You can drive in ways that don't put enough heat in the tires, especially the rears of a Carrera. Or you spend too much time spinning those wheels out of corners, they will heat too fast. Much the same in front.

All that is about driving technique though. And if we think tire pressure settings are contentious, you can imagine how many opinions exist on driving technique. Well, sure you can: that's why we have races, to avoid pistols at ten paces in the old-fashioned way.

Gary, who has decided to file cold-pressure-setting with "religious preference" and declines to extend the debate on that;
and on a completely unrelated subject, finds that he hasn't time to proof this because Cindy needs him but he'll look for any embarrassing oversights later

Zeus993

So in reviewing the extensive opinions stated... 33 / 39 right?

gota911

But those numbers are correct only if the font of your car is to the left of your view point. If the front is to the right of your view point, then the correct pressures would be 39 / 33, assuming you read left to right.

Zeus993

ROFL!!!!!!! Yes! Exactly!

simsgw

Well, alright. I've had a question off-forum that was equally sincere and polite. I do not offer to extend the debate, since we've been friendly so far and found a civil ending as well. But just for those who care about cold-weather tire management, here's what we just discussed in private, quoted at the suggestion of my interlocutor who had said:
I did, but I was much younger then and did not at first. I freely admit that the Midwest is where I learned this. I was just going out, looking to see if any tire looked 'low' (which was a plausible method with those high-profile tires) and if one of them "needed air" then I would stop at a gas station and add some air, bringing the others up to match if need be. Then one of my students... (I was a professor of Mathematics and Computer Science) asked me about tire pressures on aircraft and "even our cars" that made me think about this and research the answer. You know that effect of teaching. Forces you to think about things you've always taken for granted.

There in Duluth it might be -50F overnight, but only at the worst times. Let's say a balmier -30F night. By morning when I left for campus, it would be around zero. So that tire that looked low had a reference pressure of 32 psi, let's say. Sitting in the driveway at 0.0 F, it would be 70 degrees under the reference, so its gauge reading would be 27 psi if it had the right amount of air. Looking "a little low" might mean it was down around 22 or 23 psi. If corrected sitting there to a pressure that instinct told me to set, I might add enough air to reach 32 psi again. Discounting the warming effect of compressed air, that tire would be at 32 gauge, but actually 37 psi once it warms up to 68F. Now it's over-pressured because it will reach 68F in about a block from flexing. And then it will have five psi too much.

The pavement is warming from sun but basically still at no more than 10F, but it doesn't have enough contact with the tire to drain the heat of flexing. I'm pressed for time and can't stop to do the arithmetic, but as a guesstimate, the contact area is less than one part in thirty, so that thermal path is pretty constricted. That means the tire continues heating. Not as high as summer tires of course because the air mass flowing over the tires is cooler and a bit more dense as well. Naturally, the stabilization point is lower even at the same speeds. On the other hand, the tread is thicker so it generates more heat when it flexes. Net effect is cooler stabilization point, but not what we'd call 'cold' in any other context. Just not as 'hot' as summer tires coming off a freeway run.

The compounds are chosen to work at a low temp for winter tires, but the tires will warm up amazingly. I remember watching the tires steam in that sort of weather, and if you sat for a minute in a freshly snowed parking lot, the snow would melt into a warm puddle around each tire. So the tire keeps warming up and reaches... another guesstimate, but probably 120F in urban driving. That's very likely where the target temp for the compound lives. Give or take twenty degrees to handle slow driving and highways as well. [Supplemental to my original reply off forum: We know the compounds must have a low bottom end for their effective temp range because in slow driving, we've all seen tires with snow clinging to them and even filling the sipes.] At that temp[it was 120F - pardon my interjection], it's supposed to be at 37 psi if the ref pressure is 32. In fact, I started at 37 because I over-pressured it. It won't warm as fast because it isn't flexing enough, so it won't reach 120F. Maybe it stabilizes at 110F or 100F, so the gauge pressure would read 39 or 40 psi. Still over the design intent, but not outrageously. Not by as many psi as I overfilled it back in my driveway. This feedback loop not only has dynamic effects, but it also helps statically by compensating for driver error. If you overfill, less flex means less heating, so the system is forgiving.

In truth, that never happened for most of us. We didn't keep air sources around the house. By the time I got to the gas station of choice, the tires had been driven at least a few minutes and that early flexing ramps the temps fast. Even a couple of slow miles will see them up to at least 90F I'd say from memory. [About 35-year-old memories I'm afraid.] That is a lousy source of data, but I'm picturing the interesting effect of a very cold crisp morning and the tires being 'cool' at most to the touch. So in fact, I'm setting the pressures when the tires are at least reference temp, even if the weather is not. If I know enough to correct to ref temp (as few did in those days) then we're fine. If I don't know that, then I use 32 psi and at worst I'm a couple of pounds low because the tire has risen above the reference temp.

That pneumatic element is a godsend for making practical tires, ones that don't require professional training for the maintainers, who are normally clerks and housewives if we're honest.

[As I said, I don't offer this to stimulate an argument. Merely to explain how the theory works as well for winter tires as it does for high-performance ones. As for the specific numbers that I'm guessing from memory of visual appearances, pyrometers were much more expensive in those days. In fact, all electronics were. My big personal splurge was an HP-65 calculator from the university book store. I'd be interested in specific values if anyone has a contact pyrometer and lives in that sort of climate.]

Gary