That's pretty much the way I read it. The pressure spec is what the gage pressure at the tire should be at 20C (68F). If the tire temperature is less than that, you need to set the tires to a lower pressure; if it is higher, you need to set the pressure higher. That's a troublesome error prone process unless you have a race engineer whipping out an infra-red thermometer and a calculator for you everytime you want to check the tires. So Porsche built the calculator (and a thermometer for each tire) into your car.

The info-pressure menu item tells you how much to add or subtract from the current pressure, and that screen gives the correct change in pressure whatever the tire temperature is, whether the tires are at a cold or hot ambient, or even if you've just come off the track. That screen will tell you how much air to add or remove, and you are supposed to ignore the gage pressures being reported on the other screen. They say that at least twice. Ignore the pressures that read 32 or 34 or 43 or whatever. Just pay attention to the plus/minus values in the info-pressure display. Those are correct no matter what the temps are.

The first display is the one that has warnings about not using it to change your tire pressures. But not the plus/minus one. See page 146 of the manual where it says of this latter display, in bold face no less: "You can read the tire pressures to be corrected in this display." Then you can see the graph of temperature correction used, and the base temperature at which the specified pressure was computed, over on page 154.

Gary

 

Gary: I do not agree with that. The pressure spec is the pressure set on cold tires (as a rule first thing in the morning), whatever the ambient temp is. If the pressure is set when the ambient temp is 68F, air must be added to maintain the same pressure on a subsequent colder day or air released on a subsequent warmer day (all ruled by PV=nRT). In other words, ideally, on any given day, when tires are cold in the AM their pressure should be set per spec (33/39 997.1, 34/40 997.2). This implies that when seasons change, tire pressures must be readjusted.

The spec temp is the minimum air pressure to hold the weight of the car for a given set of wheels/tires. As the tires roll and warm up the internal pressure goes up and that is within the performance envelope of the tire/car. The situation is different on a track where tire temp can reach 200F+. In that case air is taken out of the tires once they warm a bit to prevent overpressure when really hot.

Side note: in the ideal gas equation PV=nRT, P is in atmospheres and T is Kelvin.

 

I have always followed the procedure about setting the pressure after the car has set overnight to the recommended "cold" pressure irrespective of whatever the ambient temperature is. However, if you look at the pressure versus temperature chart that Gary references on Page 154 of the Model Year 09 Carrera Owner's Manual, it actually shows a green line labeled "Required-pressure line" that has a constant slope that shows a pressure of 30 psi @ -20 degrees C, 33 psi @ 0 degrees C, and 36 psi @ 20 degrees C (68 F). On that same page, it also says.

"Tire pressure specifications

Information on tire pressure for public roads can
be found in this Owner's Manual in the Technical
Data chapter or on the tire-pressure plate in the
left door aperture.
These values apply to cold tires at 68 °F (20 °C)

ambient temperature."

I'm not planning on using lower pressures than those on the tire-pressure plate even if I am adjusting the pressures at 40 degrees F, but the info in the Owner's Manual at least seems to imply that a lower pressure at a lower temperature can be equivalent to a higher pressure at a correspondingly higher temperature in accordance with the Ideal Gas Law.

 

I think that the manual is at fault there and the chart is there simply to show the temp dependence. the bias points pressure temp are just examples. No one should leave at 32F their 33PSI set at 68F. No way.

 

Agreed. The Tire Rack site has some excellent information on setting pressures and the effects of temperature fluctuation, and your advice in your previous post is completely consistent with theirs.

 

Another misconception is that atmospheric pressure affects tire pressure. It does not. At least on our stiff low profile tires. It does not matter if tire pressure is set at sea level or on top of a mountain. The internal tire pressure will be the same at the same temperature.

What is critical is to calibrate the tire gauge every time it is used and at whatever altitude the measurement is made. Most digital gauges (Accutire) have a calibration (zeroing) mode.

 

To explain a little further why it's the measured pressure that matters irrespective of the altitude, it's necessary to bring in the concept of absolute pressure versus gauge pressure. For example, atmospheric pressure at sea level is 14.7 psi. A tire with a typical tire pressure gauge reading of 30 psig (g for gauge) would actually have an absolute pressure of 30 + 14.7 = 44.7 psi absolute or psia. At 10,000 feet above sea level, assuming the same ambient temperature, the atmospheric pressure would be only 10.1 psia, but the tire's pressure would be 30 psig, but with an absolute pressure of 40.1 psia. Note: this assumes the pressure of the tire measured at 10,000 feet above sea level was set to 30 psig at that elevation with a properly calibrated gauge.

The tire pressure gauge will always read in psig, which is the pertinent measure for proper tire performance. As you say, it is always important to use a properly calibrated gauge, calibrated at the measuring altitude.

 

And that folks, is why we have so much entertainment value in professional journals. Here we have two people -- and several others as well I gather -- who are all trained geeks. Official Geeks, you might say, but on this simple question, we could have fun throwing equations at each other for several issues. About the second issue, one of us would assign a graduate student to do the grunt labor setting up experiments.

Always assuming this were not old stuff, which this particular question is. If I may be excused for putting words in adias's mouth, I believe his concern is that modern low profile tires deviate a great deal from the pure, completely flexible bladder that we use to illustrate this question for students. The classical explanation is roughly this:

Start with a spherical bladder, shaped like a ball that is, inflated at 20C to a gage pressure of 10 psi in a fully evacuated vacuum chamber with an arrangement to load the bladder with any weight we choose. The gage pressure will also be the absolute pressure because -- by assumption -- no atmospheric pressure is complicating the question.

Now we put forty pounds of weight pressing that bladder down onto a glass surface and look up at the ball from beneath the glass. The bladder will be flattened. It will no longer be a perfect sphere, but a truncated one. That is, the weight will force it down onto the glass. (In this thought experiment, let's suppose we have suspended the weight within the bladder somehow without distorting its shape by the connection. Now the only distortion will be that imposed by the pressure between the flat surface and the erstwhile sphere.)

Given all that, which is much longer to describe without a graphic at hand, what will be the area of the bladder's surface that is pressing on the glass? In this pure thought experiment, the answer is simple arithmetic: 40lb / 10lb/si = 4 si, or four square inches. From that known area of the flat segment, we could compute the new diameter of the rest of the bladder. The internal volume will stay the same (given these assumptions) so the remaining surface must expand to allow for the flattening of that one segment.

Let's double the gage pressure. At 20 psi, the bladder does not flatten as much. Since each square inch now supports 20 lbs, we need only two of them to support the weight. The flat segment will be only 2 square inches now.

Now raise the ambient pressure in the chamber to 10 psi. The outside of the bladder will be subjected to ten pounds over each square inch of its surface. That leaves the net pressure available to resist a load being back to 10. We had 20 psi absolute pressure, that is pressure measured in a vacuum, and now we still have that, but the gage pressure has dropped to ten because a gauge will always read the difference between ambient and the pressure at its opening. The bit we apply to the tire valve that is. So as the ambient pressure rises to 10 psi, the flat segment will grow back to its four square inches again. It must. If we raise the ambient to 20psi, the bladder will be flaccid. It cannot support weight because it has no pressure differential to work with.

This is all traditional physics of pneumatics. At least as explained to freshmen. So how can several Official Master Geeks reach different answers about a real world problem that seems directly related? Well, the real world ain't that simple.

Just to name one obvious hole in the thought experiment, I said the material of that bladder was completely flexible. And it expands and contracts perfectly. We often use mythical 'perfect' materials to simplify issues. But go back to the first condition. The vacuum chamber is evacuated, right? If the bladder is a perfectly flexible material, why doesn't it respond to the unresisted internal pressure of 10 psi by expanding indefinitely? Well, a perfectly flexible material doesn't mean one that does not resist expansion. For this experiment it would mean one that expands with resistance that increases in a linear fashion I suppose. Or more realistically unrealistical, it could be like a perfect spring whose resistance... oh never mind. You see? See how this is growing more complicated as I introduce the factors that every upper division engineering student learns, but would have confused the basic lesson for freshmen?

Fine, so we can compensate for the resistance to expansion of the bladder's skin. Simple arithmetic won't do, we need calculus, but those upper division wannabe engineers are supposed to be getting a grip on calculus by that point in their adventure. Tain't enough.

Now I'll just sketch the issues. The reasons it isn't obvious that traditional simple pneumatic rules apply. First, viewed statically, a modern tire, unlike those fish skins they used to put on Model T's, is not very close to a pure bladder. Not anymore. It is a mechanical structure of some significant complexity. Very high performance tires like those you're supposed to put on a Porsche are as complex as it gets for street cars. (Aircraft pose even more difficulties that we can ignore. Race cars of all sorts take these road complexities to their extreme.) A high performance tire uses several components to do it's job, which includes not only supporting the static weight but also controlling the behavior of that flat segment as other loads are applied. We call that flat bit the contact patch of course.

Consider that a tire is distorted not only statically by the weight but to an amazing degree by dynamic forces. Some of them are obvious. I'll skip those. But here's one: That flat spot does not stay in one place. It moves around the bladder. (i.e. the wheel rolls. Wow. Big deal.) But that is like ripples in a straight object like a guitar string, right? When you speed up the ripples, you change the frequency. Vibrations. That is a dynamic effect. Lots of that going on and the harmonics of those frequencies affect the tire structure and the suspension as well. (Adias tells me off-list that they actually measure those harmonic effects on tires like these, and they reject any tire whose body is not regular enough. Lumpy, to over-state the case.)

The structure of a tire must deal with all those dynamic effects including the obvious ones, like centrifugal force (and we'll skip the terminology debates, thank you), side deflection from cornering forces, slip angle on the tread when cornering, fore and aft forces when accelerating and braking, and so forth and so on. A tire for even a simple road car is a creation that works only because we've thrown away all the methods of construction that failed over a period of more than a century. Conventional tires are a design arrived at by the classic method of ad hoc, cut and try, with regular injections of ingenuity. When you deal with cars that go into realms that earlier cars could not, then the dynamics of designing the tire become a nightmare quickly. In fact, although this is not my field, I very much suspect they couldn't design a modern high performance tire at all without modern computers.

The current state of the art uses synthetic rubber compounds as the matrix for a composite material. But the compounds vary across the width of the tread and on the sidewalls. They don't just hold air in. They have to deal with the shear forces and... well, that's enough of that. Then we have the belt beneath the tread which must limit expansion despite very significant centrifugal force at high speeds. Remember these tires are designed to accept 150 mph routine and they have to deal with the thermal migration from the tortured surface of the tread to other parts of the tire, else they would heat up without limit. Measured in the pits after a lap, tire temperatures are usually 200F or less. A car on the highway will be much cooler. But if you put a non-contact recording gauge to read the tire temperature at several points across the tread, you find the peak temperatures while cornering are nearly four hundred degrees.

Valkenburgh considered 370F to be a routine maximum. The temperature when the tire was producing its greatest side force in a high-speed corner. He was writing of race tires a generation ago when they could only produce a side vector of about 1.25 times the load. (That is what we usually call 1.25g in a road car, but if you have aerodynamic down force pressing the tire to the track with a load that is three times the car's static weight, that 1.25 will turn into nearly 4 g of cornering power, as it does on serious open wheel race cars.) Current tires produce more side force, and though I haven't been on a track in years, I believe they operate at higher temperatures to do that. No matter. Settle for 370F. That's a medium oven that will cook a roast. The tire has to be able to dissipate the heat somehow, just as brakes must dissipate the heat they create when absorbing the kinetic energy of the car.

In an open wheel race car, you can see this happening in each corner. The hottest part of the tire gets a slick shiny look when you're cornering at the limit. The cooler parts of the tread are dull. You have other things to do of course, but it is fascinating to let part of your attention watch the shiny part move from side to side in a complex of corners like turns two through six at Willow. Mostly in practice of course. Even more important, you can see this shininess develop across the whole tire while braking. When it does, you are near the limit of adhesion. Right where you want to be. Any more brake force and you'll see that tire start to slow and then stop turning completely. There. You just flat spotted a $200 tire. And wait 'til you see what that does to the dynamics.

Now all of that -- and lots more I haven't mentioned -- creates a complex dynamic set of forces that the structure of the tire must absorb. Not just absorb. It must manage those forces in ways that contribute to the car's handling. Resisting static weight is a minor part of that, and in fact our very low profile tires can do a pretty good job of resisting the static load with "no air" in them, or to keep our terms precise, with the internal pressure no greater than ambient. The radial cords and the stiffness of the tread both resist going 'flat'. I quote that word because the contact patch is always intended to be flat. As flat as the designer can manage, given the dynamic situation. That's why you can't rely on appearance to decide whether the tires need air. But the tire structure must resist going too flat, and on low profile tires, it does that fairly well, even without a run-flat rating.

Then why worry about the air? Well, mostly for cooling I suspect, though we'd have to arm wrestle a Michelin or Bridgestone engineer to persuade one to release company secrets. Of course, once you put the desirable density of air in the tire for cooling purposes, that fluid filling the tire body now has other effects that are mostly desirable. The contribution to resisting the "flattening of the bladder" effect comes into play and lets the side wall of the tire be thinner than it must be on a run-flat tire. Not to complicate this already difficult picture, but it may not be thinner, just lighter in weight. We still have to get the heat away from the tread, and while the air contributes to that, its thermal path is not enough alone. We have to have the sidewalls carrying away heat to the wheel, which is a massive heat sink compared to them.

So what I suspect is that Adias is concerned about the mass of air more than its gage pressure. Again, I'm not speaking for him, just speculating. But you see how he can arrive at a different answer than I do from the same starting point. The problem is too bloody complex to know the right answer without a lot of expensive research. (Or some fairly cheap grad student labor...) Michelin and Bridgestone have already done that research for us, but the owners manual is hardly what we like to see as a report of research.

Intuitively, I think Porsche and their tire suppliers have considered this problem and boiled it down to a target air mass that creates a particular gage pressure at a particular ambient. Using that method gives the tire the air it wants in all the dynamic situations that arise. I don't think the manual is wrong, just a very terse summary of a complex situation. It amounts to saying all that stuff above and then adding: "We want a more precise amount of air than you can estimate with instructions to set the gage pressure at an arbitrary ambient you personally consider 'cold'. Therefore we chose a nominal mid-point, 68F, and we are measuring how much the temperature deviates from that as well as the gage pressure in each tire. Just do what we tell you and either add air or remove some. Whatever it takes to make the computer happy. We programmed it to do what we want."

Personally, I'm used to that. It's what a race engineer does at tracks. Rarely is the direction "put in 32 psi" except to have some reference when starting a series of test laps. What you hear is "take out 1 lb from the left front. Leave the other alone." It doesn't bother me to have a computer calculating that for me. Even without a dozen test laps.

My own reasoning is this. As a part of the structure, air has the significant advantage that it moves in the right direction in response to dynamic loads. When the tire is working hard, the heat that moves into the internal air causes the pressure to rise. That stiffens the tire, as if the structure itself were made with tougher (or more) material. But temporarily. When the tire quits working so hard, the internal air cools as soon as the tire body does. This lets the gage pressure drop again, the body of the tire becomes less stiff, and that is desirable. Not just for comfort. Comfort is a secondary question with a race tire or a tire for a car with the performance of a 997. The tire is part of the suspension and it has to be supple to let the contact patch follow the road surface optimally. Again, when the temperature is lower than nominal and the tire is not working hard enough to warm up, the gage pressure needs to be lower to take away some stiffness. The solid tire materials are already more stiff than we want them, so we don't as much contribution from pneumatic pressure.

So: a tire in nominal conditions must be supple, it must flex with relative ease to provide the performance we seek; but when taken to the peak levels of stress, the tire needs relief. When the cornering or acceleration forces drive the surface temperature up near 400 degrees, the body of the tire must stiffen and flex less to reduce heat build-up from that source. Air makes all that happen as a natural result of gas physics.

Put another way, the air isn't in the tire to keep the wheel off the ground. We could do that with the other solid components of the tire. Air provides the response to dynamic loads that helps the solid components be lighter and more flexible. To play a more effective role as their part of the suspension. It takes a certain amount of air to do that. And yes, I do think gage pressure is more important than absolute, though I'm not going to muster the research to support my intuition.

I agree that two lines in the manual are not enough attention to be given to such a complex question. They have a computer that will occasionally tell us to do the unthinkable: let air out of a warm tire. That calls for more explanation than a quick graph in my opinion.

It is too terse, but I don't think it's mistaken. What I do suspect is they use a technique common to other engineering fields. Explain the topic to a "technical writer" who presumably speaks English, not Geek. Or at least German or some other human language, not Geek. Then explain it again. And again. Once the technical writer seems to nod at the right points, go back to your real work. This is a revered technique of engineers and other Geeks when someone invades their domain and wants an explanation without first learning the language. The Geekish, that is.

What we have in the manual is a technical writer's version of this stuff above that an Official Geek considers a 'simple' explanation. Aren't you glad?

Personally, I'm going to believe the manual, and therefore the computer -- but I can fully appreciate the position of others who hold to the traditional methods.

 

I think what this implies is, if I set Cold TP at 30F (and set it to 68F spec pressure), that 1/2 hour or so into driving, once the tires warm up and ambient temperature is no longer having as much effect on the tire's temp, that I'll need to let out some air or the tires will have too high pressure. I think the tire's relationship to ambient or seasonal temperature tends to fade pretty quickly once on the road. I would guess this is why the TPM system measures tire temperature rather than ambient temperature.

I do agree that "warmed up" tire temperature in cold weather is likely to be lower than in hot weather, but I never went far enough with thermodynamics to have a good feel for what role each bit plays; the tire, the road friction, the wheel a heat sink, and the heat transfer to/from the atmosphere. Obviously the tires get hotter, up to a point - they don't continue to heat up and then melt - normally. So something is effectively removing some heat - even in the summer.

Setting the tire pressure always at the same pressure at cold, regardless of what cold is, seems to be ignoring all other sources of tire heat other than outside air temperature. Doesn't seem right to me. Not unless the operating tire temp is lower by the same amount ambient is - and maybe it is, just seems unlikely.

At any rate, my reasoning says go with what the Owner's Manual says. Not only because it make sense to me, but also I suspect someone at PAG put way more time and engineering into that TPM system than I have - someone who has a better grasp on the thermodynamics.


BTW, I did verify yesterday that the +/- Tire Setting indicators do remain constant when driving, while the absolute pressure indicators change continuously.

 

most people I know who bought cars with TPMS - Mercedes, Porsche, etc - have some issues with it. It does not seem to matter who makes this, to me it seems that concept itself is kinda flawed.

All it is good for essentially is to notice and inform you if you got a flat tire.

Worst you can have is to have some brains in this TPMS thing when it suddenly decides you got too much pressure and starts draining air out while you drive the car.

It was one of options I looked specifically to avoid to have on my car.

 

The graph on the manual shows the pressure/temp relationship but does not say - or if it does it is wrong - that cold tires should be under-inflated on cold days and overinflated on hot days, because they should only be at spec at 68F.

There is no way that the spec pressure should be referenced to an ambient temp of 68F. If that were the case on a 38F day the tires should be set 3PSI down (pressure varies roughly 1PSI/10F). We do not do that - on a 38F day we still make sure that the cold pressure is the spec pressure.

It is true that if a cold tire pressure is adjusted on a 68F day at the spec pressure (barring air leaks) on a subsequent 38F day the cold tire pressure will be 3PSI down, but its driver should bring out its compressor and raise the cold tire pressure 3PSI to the correct spec pressure.

Furthermore, the TPMS diff +- mode takes the temp variations into account and shows the diff temp on the various tires at cold temp. Be forewarned that if it shows a -2PSI diff that diff is at cold not when tires are hot (even though it shows the same -2PSI diff when hot).

 

But why?

I do not understand why there is a reliance on cold tire pressure, when in the general case of driving the car, the condition is nearly irrelevant. I do see how it simplifies the notion of airing up the tires and provides a consistent process, but otherwise it seems overly "simplified". As well, the notion of "cold" is completely arbitrary. Mixing specs and arbitrary measures creates goo for me.

A better question is: what pressure is correct for the tires at operating temperature? Since this is how we are actually using the car, versus storing the car. I know this is important - you can feel it when driving. I think our cars give us a better answer and a more sophisticated approach to setting tire pressure.

 

Cold temp setting sets the pressure baseline (weight holding) for the vehicle, that is why. It also assumes that the tires will warm up 30-40F in normal driving conditions.

On the track it is different. There tires heat up substantially and on subsequent runs air is bled to compensate.

There's no 'oversimplification' in what I said and nothing 'sophisticated' in the TPMS reading which when interpreted correctly is the one and the same.

 

This is a good article - long and detailed, and more track oriented, but it does discuss the contribution of ambient temperature to operating tire temp.

http://www.turnfast.com/tech_handling/handling_pressure

According to that author, ambient temperature contributes more than I would have thought.

By more sophisticated, I meant that the TPM is adjusting recommended pressure offsets with tire temperature compensation. This should allow for corrections even when the tires are not cold. Of course you could do those calculations manually yourself as well, the TMP just makes it more convenient.

 

The TPMS is not very sophisticated at all. It is simply a pressure and temp sensor and a buffer memory displayed on the dash. Nothing more. The Physics still need to be understood.

I urge all to really make an effort to understand this issue. I have seen too many tires blow because people run them too low. Especially low profile summer tires.