There is no doubting what you say as it makes sense.

But on track in real world use on road tyres such as ps2 and PSS leaving pressures at stock results in things turning greasy and slippery.

On my first track day I went out on 33/39 cold settings and the car was great for several laps but then felt dangerous with less grip in the turns and the car feeling squirrely under braking, so much so I came back in. My pressures werevapproaching 50psi in the rear and the tyres burnt to the touch. I'd seen other guys dropping pressures in the pits and read online how anything over 40psi hot is bad news.

Fact is reducing pressures the car was handling great again and with pressures managed down to circa 38psi hot I was able to do 20-30 laps with ease with no lack of grip or odd handling characteristics.

Fact is my car drove and handled far better with hot pressures circa 38psi, leaving the car at stock pressures it was without a doubt worse, almost to the point of feeling greasy .

Infact doing a search here finds several other guys who have done track days finding the same, that when rear pressures on track exceed 42psi there is a loss of grip, so it's not just me.

So what your saying may make sense yet it seems in real use anything over 42psi on road tyres on a track day results in loss of traction and poor handling as found by myself and several others here from using the search.

Let's face is a tyre at 50psi is goning to have less contact patch area than when at a lower pressure and a lesser contact patch means less grip.

 

They never dropped the rear pressure on the GTS.

 

Please check your Owners Manual. They did drop it (lower it in the rear) from the C4S/C2S to the GTS. They all weigh basically the same and use the same tires. The C2S/C4S are 34/40. The GTS is 34/37.

 

That is not a good description of the relationship between contact patch and grip. As I said, the basic relationship is that the tire's coefficient of friction improves with rising pressures. Looking at the other part of the relationship, when the contact patch grows smaller the clamping pressure between the two surfaces rises, between the tire compound and the road surface. The material-to-material grip improves. The other relationship has to do with getting those materials into contact. As the clamping pressure rises, two things happen, one at the micro level and one at the macro level. Both mean that irregularities have a greater effect on reducing the actual area of the two materials in contact.

For a really macro image, picture the tire running over a stone the size of a peach pit. The pressures at the top of that obstacle will be higher, which is supposed to improve grip on that small area, but the truth is it moves the clamping pressure way beyond the working range of the compound so that benefit does not exist. Meanwhile, the obstacle has lifted a largish part of the tire's surface away from the road, so the one confined area at the top of the pit is trying to do the work, to provide the grip, of a large fraction of the contact patch.

We don't spend our lap on top of peach pits or stones (well, some do, but we don't) but the same thing happens at a micro level. Every small protrusion of the track surface lifts a certain amount of the tire away from contact with the track. Low pressure, like a softer compound, lessens that effect, just as softer suspension keeps the tires on the road better on a bumpy track. We're always shooting for a compromise between getting the clamping pressure up into the range the compound wants, heating the compound to its optimum temperature range, and avoiding the loss of contact area because of mechanical effects, because of micro-miniature equivalents of that peach pit.

Thus the track surface is critical to our decision. On a completely smooth track, like some skidpads made of polished concrete and 'smooth' for this purpose, we want higher pressures to achieve higher clamping forces between the compound and the polished concrete. That means a smaller contact patch for more grip, not larger. This is particularly true if water is present.

On a normal track, with more 'tooth' to the surface, we want less pressure. And as we get down to rough tracks we not only lower the pressure but we also soften the suspension. Again, the goal is to get as much of the contact patch into full contact material-to-material and to do that with the highest clamping pressure the compound will tolerate in its optimum temperature range.

We do have frequent comments about people finding the track 'greasy' when they ran book pressures. On the other hand, we have contrary reports of professional drivers and amateur racers with wheel-to-wheel experience. I found that the book pressure worked fine for me, but I preferred the dynamic response when I bumped the fronts by two psi. The Porsche drivers, according to Michelin engineers who set up the cars for them, like the tires at two psi over book, as Michelin sets them, and then they adjust the front/rear balance a pound or two to match their personal driving style.

I can't speak to the reports of less experienced drivers because I'm not in their pit with a pyrometer, and of course we'd prefer either telemetry or multiple recording pyrometers mounted at each wheel. My first guess would be the Top Gear effect. Jeremy Clarkson et fils spend their test time on tracks going sideways. That isn't the fast way around a track, but it sure is the fast way to heat tires and you might well push the compound way past its optimum temperature range unless you reduce pressures to increase the contact patch.

I'm not suggesting you or anyone else spends track time shouting "Power!" the way Clarkson does, but it is easy to get a mental image of fast cornering that includes a lot of sideways time. That's a lot of fun, and if you can afford the tires there's no reason not to enjoy yourself that way as any other. No problem there, but the biggest difference I see between that and those wheel-to-wheel racers and Porsche's professionals is that the stopwatch rules us. We worry about the fastest way round the whole track and even slow down at points where people just having fun will not. We slow here to go faster there.

One of the side effects of that stopwatch-driven technique is that tires do not heat as rapidly. It doesn't look as fast either. On Top Gear, the Stig often looks slower than the presenters. (Come to think of it, watch a bunch or us -- or better yet the Formula One drivers -- on the 'parade' lap. We're all over the place, with no resemblance at all to the racing line. What we're doing is putting side loads into the tires to heat them before we start racing.) When we set the tires to book pressure, they don't get much hotter in the course of an afternoon than they were after that first warm-up lap. The temps on track rise with each corner and each braking zone, and they fall on straights, but they stabilize early and the pattern stays the same all afternoon. The only general rise in the average temperature (which is what you read in the pit) comes from the track surface heating as the day goes on. That doesn't necessarily happen of course. Depends on the weather on your day.

What does muddle the data on all that is the use of racing compounds most times you see 'serious' drivers at work. Those compounds are chosen to work at lower clamping pressures, hence lower air pressures. We want the greatest amount of interlocking effect we can get because we know darned well we won't be on polished surfaces. That means lower pressures and bigger contact patches to take advantage of surface irregularities at the micro level ('tooth') and at the macro level to some extent. I've seen race tires run at pressures around ten pounds, though I never was tempted myself.

Basically, Gibbo, my guess would be that people who run only track days and autocross are doing a more efficient job of heating the tires than does our undramatic style. When I went to a track day to try out this Porsche when we first bought it, I'd been off tracks for seventeen years, so naturally I was assigned 'novice' status. But since they knew I'd been a past racer, my instructor was a local autocross champion. I'll bet local autocross drivers are pretty usual instructor choices. That makes me consider a technique difference that surely filters over to their students.

When you run autocross, heating the tires is not a problem. On the contrary, it's an immediate necessity because you have only one lap from a standstill to set your best time. And in many autocross setups, only low speeds are possible. Seventy mph tops I suppose? In autocross, we would benefit from using techniques that put heat into the tires from the outset. I make no conclusions about that, it just occurred to me as a possibility. I know my instructor chose a different line through several corners than I would use. I humored him at first, but once we had confidence in each other we began exchanging tips and by the end of the day he was trying new lines in his own Porsche because the track was at Willow Springs and much faster than a typical autocross.

When you heat the tires that effectively in one lap, you probably should expect them to go past their optimum temperature range if you drive that way for lap after lap. Lowering pressures does not completely help, since the greater flex puts more heat in tires, but if you're not running three-digit speeds for most of the lap, that wouldn't matter as much as the larger contact patch to accept the stress.

We're back to my primary advice. Set the pressures to make you feel stable and safe, because it's your *** in your car. No one here is going to take up a collection for your bodyshop (or your widow in the case of track work), so it's got to be your judgment.

Gary

 

^I vote Gary's post above as the best post of the thread and a top post in the forum overall. All bases were checked.

 

Absolutely. We are lucky to have that kind of insight on this forum.
For me, just thinking about these issues and making them part of the overall mechanical analysis will help my driving in the long term.

Still, Gary's message (e.g., "tire pressure is not directly significant to grip") is a really significant departure from what most of us practice.

And so I wonder how many of us, the next time we hit the track, are going to do so on book specs +2psi.

 

A good question, as you can tell since I spent a day thinking about my answer.

I'm not a tire engineer myself, we just speak the same language. Satellites and boosters were my métier, but it's all numbers and physics. And I've spent a little over fifty years just playing with cars and tracks to relax, which for a numbers guy means doing arithmetic for fun.

Back to your question. Senior engineers spend a lot of their time trying to balance competing project goals. Sometimes cost, but that doesn't apply at the level of choosing components for either Porsche or the space program. (Talent, yes. Talent is always in shorter supply than money.) One of the big conflicts between goals these days, especially in Germany, is Go versus Green. I'm very sure Porsche engineers are thinking about fleet economy statistics with every design change. Tire life, probably not enough to influence a pressure spec. I can't think of a way ten percent difference in tire life (and that number is arguable) would influence Porsche sales enough to bug the engineering team. Now if they started burning tires like the NSX did, that would be different, but that was a suspension setting, not tire pressures. With Porsches, the differences in our driving styles overwhelm any effect of pressure setting. But fuel economy... Now that is a constant matter for political attention. What I can't decide is how much that would influence the choice of tire pressure spec.

You see, that trick of higher pressures works dramatically for cars with an economy emphasis. The Prius of course, but the Leaf and others of their ilk. The Civic HX or whatever they call it. Cars like that always use skinny tires and fairly high pressures. But when the goal is low running resistance, we don't just bump the pressures, we also choose a different compound as well using skinnier tires. The tires on those cars have very hard compounds. If Porsche were going to shoot for higher economy with the choice of tire and pressures, then the two should work together.

Did they maybe favor just a higher pressure and ignore the compound choice to preserve the performance we expect (and magazines test on skidpads)? Just for that reason? Well, maybe a pound or two worth. You might be right.

In addition, a lot more lawyers have been sitting in on design reviews for consumer products, and the business of people letting their tires go soft and rolling over the family SUV put the fear of God into a lot of people in new product design. Higher pressures are really no defense against someone who doesn't check their pressures. The same size leak might make a tire go flat in nineteen hours instead of ten hours, but flat is flat. And soft enough to roll off the rim is soft enough. Starting at a higher pressure is effectively useless for safety purposes. But. Lawyers are not engineers. "Ford Explorers were rolling over because their tires were too soft, right? Let's bump the pressures just in case I have to take this to a jury." The first time a lawyer says that, the engineers explain politely. The second time, they show a little irritation. The third time, it becomes a bathroom joke during breaks, but the senior engineer starts thinking how to shut this guy up. He might be the one called to the stand to explain why he turned down a repeated suggestion to specify higher pressures. "Did you never consider how terrible it would be if this car rolled over?" Aaaaagh.

Finally, we end up with engineers trying to solve a perception problem. Could that include a quiet rump meeting where they change the compound choice to keep the performance the same, but raise the spec pressure to calm the legal staff? Yeah, maybe. I've seen bigger accommodations made to calm fears of people without the technical background.

So is that two pounds we should take off their recommendations immediately? Well, I do ignore that little arrow that keeps telling me to shift up. I'm not averse to the idea of second guessing the owner's manual about what is real and what is political accommodation. But in this case, I suspect a liability adjustment in pressures would have been balanced with a change somewhere else in the suspension or tire choices, so we'd be destroying a balance that the engineers tried to restore in their hypothetical rump meeting. (The one without the lawyers distracting them.)

Fuel economy? Yeah, maybe. That goal is set politically, but definitely an engineering metric. They obviously spend a lot of time at Porsche trying to achieve mind-boggling performance while at least showing consideration for fuel economy and other social goals. If I could convince myself that one or two pounds pressure rise made any measurable difference in the fuel economy tests, I'd be sold on this idea. As it is, I'd say a couple of pounds is well within our personal discretion as drivers. It can't hurt to run at sub-autobahn speeds with ten percent less pressure in your tire. I know a couple of engineers who do that for several reasons.

That does not mean I agree with lowering pressures when the tire is hot. I cringe every time I see somebody at a track day doing that. Pick a lower 'cold' pressure if you're making a conscious decision for your own reasons, including this discussion, but please do not randomly lower your tire pressures to "see if it handles better." Please. Not kidding about this.

Gary

 

Gary,

IMO your wear life / fuel economy analysis makes a lot of sense. What I don't understand is the above quote. Are you making a safety argument? If so I missed that explanation in your other posts on this thread as I did not take you to mean that lowering pressures is patently unsafe, since you also encouraged folks to make their own choice based on their judgment and driving. If dropping weight is just a bit counterproductive/irrelevant/misguided and these tires rarely burst or de-thread why not go trial and error (within reasonable limits of course)? Especially at lower speed applications (e.g., auto-x) what's the harm in dropping say 5-8 psi, potentially degrading the handling a bit, realizing the folly and toasting a hefeweizen to porsche engineers? (Sorry, it is Oktoberfest after all ).

Thanks again for your knowledge share.

 

I don't think a pound or two is a problem, at least not if you let the tires cool off from the latest lap for twenty minutes so you can be fairly sure they are even from side to side.

My concern is that "a pound or two" is rarely what people do, especially those who don't have an air supply along. (Like adjusting table legs: Lower one a little bit; try to match the other side and get it a little too low; go back and take "just one more pound" off the first one; "oops, that was half a pound too much"; and so forth.) And most people have no proper pressure gauge and no idea of the correct procedures. All the procedures you read assume you're working in a 'cold' garage with tires that haven't been worked for hours. That's because the procedures for working with hot tires are so complex people just ignore the guidance.

Worse yet, we specify tire pressures 'cold' for good reason. (Actually, it's 68F which is only 'cold' by comparison to trackside temps.) It's the only easy way to know how much air mass you're putting in the tire. Or removing. If you take out eight pounds at the track, the questions begin: How long since you left the course? How much lateral difference is there? Does the person letting out air know the target pressure is not some specific reading, but X pounds lower than the initial reading because tires on different sides heat differently? What is the track surface temperature? The air temperature? The gauge quality? Is the track a couple of thousand feet higher than your garage where you set the reference pressure?

Just consider that last one a moment. If you come to Willow Springs from Los Angeles, your gage pressures will be a pound higher even after you let the car sit overnight in the hotel parking lot. Now you lap for a couple of sessions and decide to lower the pressure eight or nine pounds so you choose ... what did you have in mind? 38 let's say. And the tire is hot. What is the reference pressure that equates to that? Darned if I know. We have no way to know. Too many variables without data.

Going home, the tires began cooling as soon as you quit lapping. The freeway doesn't load the tires enough to add more than a couple of pounds of 'work' pressure to the reference pressure and often not that much. At 75 mph, my pressures read the same as they did in the garage after the spike from the on-ramp cornering goes away. So you're rolling down a Los Angeles freeway at a gage pressure of ... who really knows? But if you check your TPMS on a dot two, you might see 30 pounds or you might see 25 pounds. Is that safe? Probably. In absolute terms, certainly, though the tire life will suck. But how will the handling be with cold tires set at 25 lb reference? Do you know? I don't, but I suspect the transient response will go to hell.

At some point, it does become a safety issue and I'm sorry I didn't explain that before. These aren't track toys we're talking about for most of us. We drive them to and from the track, and sometimes with others aboard. You aren't using the car's handling potential in routine freeway driving, but if a crisis arises, you expect a certain response from the car. If the pressures are that much below the recommended reference, you won't get it. The response may be safe in some intrinsic sense, but it won't be the response you expected from your past experience with the car. It won't be the track-day handling because that was with hot tires and a lighter load. Most of us have 'stuff' we take to the track that remains in our pit when we lap. All that goes back in for the trip home, maybe along with a significant other, some kids, or a pet. All that adds up. You may go home with a 3700 lb GT while you lapped a 3100 lb race car. And the GT is running 25 psi cold, while the race car had 38 psi hot.

At least four times that come to mind immediately, I have saved our lives with a quick response in a car. Two mountain roads, one truck that lost control on a rainy highway, and one heavy object dropped off a pick-up. Alright, the last one we might have survived. The other three unquestionably depended on my car reacting just as I expected it to. Sometimes 'expected' is just as important as absolute measures of performance.

I don't insist you can't change pressures at the track. I just encourage you to be as cautious as you would with any other life-critical component of your car, and tires take special attention to get right in a track setting. Setting the pressures at a reference temperature is the only way to know how much air you're really putting in them.

When we adjust race tires, we have elaborate procedures for testing to decide we should, along with notes from previous visits to the same track, and often a suspension adjustment going on at the same time. If you carry a good quality tire gauge and some sort of pyrometer that suggests you need a couple of pounds less then you're on the right track. But you need a good quality gauge that has a release valve for atmospheric compensation, and you really should have a pyrometer so you can set up a personal procedure. "I won't change the pressures until the carcass temp is such and such." Then wait after each session until the tires cool to that temperature.

Otherwise, I would set the pressures at the hotel or at home if you live nearby. I'm not entirely comfortable running tires that low even for the trip to the track, but it's better than the essentially random result you get from changing pressures at the track without a pyrometer.

Forgive me. I know I'm sounding old and fuddy-duddy here, but I've lost two friends in track accidents and I've had a couple more get hurt trying to follow me on public roads. Do this long enough and you get leery of people following one highly visible part of what you do without knowing the training and back-up procedures that make it safe for you, but maybe not for others who casually try to follow.

I don't say don't do it. I do suggest it's important, so you should know what you're doing. Both in the sense of occasionally listening to old men yammering on like this and also in the sense of using procedures and equipment that ensure the result is the one you intend.
Gary

 

I've been trying to figure this one out, Gibbo. I did find one Michelin recommendation of a specific hot pressure, and it was a target for track work. In the brochure for the Pilot Sport Cup tires they suggest a target hot pressure of 32/36 psi. That is, front/rear. I feel compelled to add all the caveats they used, and then one of my own:

First: They suggest that 'cold' pressures of 25/28 will yield the hot pressures 32/36 in most cases. Next, they explain the goal of that pressure recommendation: Then they include an example of car set-up that opens with a description of the elaborate procedures we use in racing. You know, the ones that cause people's eyes to glaze? Once again, I won't bother to quote them, but I'll provide the references for anyone going into serious track work. For the rest of us, I'll just note that they require at least six laps of a road course. (Between each adjustment. )

Michelin concludes: That 'step' is the data collection procedure that requires at least six laps of the course. They do provide recommendations for autocross work, but offer no suggestions as to how one sets up for a one-time course usually driven only once in practice. (Unless they've changed the concept of autocross since the days I last tried one. That's entirely possible of course.) They do remark: Their final caveat is hopefully obvious by now: I honestly think that applies to anyone who has joined this discussion, but my quotes will end up in the archive so the caveat should be there as well.

Now my interpretation of that advice in our own context: The Cup tires employ a racing compound in a tire legal for street use. The tread is notably thinner than the PS2 or the PSS, which means the tires will not heat as fast. Those pressures were suggested as a baseline, a point of departure for set-up work on the suspension of a track-oriented model like the GT3. The actual target was to achieve a temperature range that optimized the grip from the compound, but you need a starting point. They discuss that, along with a couple of pages about track considerations "the experienced racing enthusiast" will take into account.

I can't disagree with any of that and of course I'd like even more tire data if I were still racing. What I can't decide is how to apply the information to our situation. I mean our current discussion of pressures in the road-oriented tire models like PS2 and PSS, still less the other makes of tire some of us use. I can't think of a reasonable basis to even guess what the temperature range might be for the different compounds used in PS2 and PSS tires and the point of the hot pressures they recommend was to reach the specific temperature range of the PS2 Cup.

Let me just summarize my personal conclusions from our discussions here. The possibility of liability and fuel economy influences on the 997 tire pressure specification is influential with me at least. Not as an excuse for random choice of pressures as my whim suggests, but as support for Tony's observation: My own 997 is in fact lighter than the GTS because I have fewer options than are standard in that configuration. And I have already mentioned that I found the vehicle dynamics more to my personal taste with fewer pounds between the front and rear. For the technically interested, the depth of contact patch with stock 34/40 is 1.9" front/2.1" rear. After track experimentation, I found the car to my liking with 1.8"/2.1". The heavier GTS with the new 34/37 rides at 2.1"/2.4". If I lower my pressures to that, I'll be seeing 1.9"/2.3". Without a long essay of its own, the contact patch should be 'longer' on the driven wheels and the differential between front and rear should be 10% to 20% with a Porsche. I like it on the high end of that range and the 34/37 would give me that.

I'm going to see if a revised or supplemental owner's manual is available for my 2009 just to see what it says if such has been published. Meanwhile, I'm going to set those lower pressures and try them on this new set of PSS tires for a week of road work. I have a track day scheduled for 3 October and I'll report the result of that as well.

Finally, Michelin engineers and I share an important attitude that bears repeating. This time in their words: Or as I said: It's your car and your *** when you go on tracks. Please learn to know what you're doing when you choose to do anything that isn't "standard operating procedure" for these high performance cars, but whether you do or not, it's your choice.

Gary