Any thoughts as to why in the Carrera/GTS Owner's Manual the pressures listed for 19 inch wheels are 37 psi for the 4 wheel drive models, but 40 psi for the 2 wheel drive models (partially loaded).

What pressures are GTS owners using at the rear?

 

In the supplement for the GTS manual it states 37 for the rears.

 

Another question is why the GTS pressures are 34/37 while the 997.2S are 34/40? Same weight/same tires. Is PAG revising its pressure recs down?

 

Would be very interested to learn more about this as on track when the rears go above 42psi hot they get greasy. So always thought 39/40psi in the rear was too high for cold temperature.

 

All fair questions, but irrespective of whether Porsche recommends 37 or 40 or some other number, wouldn't you adjust that up or down anyway depending on your use and ambient temps? e.g., if i'm going for a spirited drive in the twisties in July shouldn't my cold pressures be different than a highway roadtrip in March?

 

^+1 ....and don't rely on tire pressure monitoring system.

 

No. Sadly, no time for a detailed engineering explanation.

G

 

Dang...wife is out of town and I've got nothing better to do than read one of your famous essay's.

 

I enjoy reading those detailed posts as well.

 

What can I say? My wife was almost out of pocket today, so I've not got time to write one! Time or alertness.

Cindy woke me around 0430 very frightened. Looked like an onset of full tachycardia and got my attention as well. The sense lead on her implanted defibrillator failed almost a year ago and this mimicked the pattern of the event that led to full arrest and caused them to implant the wee bugger a few years ago. Now we were flying without it and I was more than a little tense until we got her to the ER where they could use an external cardio-conversion if her heartbeat didn't stabilize.

It did, but after driving the seven-hour trip to the dealer yesterday, and being wakened like that after a two-hour "night's sleep", and then standing by an ER bed for six hours today... Well, I'm not really fit to write anything. Including this note...

Gary

 

Gary,

I hope all is well with your wife. Take care of that young lady.

 

Gary, go take care of the wifey...I hope everything is going to be alright.

 

+1, best wishes for a speedy recovery. look forward to your insights when she is better which is hopefully very soon.

 

I'm staying close for a few days, so in between taking care of Cindy, I have time to explain why not.

Thinks Boyle and Charles and Gay-Lussac if you're accustomed to the older naming for the individual 'laws' of gas behavior that we started learning back in the seventeenth century.

Solid tires of most sorts are limited to very low stress applications because they are not pneumatic and pay no attention to the Gas Law. Made of a traditional solid, like oak wheels in ox carts, they respond to stress by reaching the mechanical limit abruptly. They simply fail with no obvious signs ahead of time. Except maybe the oxen panting and sweating like... like an ox. Iron tires help a little. Not much. Solid tires of modern petrochemical origin aren't much better. As the usual 'rubber' heats up, it softens. As it softens, the same load is enough to compress it further. That means that each rotation changes the shape of the tire more than before. We say the magnitude of the flexure increases while the frequency remains constant. (Or I would. Engineers who actually design tires probably have their own verbal shortcut.) Again, that effect continues without limit until the failure point: more heat>>softer tire>>more flex>>more heat>>softer yet>>more flex>>material limit reached. Bang.

Pneumatic tires are much better tools for a designer. All because the Gas Law boils down to the harder working tire getting stiffer, so it flexes less in the presence of the load that warmed it. The tire that isn't working hard (or is in more congenial conditions) will flex more and thus warm up. Both cases are movement in the benign direction. The opposite of solid tires.

Such tires are an example of an analog feedback loop and they tend to 'seek' the nominal operating condition the designer had in mind. Like this: When the tire is cold, or more accurately at the reference temperature the designer chose, it is at a nominal pressure that deals with form stiffness and still allows a routine amount of flex. The footprint of the tire is what is desired (by the designer at least) for routine operations with an ambient equal to the reference temperature. That's our starting point. The designer's starting point, and it is important.

Now go out for a ride on a cold day. Not 'cold' like "set your tires cold" which is just slang for "at the reference temperature." No, I mean a cold day, like 30F perhaps. The temperature of the air in the tire will be lower than the reference temp, so as Boyle's Law describes, the pressure will be lower as well. That means the tire will flex more than it would at the reference temperature. That greater amount of flex will warm the tire, so that despite the cold ambient conditions, the tire will incline toward the nominal amount of flex the designer had in mind. It may not reach that nominal temperature and pressure. It depends on how hard you ask it to work in those cold conditions. The important point is that it will 'try', so to speak. The harder you work the tire, the more important it is to reach the design point. Sure enough, the harder you work the tire, the more it flexes, the closer it gets to the design point. To the intention of the designer.

Now take those tires out on a spirited drive in August (assuming you live in the northern hemisphere).The garage temperature will be higher than the reference temperature in most climates. A lot higher in my home garage in the desert. The road surface will be a much higher temp. All in all, the tire will be hotter. That means that tire pressures we set at the reference temperature will have climbed. Again, just as the Gas Law describes: hotter gas, higher pressure. "Aiiigh! Will we blow a tire?" No. Because the internal pressure is higher, so the tire will flex less than the nominal pressures would permit. It will not heat as much as a tire that started at that 'low' nominal pressure. It will tend to stay at the temperature and pressure compelled by the ambient conditions instead of climbing further. That is, despite all the air flowing over it, the tire is not going to be at 59F when the road surface is 120F, but it's temperature won't climb indefinitely as would that of a solid tire. The harder you work it, the less it will flex and the less it will tend to heat. It is doing work, so it can't stay at 120F, but it will stay reasonable.

This doesn't mean pneumatic tires are invulnerable. What it means is that they tend to converge on the designer's working range. When too cold, they flex more and warm up toward that range. When too hot, they flex less and slow the rate of leaving the intended range. Pneumatic tires converge, solid tires diverge.

The convergence of pneumatic tires permits much greater limits of operation, and when they do fail, it is milder and usually comes with warning signs. If nothing else, with a non-professional driver you won't reach the point of failure at all if the tire is properly inflated at the reference pressure. That's what the designer is paid to accomplish. With an enthusiast or a professional driver, indications will be felt that the tire is working very hard. The frequent comments about a tire feeling 'greasy' sound like indications the tire has passed its optimum temperature for adhesion. Letting air out of the tire increases the footprint size and may help compensate for reduced compound performance. Maybe. It also lets the tire flex more and that means the tire will heat even further. More on that later, but hopefully enthusiast drivers can recognize that situation. A blissful driver may let out still more air. Then the driver has added a divergent component to the neat feedback loop the designer planned.

If you begin with the wrong pressures, all that goes to hell much quicker. A tire set to the specified pressure, but at garage temps much higher than reference, is at risk of diverging because the pressures are too low. Same in reverse if pressures are adjusted in a bone cold garage. A tire that has been allowed to lose air is even worse.

The absolute limit, the point of mechanical failure for over-pressuring a tire used to be the bead. Then cheap alloys came into the market and the wheel would often fail as early as the bead itself. With high quality wheels like our factory units or the aftermarket wheels most Porsche owners choose, the wheel is stronger than the burst point for the tire in my experience. (Unless of course the wheel has been damaged. All this discussion assumes tire and wheel are undamaged.)

Under-pressure has other modes of failure. A pneumatic tire has four primary parts. Subsystems if you're feeling geeky. The bead is the mechanical transition to the solid wheel; the sidewall or underlayment provides the basic envelope for the air; the belt provides a stable circumference and a flat surface for road contact; and of course the tread itself provides the armor, the cushion, and the 'adhesive' compounds for mating with road surfaces. (The bead, underlayment and belt are collectively termed the carcass' when we're discussing retreading.)

Over-inflation stresses the bead (and the wheel too of course, but we won't notice that if the wheel is our sort of unit). Underinflation permits too much flex. At first, just a little too much before the resultant heating increases air pressure and that tends toward stability, but if you're determined to "bleed a little air" you can make a tire underinflated enough that the flexing with the load you present will cause any of several failures. Tread separation in cheap tires is common, especially retreads where the new tread won't be as well bonded to the underlayment as it is with new construction. (Trucks do this a lot, even though their retreads are anything but cheap.) With new manufacture, like our tires, the basic envelope can fail. Probably not beneath the tread, absent some mechanical insult. Normally, the sidewall tears, starting at the weakest point or at some point that comes into contact with a normally innocent object like a stick or small stone that misses the tread. When that happens, the tear progresses around the tire so quickly as to seem like an explosion, though it isn't really.

I've had a bead failure, two 'exploded' sidewalls (at once), and a couple of tread fractures before the days of steel-belted radials. Cindy had a sidewall failure with a seriously under-spec'ed tire for her car. Her case was the most dramatic, happening at 75 mph or so. The principle effect was a serious dressing down of a tire seller.

All of those were impressive at the time, but nothing like as dramatic as a true explosion of a solid tire would have been. Those tend to damage the wheel as well, and so severely that loss of vehicle stability is almost certain. Pneumatic tires are more forgiving, though we're so used to their behavior that I suppose most people would have considered Cindy's coast down on a naked wheel to be catastrophic or at least terribly frightening. In fact, it only pumped up our adrenalin because she did nothing stupid.

The gage pressure at a reference temperature is an important choice for a designer. Tire manufacturers usually don't make that decision. They provide car manufacturers with performance data far more detailed than we ever see, and the car designer factors in static loading with intended operational scenarios to arrive at that nominal working range and a pressure at the reference temperature that will tend to keep the tire within the tire manufacturer's limits. With cars that push the envelope, like ours, designers work together from both manufacturers. That's where the N-rating comes in.

Bottom line, don't fiddle with the recommended pressures until you're convinced three things are true:

1. You are special. Your intended use is a special case and you'll return the pressures to the normal recommendation when routine operations resume. You're thinking track day of course, but I actually had in mind people who use vehicles in very sloppy conditions on mud roads and shoot for more 'flotation' by using much lower pressures. Not Porsche coupe territory normally, but it might arise with Cayenne owners. That only works for very low speed operations. Think fifteen mph and get nervous if you find an open stretch and start hitting thirty.
   
2. You know when to get nervous. That is, you know the signs of a tire in distress and/or you know how to deal with a failure. And don't mind the cost. Cindy dealt with the failure and then got a refund on the blown tire and credit for the others toward the sort of tire she should have been given in the first place for that car. I think the manager would have included his first-born if necessary to get her out of his shop.
   
3. You know the rules. As with any mod, you have to know the rules to know when when it's safe to break them. We actually delegate the knowing to the designers of an aftermarket mod like aero devices or exhaust work. We rely on reputation of the source instead. Porsche does not have a monopoly on good engineers, so companies that see a market niche can knowledgeably change the original design. With tires, especially if you're listening to railbirds at the track to pick your pressures, you better know what you're doing yourself. Railbirds don't have a great reputation.

Okay, track days. Well, first, for the contrast, let's talk autocross. One lap and you're done is a peculiar operating condition to say the least. The manufacturers freely admit you need a specialty compound if you really care about best time of the day. Even race compounds don't heat that quickly to their intended operating range. We were pretty informal in the days when my competitive driving was limited to autocross, so you're going to have to figure this one out yourself. Railbird advice maybe, at least if the rail birds have their own cars and a stack of trophies and still want to share. Worst case, you write off the car's bodywork in a low-speed 'incident'. Unless the 'autocross' is really just one timed lap in a day of running on a true race track. That's ... different.

Now, for race tracks. Lots of opinions. Honestly, they're all worth the same. Nothing at all. It's your car and your *** if you depart the track at three-digit speeds. If you don't plan to follow professional advice, then you should at least use the procedures professional racers follow. If you have that skill set. I seriously doubt one of us in ten is good enough to serve as the test driver component of those procedures. (I'm not at my age.) You have to be able to produce very consistent lap times through critical test sections, have telemetry mounted, or get the car in for pit measurements using a consistent and quick slowdown. The goal is to have the tread at the optimum temperature for grip in the most important corner complex. You can't evaluate an incremental pressure change unless you can produce the same results each time the same pressure is set. (Without cheating by knowing the pressure either. Just by your response to the tire's response to your inputs.) We do that all the time with race cars, but it's trickier with road cars, not easier. And when I say 'we', I mean the best teams. The rest try to crib from their notes. Or just wing it.

In case you don't feel up to that, what is that professional advice? Well, I haven't polled the manufacturers, but I asked Michelin's chief field engineer what they do. At demo days for new cars, for the likes of Hurley Haywood and Porsche's other professional drivers, they set up the car with pressures at the garage temp, the reference that is, set to two psi gage over the Porsche recommendation for general purpose. Then they adjust in small increments by direction of those drivers. A total of a pound or two he said.

For example, I'm no Haywood, but I find that my car suits my driving style better with an extra two pounds in front. I set up my C2S for track days at 36/40 which is two psi gage over Porsche's recommendation in front.

He went on to say, somewhat sheepishly to my ear, that they let out some air if track conditions and those drivers get pressures above 50 psi. I won't argue with him or those among us who do the same, but it grates on a design engineer to hear that. Pressure is not the enemy unless the operator is cranking up the 'cold' pressure sky high to carry loads beyond the tire rating. If a tire reaches 52 psi gage under working conditions, then it is doing something that requires that pressure. It needs to be flexing less, not more. If you let air out, you're going against the feedback loop and introducing a divergent tendency. Where do you go from there? Let out another two psi when the tire heats up even more? Worrisome to a design engineer, but it's your car. For the record, if that ever happens to me, that is, if the pressure stays over 50 psi after ten minutes of post-lap cooling let's say... If that ever does happen, I'd worry about what I'm doing wrong to cause unexpected tire heating, thinking of pressure as a symptom not a demon. I'm not able to hit the sustained mix of speeds/cornering that a professional driver manages, so what am I doing wrong? That's what I would wonder. But that's just me.

Getting back to the original question: For normal use as a high-performance sports car, leave the pressures where Porsche recommends them, as measured in a garage at reference temperature before you start driving. (And learn how to correct for garage temps above or below reference.) If you know what you're doing, tweak them a pound or two to suit your driving style.

The car handles beautifully in all conditions with those pressures and tire wear is fine. I just changed out our PS2 tires for Michelin Pilot Super Sports. After 28,000 in front and 13,000 in back the wear was even except for the usual inner edge wearing a millimeter or two faster, as is usual with cars like ours. The rears had at least three thousand left, but I didn't want to mix and match. The fronts were tolerable for around town, having just reached the wear bars at 2mm, but I have a track day coming up in a week and wanted to try the new PSS tires. So I changed now instead of waiting until the end of November perhaps.

Why is Porsche changing their recommended pressures for some models? Durned if I know. Half a dozen possible reasons that have nothing to do with safety. Again, I'd take their recommendation until I'm sure I know better.

Gary

 

Thank you for your insightful post. I hope your wife feels better soon.