Gary,

Tremendous post as usual. Lots here to consider and learn -- thank you!

Small question: Do the tire design engineers also consider fuel efficiency and especially the tire's wear life in setting the reference point (garage temps)? If so, and if such concerns are secondary to absolute performance for a given user, one could maybe argue that the best temp (for purely performance) is a couple psi low than spec.

Bigger Question: My T4S cold specs are 37/44. I can easily get the rears into the mid to upper 50s on a track with those starting points, without doing anything wrong to get them there. So, I had been tracking at 29/31 cold, which gets me somewhere around 38/41 hot -- which seems all wrong based on your post. The car felt fine and the tires (Super Sports) look good, but your post makes a lot of sense so I'm pretty confused as what to do going forward. In general I have no doubt that there are plenty of silly or even harmful practices going on at the track, from guys who pick up and share bad habits, but in this case, it seems like almost universal practice to bleed down the cold temps quite a bit before tracking.

Guys (I'm looking at you Damian!), I'd love to hear some other opinions on your real world experiences at different starting temps, especially on the track where it counts the most. I think a very telling piece of data would be skidpad tests on a given tire, like the Super Sports, at varying psi -- 38, 40, 42... 50psi -- to see what is optimum (for pure performance), then we could just set our colds to get us there.

 

KonaKai, your T4S cold specs for normal load with 2 passengers and some luggage is 34/37. The 37/44 spec is for 4 passangers full luggage. If you want better handling and it's just you and someone else in the car mostly...go with the 34/37 spec.

 

That explains a lot! Thanks!

 

Hi there

My stock pressures are 33/39 for a C2S cold.

On track days this is way to high.
On my first track day after 5 or so laps I was loosing traction, was massively unstable under braking and things felt really greasy. Going back into the pits my rear tyres were at 48psi and fronts over 40.

I dropped pressures down to 34 front hot and 36 rear hot, my grip was instantly returned and the car felt stable and safe. 10 laps later I was at 37psi front and 39-40psi rear.

I was finding anything over 42psi in the rear the car felt greasy and lack of traction.

This was on PS2's, I am now on PSS and have found the same, their ideal hot temperature on track days is 38psi.

Anything over 38psi grip is lost, the tyre contact patch reduces and as such you have less grip. To give you an idea on track day your tyres get so hot they almost burn to the touch and feel gooey to touch and this is mainly due to heat from the brakes.

Also emailing Michelin about track day tyre pressures who are the real experts as after all they make the tyres I was given the response the best ideal hot temperature for ps2 and PSS was 37.5psi on a track day.

For the road stick with Porsche recommendations though be good to know why some very similar models have different settings, maybe different GEO's?

 

You're welcome. This is outlined in the owners manual. Actually I see you have a 2007 so If my memory is correct your cold pressures with normal loads are 33/37. For some reason Porsche up'd the cold pressure on wide body cars in the front by 1lb.

 

AWD = extra weight on the front axles

 

Both 2WD and 4WD 997.2 had the same 34/40PSI.

GTS (weighs same as 997.2S) recommends 34/37.

991 31/34 under 160MPH (see below).

What's at play here is more than weight.

 

The lower pressure on the 991 may be due to the 20's...makes up for the increased diameter and lower aspect ratio, with the lower tire pressure...for a similar quality ride.

 

I certainly think it's your car and your *** on track days, so you want to run pressures that make you feel stable and safe.

Going on from there leaves me with some objections to your report, Gibbo. The tires are certainly hot to the touch. In fact, if you touch them after coming off the track and find the tread doesn't burn your skin, then you've either waited long enough for them to cool or you're driving too slow. Measured by telemetry the tread temperature runs as high as four hundred degrees at points where the tire is pushed to its highest thrust, either lateral or fore and aft. That's only the tread of course, and the differences across the tread that we rely on to choose suspension settings are gone within five minutes of reaching the pit. Nevertheless, the tire is a largish heat sink and retains enough of that heat when we stop that the typical pit measurement is hot enough to burn skin. Some of that heat arises from the brakes, but certainly not most of it. Not unless you've modified your car and no longer have air ducts cooling the brakes. To confirm this, spend some time on a skidpad and then measure the tire temperatures. A hard-working tire generates heat fast. The envelope, that is the sidewalls and underlayment, generate heat by flexing as the tire's radius shrinks passing through the contact region. The tread and belt are bent inward as they enter the leading edge of that contact region and bent back as they leave it at the trailing edge. Again, that flexing produces heat. All of that happens just from rolling on a smooth road surface without delivering thrust in any direction.

Now the coefficient of friction comes into play. With modern compounds, it is up to 1.4 I've heard, though I haven't done any research myself. The Pilot Super Sport has three compounds (or is it four?) and those will be chosen for more life than a race tire, which optimally should run out of tread just as the car pits for a change (or a trophy). I suppose the coefficient of friction for the stickiest of those compounds is around 1.1, maybe a little higher. The grip that implies is provided in two ways. The first is by classic intersurface friction and the second by macro 'locking' of imperfections in the road with the uneven contact surface created by vertical loading on the tire. All that gobbledy-gook translates to heat.

When they work hard, tires get hot. The brake disks and all nearby masses get hotter of course, so there is heat transfer, but it isn't as significant as the heating from performing their work. Not on a car designed well. There have been exceptions, but the Porsche Carrera is not one.

The answer you got from Michelin is a little surprising. Not because the chief field engineer of Michelin USA gave different information, and not because the value of 37.5 psi (or even worse 2.55 bar) is terribly precise for a recommendation like this. Those are worrisome but not what really troubles me. What concerns me is that tire pressure is not directly significant to grip. In general, the coefficient of friction goes up with increased pressure, while the interlocking effect improves with lower pressures. The first effect dominates on a smooth track and the second dominates on a rough track where the "air suspension" component also matters to the overall cornering potential of the car. Those are pretty general guidelines that don't yield a precise answer. Not even to the nearest pound really, let alone a half pound. Could there have been a misinterpretation of your question? Or could it have been someone trying over hard to impress you?

You see, the important point of pressure that usually keeps Michelin from making pressure recommendations is that the correct answer depends on load and suspension dynamics as much as it does those other variables I just mentioned like track surface. What really makes a tire's available thrust rise and fall is the tread temperature at any moment. On a corner that ends a long straight, you have to expect understeer because the fronts have been cooling faster than the rears which are less exposed to the air flow. Unless you have a hard braking segment before that corner, in which case the fronts may be heated more effectively and be closer to their optimum temperature, so they deliver side thrust more efficiently than the rears -- and that encourages oversteer. Most of us have watched drag racers lighting up their tires before a run. That heats the tires in a best effort to get them to the optimum compound temperature before they need to deliver their maximum thrust. In Formula 1, they put electric heater blankets around the tires when getting ready to run, even while sitting on the starting grid. Again, same problem. The tires need a particular temperature range to produce their grip.

Compounds they use for road tires are not as peaky as race compounds, not as sensitive to being under their optimum temperature, else we'd have grannies dotting the roadside bushes every brisk morning. Nevertheless, they definitely improve their grip as they reach their own optimum temperature range.

Michelin certainly could tell us an optimum temperature for each of the compounds across the tread of a PSS if they were willing to divulge that to the general public. I'm not sure how they would know which pressure would lead to that temperature in a particular corner at Brands Hatch or Willow Springs or Laguna Seca, let alone the more difficult case of an optimum average temperature to improve grip at all the corners at even one of those tracks. Race engineers would be out of a job (or hobby as the case might be) if anyone could predict an average temperature that would provide the best overall performance at all three of those tracks.

Foreseeing which pressure would achieve that somehow optimized temperature for all tracks in all weather for all drivers is... well, a crystal ball does not convey it. We care about pressure because of all the ways those earlier factors interact, and tread temperature is just one of them. Since the pressure we set not only affects the tread temperature but also interacts with the car's suspension to control the shape of the contact patch in different corners, and it affects the tire's interaction with track surfaces which vary from day to day as well as track to track... well, that was some e-mail.

I think someone was ... overconfident to provide that datum. 37.5 psi, huh?

I'll keep using 36/40 which works for me. It's based on the U.S. Michelin suggestion that we bump the Porsche recommendation for any particular model by two psi on a track day and adapted front to rear for my driving style.

Your opinion is definitely the one you need to rely on when you go track speeds, but if you ever get to Willow Springs, it'll be fun to compare our results.

Gary

 

They up'd the front on Carrera S also to 34 cold. 2 wheel drive car

 

Yes, but why did they drop the GTS rear to 37?

 

It occurs to me that I haven't seen a weight distribution for the 991. It is lighter than the 997, but how much of that reduction is in the rear?

Gary

 

The 997.2 is 34/40

The GTS is 34/37 - same weight/same tires

The 991 is (on paper) 100lb lighter, but that depends on configuration. For instance if it has Bose or Burmester it will probably be heavier than a 997.2 without Bose. Thus I infer weight diff is immaterial in the 'new' cold pressure recs.

The 991 is 31/34 if <160MPH.

 

Close. The material they gave me says 3075 lb for the S, which is 67 lb lighter than the 997S we have, Tony. Both at the nominal weight. You're right about options of course. My example is 3160 lb, which is 18 lb above the nominal of 3142. Yours would be another forty kilos for the PDK. (I think it's forty, isn't it?)

You've all made me curious though, so I did some arithmetic. Basically, you figure the weight on each axle, divide by two to estimate weight on each tire (ignoring lateral differences in cars that aren't corner balanced after purchase), divide that value by the tire pressure for that axle, and then divide that by the width of the tires on that axle. All that gives you the depth of the contact patch. Wasn't looking for anything in particular, just noodling with the numbers. My 997S is probably typical, measuring out at 38%/62% weight distribution. At 3,160 lb total and tire pressures of 34/40 that works out to a contact patch at the front that is 1.9" deep. (That is, fore and aft.) At the rear, it's 2.1".

Now the 991 at those pressures of 34/37 and a weight of 3075. They've changed the wheel base of course and the cg of the engine as well. If they changed the weight distribution to only 40%/60% then the contact patches would work out to 1.9" in front and 2.15" in back. We don't know if they did precisely that much change of course until we can weigh one, but 40/60 isn't unlikely.

Since contact patch shape is more important than actual tire pressures, I'd say they did that intentionally. The extra five thousands depth of patch in back might have been found in testing to be helpful when you have torque vectoring going on. Or it might have just worked out that way, which is quite likely. I misdoubt five thousands is going to have a measurable effect on handling with all the other factors at work.

Off to bed. Will think about this more tomorrow,

Gary

 

Oops. Just reread your note, Tony. You said the GTS was now set at 34/37, not the 991. I should have waited until I slept before doing that arithmetic. I'll re-work that stuff for the 991 pressures you provided tomorrow. Sigh.

Gary