Zeus993

Question - anyone, in addition to Gary, played with the pressures on a 997 C2S with new Michelin PSSs? Any advice? Mine are feeling harder than my previous Pilot Sports 2s...

ADias

Try 33/37PSI set at 20C/68F or temp corrected according to the line graphed below:

Vancouver-BC

I have to replace the tires on my C2, and am interested in the PSSs, where in Vancouver did you get them.

Thanks, Michael

Zeus993

Tx. I think this is about where I have them. I purchased the tires through Blue Star Motors which I believe got them through a tire shop on Southwest Marine Drive. Check with Rob at Blue Star.
Rob Libera
Phone: 604 649 1975

Aerokitted

kitsilano!!!!

i run 30 /33-4 cold. drives fine, but then i'm not nearly as sensitive to tire pressures as some folks.

ADias

I noticed you live in Vancouver. You must have colder temps by now and, as you know, rubber gets harder. In any case the 'cold' 33/37 is set at 68F. If the cold temp on a given day is say 50F the tires should be set at 31.5/35.5PSI following the graph.

Summer tires should neve be driven below 40F in any case.

DS997S

I use 33/39 but the ride is very bumpy even on normal. Will consider 33/37 as recommended here on my N1s for 2007 997.1S. The manual recommends 33/39 or partial loading and 33/42 for full load with luggage.

Fred R. C4S

Forgive an old mechanical engineer, but I think you are misunderstanding the point of the graph. The graph indicates what the pressure in a closed vessel (tire) will rise or drop to with a change in ambient temperature. It is by NO means a chart with which to set tire pressures.

Do you really recommend setting tire pressures to 31.5/35.5PSI if the ambient is 50 deg F? The cold inflation pressure to set the tire to at 50 deg F ambient is the same as at any other temperature. With the 33/37 at 68F used in your example, you will not have enough pressure at 31.5/35.5PSI to properly support the weight of car at 50 deg F with the tire. The tire will be UNDER-INFLATED.

The chart can be used to show what your pressures drop to at the lower temperatures. This is a sign that you need to ADD air to bring the cold pressures back up to spec.

simsgw

Forgive an old aerospace engineer, but Tony is right. In fact, I think that chart comes from the owners manual. If not, its data points are identical. Your intuition is right, as well as your information of course, but your conclusion overlooks a subtle side effect.

The pneumatic element of tires is the designer's control loop, a way of maintaining the working temperature intended for the tire. Solid tires, like those on carts and wheelbarrows, always flex less when cold and more when warm because of the material's thermal response. And as a result, the temperature of the tread (or merely the outside rim in the case of wooden or iron examples) tends to follow the ambient temperature more closely than desirable for a tire whose friction coefficient is crucial to performance. In fact, that response is a positive feedback that tends to exaggerate the underlying effect, like heterodyning in amplifiers. I've never examined the issue, but I suspect this is the reason the iron tires would be thrown off a wagon's wooden wheels at awkward times. The thermal mismatch between the iron and the wooden wheel would cause the tire to loosen under heavy work. (I don't assert that, since even at my age very few wagons were around when I was young, and even those few had been changed to pneumatic tires except for exhibition examples.)

A pneumatic tire tends to stay closer to the optimum tread temperature. When cold, the tire will in fact have lower pressure, but that's desirable from a design point of view. It will be required to flex more than a hot tire. That flexing raises the carcass temperature. Conversely, when the tread is working exceptionally hard in continuous cornering or acceleration, the heat of friction is transferred to the carcass and of course the air inside. That causes the pressure to rise. When the pressure rises, the tire flexes less, which reduces that source of heat.

Those feedback effects mean that a pneumatic tire can be kept within a more narrow range of temperatures when working. That allows all sorts of benefits when designing (or choosing) the compound for the tread because the coefficient of friction for those compounds varies with their temperature. As one example, for which I have the numbers handy, the relatively flat plateau of 'acceptable' track performance for the Michelin Cup tires is from 160F to 220F. That's a range of only 60F, and the compounds chosen for street tires don't really have a much flatter curve for cf vs temp I've been told, though they don't advertise the exact numbers. Without the feedback response of the air component, our tires would be too cold or too hot most of the time.

For road use, tires do have a slightly lower standard of 'acceptable' for the coefficient of friction simply because they are not operated (nor are most of the cars designed) at the levels of grip beyond 1.0 g so the 'plateau' that falls above that lower expectation will be a little wider than tires designed for track work. Conversely, true race tires have even higher expectations and a more narrow range of 'acceptable' as a result.

But each of those design goals is made practical by the reduced range of temperatures when a pneumatic tire is working. Flexing is a major source of heat in all tires, and the pressure/temperature response of pneumatic tires allows that source of heat to be modulated -- in effect -- so as to keep the tire within the target range. We would be interfering with that feedback response if we set our pressures higher when cold dropped them, just as we would if we lowered them after the tires warmed up with work.

That's the real reason they used to caution drivers never to bleed air at a fuel stop, because the correct pressure right after driving is always higher than the pressures they specify for that 'cold' setting. My tires with a 68F setting of 34/37 rise to 39/42 very soon after leaving home and they are amazingly stable at the 140F carcass temperature implied by that working pressure. The pneumatic response is what lifts them to that temp and keeps them close to it through the range of activities I enjoy.

That 68F that Tony first mentioned is one of those SAE standards I believe. Either that or an international standard.

Gary

Fred R. C4S

Gary,

I respectfully disagree. What should my inflation be at 10 deg, 0 deg, -20 deg? At some point your position would have me lowering cold inflation pressures to a point at which I could no longer maintain rim to tire bead seal. I would seriously risk popping the bead should I strike a pothole

Here in the Midwest where we experience these sorts of temperatures , we add air when it gets cold to maintain a constant cold pressure setting.

ADias

The operational range of summer tires stops at 40F. Below 40F you should not be using summer tires.

Gary answers your reply to my post, so that is covered. FYI, the Owners Manual refers 'cold temp' pressure to the absolute 20C/68F. Just check it yourself.

Fred R. C4S

Checking the owner's manual, I see that there is a notation of 68 deg on the page listing the cold inflation pressures for the various wheel and tires sizes sold by Porsche. But there is no note, footnote, or hint to adjust the pressures given by any SAE, IS0, ASTM, TUV, or any other standard. How did you conclude that the pressures should change on a sliding scale and where did you get the scale?

Just curious. I've noticed if I'm not careful, I can learn something everyday.

simsgw

Oh yeah, I know. I was stationed in Duluth for four years and then Dayton and I've flown as far north as the Beaufort Sea. The slope on that line Tony posted isn't as great numerically as it looks to the eye. It is only 0.7 lb per ten degrees Fahrenheit. Minus twenty is only ninety degrees below reference (roughly), so the adjustment would be 9x0.7 or six pounds. My Summer pressures of 34/37 would become 28/31, which is well above the pressure needed to hold the bead. (Though of course my current tires themselves would be useless at any pressure in a Duluth winter.) And again, the heat production from carcass flex is very significant. My tires rise seventy degrees in little more than a mile, though admittedly even our winter temps in the desert don't get that cold. We're not below zero very often. But the tires will warm up to decent working temps quickly, whatever the temperature of the road surface. The contact patch just isn't enough square inches. With race tires, we do compensate for the track surface temperature, as you know, but only because we're pushing them to a limit road tires never see, and we consider 'acceptable' to be a much narrower target.

In fact, I know some people drop their pressures that far with road tires in the hope of better handling at track days. Somewhere around here we have my report of a day spent evaluating pressures at the local track using race techniques and equipment. I don't use those low pressures and I question their value, but they don't cause a problem for the tire staying fixed to the bead. (My optimum pressures for track use turned out to be only two pounds lower than the road-use specification for the compound I'm using.)

One consideration in designing winter tires is to keep them from overheating with that thick tread they need. A cold road surface and a cold airflow do not make up for the flexing the tire encounters. That's the design benefit of course. Can you imagine the friction coefficient of a solid rubber tire at those temps? Bumping the pressure above the manufacturer's spec can have a little bit of that effect. The reduced flexing can keep the tires from heating to the intended operating range and the compound will lose its proper grip level. A winter compound has a lower operating range than summer tires, but it still must be allowed to heat into that range which is way above ambient. I don't have numbers, but I'd be surprised if the design goal for the range was below freezing on the bottom end. I greatly suspect that the grip of winter tires is almost completely down to the sipes and road roughness until we drive a block or so and get the compound warmed.

I haven't checked, but I know we have special procedures for Air Force equipment in Arctic conditions. Quite possibly, a different set of standard pressures, a different reference I mean, is published for tires intended for use where 'storage' conditions are routinely below -40. Nevertheless, when your garage occasionally gets that cold, and I remember that it does, if you have to check the pressures, then the adjustment from 68F would only be eight pounds below the manufacturer's specification.

Gary

simsgw

Yeah, ain't it a bitch? At sixteen, I knew everything. What the hell happened to the world? Who's going around inventing all this stuff?

Gary, who doesn't remember the page number but it's where they explain how the TPMS works in a 997.2

ADias

Fred: This has been discussed several times. Gary and I long exchanged views on this. I, like you, was surprised when first saw in the 997.2 manual the 68F 'cold temp' absolute reference, as I was used (like you) to a relative 'cold temp pressure', cold temp being the cold temp of the day. Then we noticed that the second version of TPMS (in the 997.2) tracks the line in the graph to a T. BTW... that line is the classic perfect gas law - PV=nRT. I can send you the Excel spreadsheet behind that graph, if you send me a PM.

So, short of air leakage, one can set tire pressures on the curve and let it ride. As I stated before, the method breaks down below 40F, when these tires should not be used anyway.