BlackBeauty

The air in the tires supports the weight of the car. I'd like to think the recommendations in the manual are supported by some level of engineering understanding, of which I have little.

I'm inferring the difference in vMax speed will translate to some degradation in handling/safety characteristics, so the lower the pressure, the lower the performance, which is in part why I don't want to go to a pressure below what's recommended, but again, what do I know.

On the ambient temperature, I was suggesting that if the recommended pressures were at X and you're filing at a lower ambient of y, should the recommended pressure be even higher? I wouldn't go higher than the placard but it made me think about the issue.

Valvefloat991

[QUOTE=I'm inferring the difference in vMax speed will translate to some degradation in handling/safety characteristics, so the lower the pressure, the lower the performance, which is in part why I don't want to go to a pressure below what's recommended, but again, what do I know..[/QUOTE]

The pressure required for top speed is based purely on tire durability. As was pointed out earlier, a tire flexes as it rotates--the flat spot on the bottom, where the tire contacts the pavement, goes round and other round parts get flattened. This constant flexing produces heat, which increases temperature in the tire carcass. As you go faster, the tires rotate at higher speed and there's more flexing, and therefore, more heat, in a given time period. If the temperature gets high enough, the tire disintegrates. One way to reduce the flexing is to increase the tire pressure. The higher the pressure, the smaller the flat spot and the less flexing there is. That's why tire pressures are higher for top speed runs--and also for full loads (more lead means more flexing).

All of this was demonstrated by Ford and Firestone around the turn of the century, when owners neglected their tire pressures, which were specified at a low 26 psi and then proceeded to drive their Explorers, heavily loaded, at 80+ mph on hot summer freeways. At 18 psi or so, the tires failed with unfortunate consequences, particularly when the owners panicked and slammed ont he brakes or swerved towards the shoulder.

Porsche is particularly conservative about this because they must allow for an owner who lives a quarter-mile from an autobahn exit and starts his cold car, with cold tires, and immediately proceeds to accelerate to top speed. Thus the cold pressure is rather high.

None of this has anything to do with the grip or handling of a tire during back-road cornering or tack events. In fact, the ideal pressure for track events is way closer to the comfort settings than to the top speed settings.

Class5Kayaker

I was running what the placard said the first day I had my car and the ride was very "stiff/jarring" (felt every bump, etc.). Dropped down to comfort settings (31/34) and MAN what a difference! Unless you're hauling passengers and a whole lot of crap around, I recommend something close to the recommended comfort pressures. Unless, of course, you plan on driving over 165mph. Then you need the higher pressure.

BlackBeauty

worf928

Thanks for answering the question. Now we can and did get to the 'meat' of the issue.

There is and the manual presents that understanding in a very simplified way.

As Valvefloat991 expounded upon, heat is what kills tires. The 'math' discussion gets very complicated because we're dealing with a gas-filled, moving and deforming object that dissipates heat as it does these things. The slower the tire rotates - all other things equal - the more time it has to dissipates heat from flexing at the contact patch and it accumulates heat at a lower rate. If it rotates faster it has less time to dissipate heat, accumulates heat faster, and therefore its core temperature increase. Of course the gas (air) in the tire becomes more or less dense as the core tire temperature changes. And the gas density determines pressure. Remember Pv = nRT?

Bottom line is that there are a lot of non-linearities. Drop speed by 50% and you get a greater than 50% increase in heat dissipation. Double speed and you get less than half the total heat dissipation. The math gets messy quickly.

This is true if performance is defined as the highest possible sustained speed and the highest possible fuel economy at the expense of traction and comfort. (With more pressure, fuel economy goes up because you're spending less energy from the engine on flexing the tire sidewalls. This is simplified by calling it 'less rolling resistance.')

On the other hand...

... decreasing pressure will - up to a point - increase grip (cornering, braking, and acceleration traction) and increase 'comfort' at the expense of fuel economy. For the former more flex means that the tire is 'flatter' when it contacts the road - thus more contact patch and thus more grip. For the latter more sidewall flexing increases rolling resistance and that consumes extra fuel.

The higher the pressure the less side wall flex. Or, think of it as 'bulging the tire in the middle.' Effectively the part of the tire that is in contact with the road gets narrower as the pressure increases.

Let's put this in the contact of track events. The goal in these cases is to try to maximize contact patch size.

Autocross is much like Valvefloat991's Porsche Scenario:
... except that you don't even get a quarter mile to 'warm up' the tires. Thus, you typically want a bit more pressure to 'simulate' a warmer tire since more pressure is like more temperature (Pv = nRT again) and the AX course is usually so short that you can't 'over heat' the tires. So, you want to start out with a pressure that maximize contact patch. You won't be driving the AX course long enough to get any temperature (and therefore more pressure) into the tire so the contact patch will stay more-or-less the same.

However, for road course events (Watkins Glen, etc.) where you'll be hot-lapping for 20 minutes you have way-more than enough time to get core tire temperature. Here, you want to set an initial pressure that maximize contact patch once the tire gets warm. Thus, you need a lower initial 'cold' pressure. Otherwise you'll end up with a tire that has too much 'hot' pressure and a much smaller contact patch.

More the reverse of that.

If you fill at - say - 32 degrees with a recommended 30 psi - and then drive at 32 degrees all's well. If, suddenly though, the ambient temperature increases to 80 degrees then the static pressure in the tire will increase by 4-ish psi and would therefore be more-inflated than you intended. If your 'ambient fill temperature' is much lower than your 'ambient drive temperature' you want less fill pressure.

The reverse is what usually triggers posts at the start of winter: I left home/work after it got cold and my TPMS system says I'm down 2 pounds. Do I have a slow leak? No, the lower temperature resulted in the air in the tire becoming denser thus lowering the pressure.

And thus my quip about the temperature of your garage.

Bottom line: Ambient conditions will not change the static pressure by more than a few pounds. There are so many other factors that effect tire 'warm driving' pressure that it isn't worth it to obsess on fill temperature-driven pressure except twice per year.

Personal anecdote: My garage is rarely below 55 degrees. One night I set my tire pressures. The next morning I drove to work. The outside temperature was in the 20s. During the afternoon, with the car parked outside, the temperature dropped into the negatives. Leaving work the (non-Porsche) TPMS complained bitterly.

When I got home I forgot to check the tires while they were still outside-cold. I remembered the next morning. All my pressures were fine. So, I put in an extra three pounds. Problem solved. Until Spring.

Curvehound

Informative thread.

Anyone know why Porsche does not have comfort settings for winter tires?

BlackBeauty

jadatis

About the ambiënt temperature.
The pressure advice is calculated for when outside tire temp is 18degr C/ 65 degr F. others say its 20degr C/ 68degr F, but this gives that less difference that we dont have to discuss about who's right.

When filled at 65 degr F when starting drive the tire has inside same temp as outside tire, and by that deflection is more then when heated up.
When driving long enaugh at speed, the temp rises in tire, and by that the pressure rises, and by that the deflection gets less.
At a sertain point there is a balance beween the little less heatproduction by the little less deflection then cold, and the cooling down by the temperature differences between rubber and surounding rubber, and rubber and in and outside air, and temp difference beween in and outside-tire air.

When its colder outside, more temp differences , so inside tire is a bit colder , and cooling down of rubber is more.
So when colder you can do with more deflection , so lower pressure then for 65 degr F, only for savety.
For fuel saving and road feeling ( read discomfort) you can keep to the advice pressure , but for savety not needed.

When its hotter outside, the cooling down of tire is less, but by the lesser deflection because of the higher inside tire temp, the warming up is also lesser.
My conclusion is that within a sertain rainche of outside tire temp , the cooling down and warming up is in balance so the rubber wont reach the critical temp at wich it hardens ireversible, and crackes every deflection ( bending) a bit further . Once the structure is damaged , mayby only after 3 years the tire is that much torn further that it blows and cources accidents with the missery that goes with it.
My estimation is that if you determined the cold pressure for a situation to the best way for mother nature, you fill tire to that pressure calculated back to 65 degr F, and you are save within a to use ambiënt temp of up to about 100 degr F, but discussable this border.

How can we get more deflection and still not overheated rubber.
In Formula 1 they use hollow spokes so there is more airstream inside the tire. moving air cools better then when standing still.
I once supposed in reaction to an article about the lowest pressure Pirellli allows for Formula 1 races, to make the spokes in shape of propellor.
This gives airsteam trough the rimm, wich is the place where the inside tire temp is best transported to the outside.
This would give a lower temp inside the tire, wich cools down the rubber better, so they can allow a lower cold pressure so more deflection so more gripp, and still the rubbber wont reach the critical temp at wich it hardens.

Will give tabel of how the pressure rises and lowers for temperature.
The 1 psi /10degr F rule is a rule of tumb and better would be a sertain percentage of the pressure for every 10degr.

worf928

Valvefloat991

What you say is true, but bear in mind that the load index is per tire. So we are talking about each tire alone having a load index roughly equivalent to the entire weight of the car on that axle. In other words, with a 911 loaded with two non-corpulent persons weighing around 3600 lbs, and 60 percent of that weight being on the rear, that comes to a total of 2160 lbs on the two rear tires. In that context, whether the individual tire load capacity is 1800 or 1900 lbs seems immaterial.

Admittedly, that load capacity is usually specced at around 50 psi, but even if that capacity declines linearly with tire pressure, at 35 psi, each tire can still carry: 1819 x 35/50 = 1273 lbs. That's well over half of the total rear axle load of 2160 lbs. Now perhaps I wouldn't want to explore top speed at this pressure, but the normal load pressures for the winter tires (19-inch) are 31/33 F/R, which is not harsh at all. And their listed load capacities are 1356 front and 1764 in the rear.

jadatis

Reaction to above.

What you write about one tire being able to carry almost the complete axle, is what I already expected.
I also expect the tires to give on sidewall ZR and That Y speedrating, and being XL/Extraload/reinforced.
The pressure for wich the maximum load is calculated for referencespeed is always 42 psi /290 kPa in European system wich they probably arre, because I think no P before the sises.
So you assumtion that the 875/825kg is calculated for 50 psi is not true.
If you follow the calculation of the tyremaker, you have to highen up the referencepressure with 50 kPa to 340 kPa wich is about 50 psi. and then max speed is 270km/h.
But I already explained my system wich comes to higher pressures.
Then for that 270km/h you have to substract 11 LI steps to give 103-11= 92 LI wich is maxload of 630kg/1385lbs a tire.
Then a axleload of 2160 lbs would need a pressure for that speed of 31 psi.
But I have not read still, what speeds you Porche drivers maximaly use ( and wont go over for even a minute).
This is probably over the legal speedlimit, but when you buy a Porche you get dispensation for that
If by any chanche you keep to max of 160km/99m/h. you would need for that same axleload a pressure of even as low as 21 psi.
The official european formula sheet does not allow a pressure below 150kPa/22 psi.
So what I expected is true, the pressure needed for the parly load and even up to that 270 km/h is as low as 31 psi.
That is if all the data given are right.

But then I did not add a reserve for unequal load R/L and did not lower the loadindex because of low Aspect ratio ( fi 235/30ZR20 103Y)
So then load on axle should be equally divided , wich it never is.

Curvehound

OK, thanks. that makes sense

chuck911

Attentive readers may well wonder, "around the turn of the century"??? WTF?? Oh wait, that was only 18 years ago.

Nevermind.

Attentive readers might well be asking themselves, "If Firestone recommends 26, then how in the world driving 80+ on a hot summer freeway do you get to 18???!"

This was indeed the story. Crazy Saudi's (but I repeat myself) would take their Ford Exploders out on the sand dunes. Then for fun they would drop them down to like 12 psi because hey, sand. Nowadays of course they would swerve the Exploder up a ramp, get the thing up on two wheels, and drive along on two wheels while a couple of their buddies climb out and pull the wheels off and put them back on. But way back around the turn of the century, primitive technology, no Go-Pro, hardly even any YouTube, just a lot of sand and alcohol, they had to make do with letting the air out and jumping sand dunes.

After which they would load up their Exploders and see how fast (or if?) they can get back home on 12 psi.

Now the really crazy thing is, when the inevitable happened and these lunatics sued they did not get laughed out of court but instead were actually paid a huge settlement. Which while hard to prove probably does a pretty good job of explaining why the numbers they give us have so little relevance to the sort of real-world driving experience that would actually matter to an even halfways responsible or mature adult driver.

salayc

perhaps this explains it better: https://www.edmunds.com/porsche/911/...-pressure.html

Valvefloat991

It wasn't just the Saudis. Plenty of Americans check their tire pressure once a year, whether they need to or not. When the recommended pressure was the more typical 35 psi, the loss of 6-7 psi wasn't critical. However, when the recommended pressure was 26, the loss of several psi was a little more dicey.

As tire temperature depends on both ambient temperature and speed, it's no surprise that most of the Explorer/Firestone blowouts occurred on the highway and in the summer in loaded vehicles. In fact, over 50 percent of the blowouts were on the left rear tires, because in a loaded Explorer, that is the most heavily loaded tire.

The other part of the story is that it turns out that an Explorer with such a blowout will travel more or less straight with reasonable stability, absent spurious input from the driver. The sad part is that a number of people died, mostly because they did something sudden and unnecessary to destabilize the car.