Doorjamb OEM recommend tire pressure--51psi

[limeypride]

I've asked a few questions on this topic but I still don't have an explanation as to "why". So, does anyone know why Audi (or is it RS7-specific) specify such sky-high tire pressures--all 4 wheels @51 psi is both figuratively and literally off-the-charts. For comparison, my F10 M5s were 33 psi as were my F06 M6s. The X5M was a little higher at 36 psi. My wife's new Ford Explorer is about the same. The SUVs are both far heavier than the RS7 both approaching or cracking the 5,000Ib mark. The M6 GC is about the same weight as the RS7.

The stock tire sizes between the bimmers and the RS7, though, differ wildly. M5/M6s were 295/30/20 rear and 255s up front. The X5M looks like a steam-roller from the rear with 315/30/21s. Given that the RS7's tirewall is 30% of 275, that is one possible reason but it doesn't seem enough to warrant a 50% pressure increase... blows my mind... and I suspect my tires if I drive it like that through the NV desert.

I've lowered the pressures now and the ride is a little less harsh and nothing negative to report: I'm running 43 cold psi rears and 45 cold psi on the fronts.

Anyone got any facts/thoughts/opinions/insights?

[wwhan]

The door jamb stated pressure is for absolute maximum load of passengers and luggage.... 4,497 lbs plus load, well over 5000lbs.

Check the manual, light load pressure is lower.

The stated presure is dependent on the tire size also, with lower profile tires requiring higher pressure.

Are the stock tires 275/30, 21"?

[limeypride]

The door jamb stated pressure is for absolute maximum load of passengers and luggage.... 4,497 lbs plus load, well over 5000lbs.

Check the manual, light load pressure is lower.

The stated presure is dependent on the tire size also, with lower profile tires requiring higher pressure.

Are the stock tires 275/30, 21"?

Stock tires are Contis at 275/30/21.

Understood RE: the doorjamb and cited air pressures being for daily driving with 4-passengers but even the owner's manual says 46 psi under light load--that's still sky high.

[warp_ix]

It says 45 psi on my 2016 RS7 Prestige. I don't know why the Performance would be different.

Sent from my XT1096 using Tapatalk

[wwhan]

Stock tires are Contis at 275/30/21.

Understood RE: the doorjamb and cited air pressures being for daily driving with 4-passengers but even the owner's manual says 46 psi under light load--that's still sky high.

Since it is a RS7 Performance, maybe they assume your driving 150mph with a full 1146lb load (GVW ~ 5643 lbs).

[limeypride]

It says 45 psi on my 2016 RS7 Prestige. I don't know why the Performance would be different.

Sent from my XT1096 using Tapatalk

The owner's manual and the doorjamb both say 51psi. My owner's manual also lists the A7, S7 an RS7 non-P which it lists per your picture below. Now that's another question to add to my existing pile, why would the two be so different--perhaps the different suspension setups.

[gk1]

Since it is a RS7 Performance, maybe they assume your driving 150mph with a full 1146lb load (GVW ~ 5643 lbs).

I would think this is right.Door jam ratings for max load at ultra high speed to avoid any Ford/Firestone issues again.

[limeypride]

I would think this is right.Door jam ratings for max load at ultra high speed to avoid any Ford/Firestone issues again.

Ahh, great point--I'd neglected to consider liability-driven engineering.

[jadatis]

Finally my registration is complete, so I can react to this topic.
Live and was born in the Netherlands , and end 2007 I got hold of the official formula ans system , to determine pressure for load and speed on tire, and went running with it.
Now call myself "Pigheaded Dutch Selfdeclared tirepressure-specialist".

What someone already wrote , about the Ford/Firestone affaire , he sertainly has a point to my conclusions.
that affaire had a large impact on how tire- and car-makers handled tirepressure advice also in Europe.
Nowadays tire- and car-makers can be held responcible if a tire blows and cources accident or only damage, even if user goes above the maximum loads given or uses speeds higher then pressure advice is given for.
That is why they now give such a high advice , that you must be doing something extremely wrong to get tiredamage.

OK enaugh for now about that, now about your tires.

the 275/30 ZR 21 98 Y XL you have and I read from the picture given, has next specifications to the tiremakers system.
ZR means that the tiremaker did something off standard in the tiredesighn , and for that reason has to give highening up of AT-pressure ( will explain later) for higher speed themselfes, but mostly they also give the speedcode V/W/ or Y ( or (Y) ) and then that system can be used.
This means for you that Y speedrate system can be used.
But first back, 98 means loadindex 98 / maximum load 750kg/1653lbs AT 2.9 bar/42 psi ( because of XL I found out with google) Up to 220km/137n/h without any part of ther rubber of tire overheating so it hardens and slowly or soon damages by driving.

But to my conclusions in time, it must be this, The 220km/137m/h referencespeed must be yust as Q speedrated 160km/99m/h, and the Loadindex best be lowered for reasons of the low aspect ratio ( hight/Width division of here 30%, mayby even by 8 steps so maximum load 20% lower.
Then new will be LI 90/ML 600 kg/1323 lbs AT 42 psi AT 160km/99m/h .
These data should be filled in the formula to give the needed pressure.

For higher speed the official system is that the AT-pressure must be highened up with a system depending on speedcode Y and 30km/h below maximum speed of tire of 300 km/h the maximum is lowered 0.5% a km so for the 30 km 15% .

My determined system is simpler and is for every 10km/6,5m/h other speed then referencespeed of tire, 1 Loadindex step different.
Lower speed> higher loadindex, higher speed > lower loadindex, and this goes for every kind of tire, from standard load up to truck-tire.

Pressure advice is nowadays only give for fully loaded or even a bit overloaded on rear-axle, up to maximum technical car speed .
But who uses his car in normal use to the maximum loads and speeds.

So carmaker has to go from maximum ever possible or even a bit higher, but the more acurate you know weights on seperate wheels, your used speed and wont go over for even a minute, and tire specifications used to laws of nature, your can determine a lower pressure , wich wont damage your tires by overheating, and you have better comfort and gripp.

So the 51 psi could be yust needed for savety , and that other give lower , is more because they did not realised their responcibility enaugh before.

So first work this already to long first post of mine out, before we go on.
About AT pressure and maximum cold pressure.
The At pressure is called reference pressure in the official formula and is the pressure for wich the maximum load for the refernce-speed is calculated , cold measured = when inside tire temp = outside tire temp, and outside tire temp is 18/20degr C/65/68 degr F.
for Standard load tires this is 35 psi in American TRA system ( EUR system 36 psi with exeptions to lower) , XL/reinforced/Extraload USA AT 41 psi and EUR 42 psi with exeptions.
The maximum cold pressure is the maximum cold pressure the tiremaker allows , and mostly between 44 psi and 51 psi and sometimes for XL// 60 psi.
Tires are tested to can stand a pressure of 2 to 3 times the AT-pressure , so I once read, and this is for your XL between 2x42=84 psi and 3x42= 126 psi, also the valves are given maximum pressure cold and must be able to can stand much higher pressure .
On LT tires AT-pressure is given on sidewall, On SL and XL// tires maximum allowed cold pressure is given on sidewall

Greatings from Holland,
(ja dat is ) Peter ( from a Dutch name song tranlates as Yeah thats Peter)

[limeypride]

Finally my registration is complete, so I can react to this topic.
Live and was born in the Netherlands , and end 2007 I got hold of the official formula ans system , to determine pressure for load ans speed on tire, and went running with it.
Now call myself "Pigheaded Dutch Selfdeclared tirepressure-specialist".

What someone already wrote , about the Ford/Firestone affaire , he sertainly has a point to my conclusions.
that affaire had a large impact on how tire- and car-makers handled tirepressure advice also in Europe.
Nowadays tire- and car-makers can be held responcible if a tire blows and cources accident or only damage, even if user goes above the maximum loads given or uses speeds higher then pressure advice is given for.
That is why they now give such a high advice , that you must be doing something extremely wrong to get tiredamage.

OK enaugh for now about that, now about your tires.

the 275/30 ZR 21 98 Y XL you have and I read from the picture given, has next specifications to the tiremakers system.
ZR means that the tiremaker did something off standard in the tiredesighn , and for that reason has to give highening up of AT-pressure ( will explain later) for higher speed themselfes, but mostly they also give the speedcode V/W/ or Y ( or (Y) ) and then that system can be used.
This means for you that Y speedrate system can be used.
But first back, 98 means loadindex 98 / maximum load 750kg/1653lbs AT 2.9 bar/42 psi ( because of XL I found out with google) Up to 220km/137n/h without any part of ther rubber of tire overheating so it hardens and slowly or soon damages by driving.

But to my conclusions in time, it must be this, The 220km/137m/h referencespeed must be yust as Q speedrated 160km/99m/h, and the Loadindex best be lowered for reasons of the low aspect ratio ( hight/Width division of here 30%, mayby even by 8 steps so maximum load 20% lower.
Then new will be LI 90/ML 600 kg/1323 lbs AT 42 psi AT 160km/99m/h .
These data should be filled in the formula to give the needed pressure.

For higher speed the official system is that the AT-pressure must be highened up with a system depending on speedcode Y and 30km/h below maximum speed of tire of 300 km/h the maximum is lowered 0.5% a km so for the 30 km 15% .

My determined system is simpler and is for every 10km/6,5m/h other speed then referencespeed of tire, 1 Loadindex step different.
Lower speed> higher loadindex, higher speed > lower loadindex, and this goes for every kind of tire, from standard load up to truck-tire.

Pressure advice is nowadays only give for fully loaded or even a bit overloaded on rear-axle, up to maximum technical car speed .
But who uses his car in normal use to the maximum loads and speeds.

So carmaker has to go from maximum ever possible or even a bit higher, but the more acurate you know weights on seperate wheels, your used speed and wont go over for even a minute, and tire specifications used to laws of nature, your can determine a lower pressure , wich wont damage your tires by overheating, and you have better comfort and gripp.

So the 51 psi could be yust needed for savety , and that other give lower , is more because they realised their responcibility enaugh before.

So first work this already to long first post of mine out, before we go on.

Greatings from Holland,
(ja dat is ) Peter ( from a Dutch name song tranlates as Yeah thats Peter)

Hi Peter,

Welcome, and thanks for all the info.

Can you provide your conclusion one more time, please.

[jadatis]

Already edited my post , corrected some miswritings and added AT and maxcold pressure.
But what do you mean with providing my conclusions one more time?

[limeypride]

Already edited my post , corrected some miswritings and added AT and maxcold pressure.
But what do you mean with providing my conclusions one more time?

I wanted to know if you're making a recommendation. There's so much information in there, it's not clear to me what my conclusions should be.

[OlyS6]

Great topic- I've been researching this myself, as I'd felt that my new tires were 'riding' underinflated. Turns out there's some fairly intricate background from an engineering aspect. If you care to research, the European Tyre and Rim technical Organization has some great information. http://www.etrto.org/page.asp?id=1594

Bottom line up front, you can use load inflation tables based on your load rating to determine the psi for your tires under given conditions (the door label recommendations are indeed for maximum load at ideal temp of 68 degrees F). Toyo tires has some good load inflation tables to follow, as do others.

A caveat to this is that after 42psi, the recommended max load does not change for these performance tires (so the recommended psi listed is not based on max load alone). To the best I've been able to read, some European manufacturers of high performance vehicles will make higher recommendations to maximize 'performance and handling', with 51 psi being a common recommendation. I have to say I don't understand this part, as the way I've always understood it, is that the more inflated the tire, potentially the less surface area patch of rubber touching the road, and therefore potentially less grip. Exactly how the 51psi is derived (since 42psi generally hits max load limit for most tires), I don't know.

I'd spent a few hours researching this (bored), as my door jamb on my S6 recommends 46psi up front and 48psi in the rear tires, making me ask the WTF question as well.

tirepressure.com has some interesting load tables you may want to peruse as well.

[tenspeed]

To the best I've been able to read, some European manufacturers of high performance vehicles will make higher recommendations to maximize 'performance and handling', with 51 psi being a common recommendation. I have to say I don't understand this part, as the way I've always understood it, is that the more inflated the tire, potentially the less surface area patch of rubber touching the road, and therefore potentially less grip.

Steel belts keep the contact patch about the same size in today's tires. In the bias ply/nylon belted tire days, it was a different story. The increase in pressure does stiffen the sidewalls which results in crisper handling and reduces the tendency of the tire rolling off the rim in a tight corner. Learned that back in my autocross days with my Opel Rallye.

[limeypride]

Steel belts keep the contact patch about the same size in today's tires. In the bias ply/nylon belted tire days, it was a different story. The increase in pressure does stiffen the sidewalls which results in crisper handling and reduces the tendency of the tire rolling off the rim in a tight corner. Learned that back in my autocross days with my Opel Rallye.

Thanks to the three of you; I've got a far better picture of things now.

[jadatis]

I wanted to know if you're making a recommendation. There's so much information in there, it's not clear to me what my conclusions should be.

Best is to give me all the data of your car, so weight on wheels , determined as acurate as possible for the situation ( succes , will be hard to do). and speed you wont go over for even a minute in that situation, then I can calculate a pressure for you. If you give empty weight of car, GAWR's and GVWR, and I asume motor in front for determining weight-division over the axles, I can estimate ( ooho dangerous) the axle loads and together with your used speed, it will give a save pressure advice.
Then in this topic I show how I calculated it, so you can do it yourselfes the next time.

But because your tires are Y speedrated with maximum speed of 300km/??m/h , your cars maximum technical speed ( given in the books), is most likely also in that direction.
So if you realy drive 300km/h with fully loaded axles , you sertainly need the 51 psi. But who does, mayby the bank robber with 20 goldbars in the back , trying to outdrive the police.

Normaly your absolute maximum speed(wich you wont go over for even a minute) will be about 160km/99m/h, and with only driver and a little load in car, mayby 2 persons.
For this speed and load , a much lower pressure is save, and you have better gripp and comfort .
When you then have other conditions, determine the new weights and speed, and you can determine pressure needed for that situation.
When 5 persons and a lot of load , you even wont make the 300km/h so this compensates a little, but then rear axle probably overloaded wich needs higher pressure.