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E90/E91/E92/E93 (2006 - 2013)
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  #1  
Old 12-23-2013, 06:41 PM
kellern kellern is offline
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Recommended tire pressure vs. dealer tire pressure

I got the 30k service performed on my 2012 E91 earlier today and the service advisor told me he had put all tires at 38psi for better handling... Recommended pressure is 33 in front and 41 in the back - The car feels a bit noisier and I did skid a bit getting out of my neighborhood in a spot where I am usually on rails (could have been pilot error) - Thoughts? Should I go to the recommended pressure settings?
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  #2  
Old 12-23-2013, 06:52 PM
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So you have three choices.....

Manufacturers rec based on your car design.
Some dealer in south Texas.
People you don't know on a message board .....

I'd vote for door number 1
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  #3  
Old 12-23-2013, 06:54 PM
ctuna ctuna is online now
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I would go with what on the side plate in the car.

I would go with what on the side plate in the car.
Mine have more preasure in the back it's 40 somethine
I believe. If you have rft's the ride is going to be stiff anyway.

Last edited by ctuna; 12-23-2013 at 06:55 PM.
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Old 12-23-2013, 07:24 PM
CitizenOfDreams CitizenOfDreams is offline
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Your lower limit is the door plate pressure. There is no point in going lower, it will only wear your tires and make the car feel like a water mattress.

Your upper limit is the tire sidewall pressure. Going above it is not a good idea, for obvious reasons.

Pick the pressure between those two points to make your car handle the way you like it.
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  #5  
Old 12-23-2013, 07:35 PM
jnew_85 jnew_85 is offline
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Also please take into consideration the season your filling your tires. Whether or not your using nitrogen fills. What the actual tire specs say. Those are just a few. The ride on your car is based on the numbers on the side plate. You want a perfect ride during normal driving operations then fill to those specs.
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  #6  
Old 12-23-2013, 07:42 PM
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CALWATERBOY CALWATERBOY is offline
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Quote:
Originally Posted by kellern View Post
I got the 30k service performed on my 2012 E91 earlier today and the service advisor told me he had put all tires at 38psi for better handling... Recommended pressure is 33 in front and 41 in the back - The car feels a bit noisier and I did skid a bit getting out of my neighborhood in a spot where I am usually on rails (could have been pilot error) - Thoughts? Should I go to the recommended pressure settings?

BMW's spot-on re: Mfg's recommended tire pressure.

Also, among the most sensitive rides - on my bike, I feel 0.5 psi off - 1 psi in my 3.

I don't like the feeling.

I have a garage compressor....you should too!

Isn't Christmas sometime soon?



Last edited by CALWATERBOY; 12-23-2013 at 07:45 PM.
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  #7  
Old 12-23-2013, 07:57 PM
gcreese gcreese is offline
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I check my tire pressures before I bring my car to the dealer, and without fail they're incorrect when I get back home. Instead of 32/39 (per the door), they'll be like 38. So they take an air hose, set it to a middling number and then put air in all four tires, so they can proudly say, "We adjusted the air in your tires." Since a lot of people never check their tire pressures, such a strategy serves those who drive around on semi-flat tires. However, it would be better for me if they left things alone.

So, no, I wouldn't trust the dealer. I'd trust the BMW engineers and lawyers who decided on the door numbers. You can adjust up or down for personal preference, but I'd start with the door numbers.
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Last edited by gcreese; 12-23-2013 at 08:00 PM.
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  #8  
Old 12-23-2013, 08:28 PM
Tom K. Tom K. is online now
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The OEM rec is for a fully loaded (1100 lb.) wagon up to 100 mph. As i normally drive solo or with only one passenger and little or no luggage, I set the rears to 37 psi and enjoy the comfy ride, reduced understeer, improved wet traction and better tire wear. When carrying a full load, I inflate to about 35/42.

Tom
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  #9  
Old 12-24-2013, 03:21 AM
jadatis jadatis is offline
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It is possible to calculate a lowest save pressure for the conditions you use the car for.
I once got hold of this formula, and in time found out many things about tires maximum load and needed pressure for that , wich is called the reference-pressure in that formula sheet, and is lower then the maximum pressure of a tire.

Also found out that the only concern of the car makers is that the tires wont get damaged by to much bending of the rubber by to much deflection of the tire.
Together with the tire-makers they yust calculated it with different formula's in Europe then in America.
America swiched over as late as 2006 to the European calculation , but only for standardload/P-tires and XL/Extraload/reinforced , and left the calculation for C-load and up to the old wrong one.

So you cant say that you have to trust the car- or tire-maker for determining the right pressure. Yust trust me and give from tires the sises/ maximum load and kind of tire and speedcode, and from car GAWR's and GVWR and empty weight and way you load it normally, maximum speed or speed you wont go over for sertain in practice, camber angle ( mosty for rear tires done , tires like this/-\ on the axle).
Then I will calculate a save lowest pressure wich you have to stay above in all conditions ( so also when its freesing cold), and will write how I came to it.

The service advices probably knows more about your tires and configuration, that he wont tell you about. A thing the tire-makers dont want us to know , but what I found out, is that for low Aspect Ratio ( AR = Hight/width division) the maximum load on the sidewall is to high given by the tire-maker. I got hold of graphics where a 35 % AR tire deflects 27 % of its free flexible part of the sidewall when maximum load and referencepressure on it. where a 80% only 20 % . The 35 should only deflect 20 % too to be save for it to laws of nature, and to my conclusions even less , about 16% because then the rubber bends the same angle as the 80% Ar.

This all the tire- and car makers discovered themselfes in the Ford/Firestone Afaire where more then 100 people died because of roll over accidents, mostly courced by blewing tires.
Low tire pressure advice was an issue then. I concluded somewhat similar then low AR tires, that the large profile blocks that covered the sidewall stiffened that sidwall, so it had less free flexible .
From that moment tire-pressure advice chached all over the world, also in Europe where the calculation was pretty alright for P-tires and XL// for decades.
If you use the ideal formula, but the input is wrong, the outcome is wrong too.
The combination of wrong calculation and wrong imput gives double wrong output.
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I live in Holland and once got hold of the system the tire- and car manufacturers use to determine the advice-pressure for cars. Learned myself Excell to make spreadsheets for it.
Next link leads to map with spreadsheets to re-calculate advice pressures for non-OEM tires, or to check the original.
http://cid-a526e0eee092e6dc.office.l...0tyre-pressure
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  #10  
Old 12-24-2013, 05:20 AM
jayb328i jayb328i is offline
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Quote:
Originally Posted by CitizenOfDreams View Post
Your lower limit is the door plate pressure. There is no point in going lower, it will only wear your tires and make the car feel like a water mattress.

Your upper limit is the tire sidewall pressure. Going above it is not a good idea, for obvious reasons.

Pick the pressure between those two points to make your car handle the way you like it.
Really??
Learn something everyday. Never saw this in writing elsewhere.
Guess my door plate was printed in error, because it does not say LOWER LIMIT. They carried that error over to the Owner Manual, Service Manual, and all the other Auto Books.

So what I have learned is that just use the Sidewall Pressure as the Upper Limit and anything below that is okay (just stay above the Door Plate "minimum"). One can ignore the type of car the tires are on, the weight of the car, the wheel base, the suspension and steering, and all the other stuff. I was under the misunderstanding that it might be a safety factor by putting close to Sidewall pressure into the tire resulting in, perhaps, longer stopping distances, based on model of car.
Glad to now understand that I can increase the tire pressure based on the Sidewall, by over 10 pounds-----and that would still be safe.
Someone should notify BMW of this finding so that they correct all their material.

Last edited by jayb328i; 12-24-2013 at 05:21 AM.
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  #11  
Old 12-24-2013, 06:50 AM
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Bob Shiftright Bob Shiftright is offline
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I use pressures in-between the 100 MPH numbers for normal and fully loaded car. I like a stiff ride (or rather, I like the most precise handling).

(Interestingly, my octogenarian uncle -- who didn't buy a Z4 because of the ride -- said he thought the ride of my car was soft enough that he would have purchased the Z4 if it had a ride like my car. The dealer must have left the shipping blocks in the suspension.)
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  #12  
Old 12-24-2013, 07:00 AM
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Bob Shiftright Bob Shiftright is offline
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Quote:
Originally Posted by CALWATERBOY View Post
BMW's spot-on re: Mfg's recommended tire pressure.
When I attended a Skip Barber performance driving school some years ago, they let us take our street cars out on to the track. (I don't know if their insurance still allows that.) They told us to pump up our tires waaaaaaaaay above the manufacturer's pressure recommendations. The manufacturers' recommendations are a compromise between ride, handling and safety.
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  #13  
Old 12-24-2013, 07:08 AM
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CALWATERBOY CALWATERBOY is offline
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Quote:
Originally Posted by Majikthese42 View Post
When I attended a Skip Barber performance driving school some years ago, they let us take our street cars out on to the track. (I don't know if their insurance still allows that.) They told us to pump up our tires waaaaaaaaay above the manufacturer's pressure recommendations. The manufacturers' recommendations are a compromise between ride, handling and safety.

True! But BMW's recommended pressures are for street use, of course. Unlike some I've found BMW pressures to be in the sweet spot for my kind o'motorsport: back road barnstormin'
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Old 12-24-2013, 07:35 AM
CitizenOfDreams CitizenOfDreams is offline
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Quote:
Originally Posted by jayb328i View Post
Guess my door plate was printed in error, because it does not say LOWER LIMIT.
It does not say so, but for all practical purposes it is. Recommended tire pressures for a luxury car are usually in the lower range, with comfortable "plush" ride in mind.

Quote:
So what I have learned is that just use the Sidewall Pressure as the Upper Limit and anything below that is okay
Yes. In the sense that, if you stay between those two limits, the tires will work in their safe range. They will not explode, break, wear out prematurely or do any other funny things.

Quote:
One can ignore the type of car the tires are on, the weight of the car, the wheel base, the suspension and steering, and all the other stuff.
You mean, like the tire brand/model/type? My door sticker completely ignores that little detail. So if I were to trust it, I would have to run the same pressure in summer tires, winter tires, touring tires, performance tires, Michelin, Bridgestone, Pirelli, Sumitomo...
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Old 12-24-2013, 01:09 PM
Pilgrim Pilgrim is offline
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Quote:
Originally Posted by CitizenOfDreams View Post
Your lower limit is the door plate pressure. There is no point in going lower, it will only wear your tires and make the car feel like a water mattress.

Your upper limit is the tire sidewall pressure. Going above it is not a good idea, for obvious reasons.

Pick the pressure between those two points to make your car handle the way you like it.
This is right according to all my training (including Bondurant and a State Patrol Instructors school).

Going softer will hurt gas mileage, compromise handling and produce premature tire wear. The only positive result is a smoother ride because the tires 'give" more.

Going up (no higher than the max pressure listed on the tires) does no harm; it will make the sidewalls stiffer and improve handling response, it may improve gas mileage, and will have no negative impact on tire wear. It will however make the ride more harsh and you will feel every bump in the road more clearly.

ONE caveat: maintain the same front/rear difference in pressures that the door sticker indicates. 33 and 41 sounds unusual - that's a lot more difference than most cars. 3-4 pounds difference front/rear is more common. Make sure you're not reading pressures from two different passenger loadings.

Explanation: if your tire max pressure on the sidewall is 44 PSI and the door sticker says 33 front 38 rear, you need to keep the fronts 5 PSI lower than the rears to maintain the handling characteristics BMW designed in. You can run up to 39 front/44 rear if you wish, although that would be a somewhat harsh ride.

If I were the OP I'd re-check my door sticker and make sure I'm interpreting it correctly.
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Old 12-25-2013, 04:57 PM
Tom K. Tom K. is online now
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Quote:
Originally Posted by Pilgrim View Post
If I were the OP I'd re-check my door sticker and make sure I'm interpreting it correctly.
As my '07 wagon has the same OEM rec of 33/41, I believe he is correct. It seems to me that BMW is very afraid that someone will set a "normal" F/R differential of 3 lbs. or so, load up the rear of their E91 and then experience dreaded over steer. This is why I reduce the differential when my car is lightly loaded - improving both ride and handling.

But setting both ends at the same pressure as his SA did will result (at best) in really squirrely instability at speed in any significant crosswind.

Tom
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  #17  
Old 12-25-2013, 06:44 PM
Norm37 Norm37 is offline
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Page 239 - 254 in the downloadable owners manual.

http://www.bmwusa.com/Standard/Conte...modelcode=123C
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