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      11-21-2013, 12:10 PM   #1
sunsweet
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Optimized tire pressure in cool weather ?

I do not plan to drive my new M3 in freezing and near freezing temperatures so I will not purchase winter tires. Here in the South East I would not benefit from them significantly so I plan on using the OEM Conti all year round.

I have a Longacre pressure gauge that enables me to release air and measure pressure at the same time. It is great. I have checked it against others gauges and it is very accurate.
Tire pressure drops with cooler temperatures, estimated to 1 PSI drop for every 10 degrees Fahrenheit. The M3 Coupe manufacturer recommendations for cold tire pressure is 35 PSI front, 36 PSI rear for normal driving conditions.

As the weather got a little cooler I've checked my cold tires pressure (undriven car, afternoon time. I don't drive the car in cold mornings). It was real low 28,28,32,32 PSI. So I've put more air in them to bring them exactly to 35 f/ 36 r. As a result the ride got much rougher. Especially on the highway you can feel the smallest minute imperfections of the road. If the road is anything short of perfect this leads to a much rougher ride even in comfort setting EDC mode. The ZCP package of course makes it more acute. The car feels perfectly planted on the road but with almost no cushion. I get limbs vibrating more with the road and also wonder if this could develop dashboard cringing noise problem in the long term.

I wonder if I could go to a little less pressure to ease the ride. But supposedly we should stick with OEM recommended pressure for maximum performance, fuel efficiency and balanced thread wear ?

What cold tire pressures is acceptable ?

Last edited by sunsweet; 11-21-2013 at 12:17 PM.
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      11-21-2013, 12:19 PM   #2
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Fill with nitrogen. Set to 33F/38R (or whatever the pressure recommended on the sticker of your door column) then you won't have to worry about temperature change anymore.
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      11-21-2013, 01:03 PM   #3
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Quote:
Originally Posted by Thik View Post
Fill with nitrogen. Set to 33F/38R (or whatever the pressure recommended on the sticker of your door column) then you won't have to worry about temperature change anymore.
Ok good to know but my question is not about how to keep constant tire pressure over thermometer changes but about the trade off between ride comfort and tire pressure and wondering what pressure people are running in their tires and how often they check it.
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      11-21-2013, 01:11 PM   #4
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Originally Posted by Thik View Post
Fill with nitrogen. Set to 33F/38R (or whatever the pressure recommended on the sticker of your door column) then you won't have to worry about temperature change anymore.
I'm no chemist but how does inflating with N rid one of the need to check tire pressures when temperatures outside are changing? I understand that there is less leakage through rubber with N but that has nothing to do pressure changes due to temperature changes. Can some one tell me what I'm missing here?
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      11-21-2013, 01:15 PM   #5
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Fill with nitrogen. Set to 33F/38R (or whatever the pressure recommended on the sticker of your door column) then you won't have to worry about temperature change anymore.
Nope.

The nitrogen folks conveniently forget to mention that regular air is 78% nitrogen anyway.

Consumer reports checked this out, and their results said: Don't bother.

Bruce
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      11-21-2013, 01:16 PM   #6
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Sunsweet, I missed your last response. I keep my pressures as BMW recommends 35F/36R for normal driving speeds. I check my pressures about once a month and I like the ride at those pressures.
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      11-21-2013, 02:44 PM   #7
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Quote:
Originally Posted by baba louey View Post
Sunsweet, I missed your last response. I keep my pressures as BMW recommends 35F/36R for normal driving speeds. I check my pressures about once a month and I like the ride at those pressures.
Ok, I will leave them at 35/36 and see if I get used to it.
Could 33/34 make the ride a bit smoother and save tire thread ?
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      11-21-2013, 03:02 PM   #8
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Originally Posted by sunsweet View Post
Ok, I will leave them at 35/36 and see if I get used to it.
Could 33/34 make the ride a bit smoother and save tire thread ?
For a car like the M3, I have to assume that tire pressures are decided upon by contact patch and safety, probably some other stuff too. Lowering PSI may soften the ride up but may also cause uneven wear and some extra internal heat.

As for ride, I love it but my wife hates it.
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      11-21-2013, 03:04 PM   #9
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Originally Posted by baba louey View Post
I'm no chemist but how does inflating with N rid one of the need to check tire pressures when temperatures outside are changing? I understand that there is less leakage through rubber with N but that has nothing to do pressure changes due to temperature changes. Can some one tell me what I'm missing here?
Air is like 78% nitrogen anyways.

The thought is that the other 22% that makes up the air expands and contracts with temperature which results in tire pressure flucation. Having 100% nitrogen would then in theorey negate that 22% and therefore your tire would change with temperature.

But since air is made of 78% nitrogen, the drops people see due to the 22% is more than just because of temperature flux.

Hope that explains it a little for you. I could go into the particle size and expansion ratios but I will spare you.
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      11-21-2013, 03:21 PM   #10
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"The M3 Coupe manufacturer recommendations for cold tire pressure is 35 PSI front, 36 PSI rear for normal driving conditions."

Where do you see this ?
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      11-21-2013, 03:32 PM   #11
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Quote:
Originally Posted by zeeman335 View Post
"The M3 Coupe manufacturer recommendations for cold tire pressure is 35 PSI front, 36 PSI rear for normal driving conditions."

Where do you see this ?
On the sticker on the door column. ( ZCP, Coupe M3)
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      11-21-2013, 03:33 PM   #12
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Quote:
Originally Posted by zeeman335 View Post
"The M3 Coupe manufacturer recommendations for cold tire pressure is 35 PSI front, 36 PSI rear for normal driving conditions."

Where do you see this ?
Driver's side door jar (on body, not door)
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      11-21-2013, 03:37 PM   #13
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Quote:
Originally Posted by happos2 View Post
Air is like 78% nitrogen anyways.

The thought is that the other 22% that makes up the air expands and contracts with temperature which results in tire pressure flucation. Having 100% nitrogen would then in theorey negate that 22% and therefore your tire would change with temperature.

But since air is made of 78% nitrogen, the drops people see due to the 22% is more than just because of temperature flux.

Hope that explains it a little for you. I could go into the particle size and expansion ratios but I will spare you.
PV=nRT therefore P=nRT/V. If everything stays the same on the rt side of the eqn except T, then P varies with T no matter what the gas. Am I missing something here?
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      11-21-2013, 03:46 PM   #14
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Originally Posted by Mr Sparkle View Post
nope, Ideal gas law has been understood for almost 200 years. But he is missing something in his explanation. He said the other 22% and the pressure changes are caused by other factors that trumps simple temperature changes. If so, what are they caused by?
The 22% is partly made up of water vapor which under temperature (high or low) expands or contracts hence changing the pressure.
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      11-21-2013, 03:52 PM   #15
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Originally Posted by bruce.augenstein@comcast. View Post
Nope.

The nitrogen folks conveniently forget to mention that regular air is 78% nitrogen anyway.

Consumer reports checked this out, and their results said: Don't bother.

Bruce
"Air is 78% nitrogen" is not a valid argument. You can say cyanide spiked water is 99.9% water anyways, but would you drink it? The other 22% of air is what mostly causes the tire pressure to change in cold or hot days.

Filling with nitrogen is more likely than not than regular air that pressure will stay close to the intended pressure. BMW recommends 33F/35R (per my owner's manual) probably because it's the best compromise between performance, comfort, tire wear and fuel economy.

At $5 a tire, I really don't see why not. Once it's filled with nitrogen, you'll have to top it up less often, which that in itself may save you money, not to mention the tire wear and fuel economy benefits by having your tire pressure being more constant.

Last edited by Thik; 11-21-2013 at 05:54 PM.
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      11-21-2013, 04:38 PM   #16
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Quote:
Originally Posted by happos2 View Post
Air is like 78% nitrogen anyways.
Hope that explains it a little for you. I could go into the particle size and expansion ratios but I will spare you.
Would you mind responding the original thread question. Thanks.
"what pressure people are running in their tires and how often they check it."
+ how is the ride.

Quote:
Originally Posted by italyix View Post
The 22%... .
Idem.

Quote:
Originally Posted by Thik View Post
" You can say cyanide ...
..and same
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      11-21-2013, 04:40 PM   #17
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Quote:
Originally Posted by baba louey View Post
PV=nRT therefore P=nRT/V. If everything stays the same on the rt side of the eqn except T, then P varies with T no matter what the gas. Am I missing something here?
That is the Ideal Gas Law.

Disclaimer: Be careful with general chemistry assumptions too. Nitrogen behanves most like an ideal gas at low pressure or high temperatures. Air is not an ideal gas but is usually assumed so (escpecially in the classroom for simplicity).

Some thoughts....
You need to look at difference between pure nitrogen expanding and how much the 22% of air that isn't nitrogen expands. This is where you will find a difference (mostly due to water vapor).

Nitrogen is also a dry gas. If you eliminate the oxygen from the system, you don't have to worry about water vapor forming which will change with heat greatly. (78% N2, 21% 02, 1% residual gases makes up air). So you have quite a bit of O2 ready to bond with H2 to form water vapor. This would require purging the air out of the tire and replacing with nitrogen. This is difficult to do as there is always residual air in the tire. Also why the process is costly in part.

Also note the tire does not seal perfectly, and all tires will leak a different about based on tolerances during manufacturing. You have to account for some leakage here as well.

O2 will also permeate through the rubber better than nitrogen will

Any water vapor inside your tire also promotes corrision on some wheel types (obviously a long process) but still a negative to air.

With all that said, I still use standard air and just watch my tire pressures. I can adjust it on the fly and the cost difference isn't worth it to me.

Again
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      11-21-2013, 04:43 PM   #18
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Quote:
Originally Posted by sunsweet View Post
Would you mind responding the original thread question. Thanks.
"what pressure people are running in their tires and how often they check it."
+ how is the ride.



Idem.



..and same
or in drivers side door.
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      11-21-2013, 04:45 PM   #19
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Quote:
Originally Posted by Lute View Post
Nitrogen is a total waste of money. Study after study have proved this. Nitrogen has no real benefit on street cars, and you can't obtain 100% pure nitrogen anyways
Yep, purity is the reason!! The more pure you try to make the system, the costlier it gets!
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      11-21-2013, 05:08 PM   #20
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Quote:
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Nitrogen is a total waste of money. Study after study have proved this. Nitrogen has no real benefit on street cars, and you can't obtain 100% pure nitrogen anyways

OP I would just go a couple of PSI over what's on your door jam.
+1, now back on the topic: Ride comfort versus tire pressure nominal spec
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      11-21-2013, 05:10 PM   #21
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Quote:
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or in drivers side door.


you are an idiot why do you think I posted the manufacturer's tire inflation spec on my original post ?
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      11-21-2013, 05:28 PM   #22
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you can adjust on a wear basis. if you wear the edges more than the center at 33 or 34 then you know the under inflation is having an effect. my guess it will make no difference in wear and thus you can use the lower pressure if it suits you better as far as ride quality.
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