Wheel pressures
#1
Member
Thread Starter
Wheel pressures
During the winter, every time I get in my car it tells me my tire pressure is low at 35PSI. After about 30min of driving it will warm up and go back up to 41. I was thinking about filling it up to 40PSI for the winter but do not want it to be 45-48PSI+ as the weather warms up next year. For the past two years I just left it under inflated until it warmed up and it seemed to put some extra wear around the outside edge of the front tires. These were filled by the dealer so I believe they have nitrogen.
Curious what you guys do here with your tire pressure during the winter?
Curious what you guys do here with your tire pressure during the winter?
#2
MBWorld Fanatic!
I keep my COLD pressure the same all year. I am super **** about that. I run 39 psi all year and check any time there is a temperature change. Excessive wear on the edges indicates underinflation.
#4
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Sorry for the science lesson, but some people kind of like the science behind our cars.
Last edited by glennhl; 12-19-2014 at 11:02 AM.
#5
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2010 C63 AMG, 2011 Mini Cooper (work)
The expansion/contraction of rubber during heat/cooling that causes microvoids in which an active gas (such as compressed air) will react to and release pressure vs the inert nature of nitrogen was my reference. Should have been more specific.
#6
Super Member
I'm not running the stock 18" anymore, but moved to 19" 245/275. That being said I now run 39/41 for cold pressures, and also always make sure that they are there when cold. All pressures go up with heat, and all pressures are listed and should be read cold. I forget what the stock 18" recommendations are, but I'd go with those. You have some flex as there are 2 recommendations, for normal or heavy loads depending on if you drive solo or with a full car most of the time.
I've passed on nitrogen myself as I think the quality is variable depending on distributor so I just stick with good old air. But if you're seeing abnormal tire wear like you report you are underinflated, which ain't good. My 2 cents
I've passed on nitrogen myself as I think the quality is variable depending on distributor so I just stick with good old air. But if you're seeing abnormal tire wear like you report you are underinflated, which ain't good. My 2 cents
#7
Super Member
That's a new one for me, thanks for the info. But isn't the air pretty inert to rubber products?
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#9
Super Member
But dry rotting and cracking happens over years. I don't think it would be enough to see a pressure change over just a few weeks???
Either way, I'll just stick with dry air. I've always read the main reason to use nitrogen is to avoid the pressure change you get from the partial pressure of water/water vapor in wet air.
#10
MBWorld Fanatic!
What is cold temperature? If 70F (20C or 293K) is a normal cold temp what about 20F (-6.5C or 266K) or -10F (-23C or 250K). How warm do tires get on a cold road do they always warm up to a given temp, or do they gain gain a certain # of degrees - I don't know.
(P1V1)/T1=(P2V2)T2 or P1/T1=P2/T2 assuming constant volume.
70F vs 20F 40PSI/294K=X/266K X=36PSI
70F vs -10F 40PSI/294K=X/250 K X=34PSI
(P1V1)/T1=(P2V2)T2 or P1/T1=P2/T2 assuming constant volume.
70F vs 20F 40PSI/294K=X/266K X=36PSI
70F vs -10F 40PSI/294K=X/250 K X=34PSI
#11
Super Member
What is cold temperature? If 70F (20C or 293K) is a normal cold temp what about 20F (-6.5C or 266K) or -10F (-23C or 250K). How warm do tires get on a cold road do they always warm up to a given temp, or do they gain gain a certain # of degrees - I don't know.
(P1V1)/T1=(P2V2)T2 or P1/T1=P2/T2 assuming constant volume.
70F vs 20F 40PSI/294K=X/266K X=36PSI
70F vs -10F 40PSI/294K=X/250 K X=34PSI
(P1V1)/T1=(P2V2)T2 or P1/T1=P2/T2 assuming constant volume.
70F vs 20F 40PSI/294K=X/266K X=36PSI
70F vs -10F 40PSI/294K=X/250 K X=34PSI
I think the amount that tires warm up during driving depends on how fast you are going, how hard to you corner, how long do you drive, etc.
#12
Super Member
the nitrogen versus air thing is this - nitrogen by itself can't seep out of tires, but the non-nitrogen components in air (oxygen, etc.) can, and so you get pressure drop over time. Tires always warm and increase pressure with use, but pressure values are always listed and should be filled cold. So no need to worry about how much values increase when warm, just fill cold and monitor cold. And my take on nitrogen is that it can often be contaminated with air depending on supplier, and I didn't see a net benefit based on that. But to each their own.
#14
Dry air and nitrogen expand and contract exactly the same with temperature change. So nitrogen is only better if you do not have a source of dry air. And the tire pressure with a nitrogen fill still goes up with higher temperatures. Both Nitrogen and Air are ideal gases in the pressure range we are using, so the pressure goes up with the ratio of the absolute temperatures. Assuming you start at 40 psig and 70 degrees (530R), then your tire heats up by 60 degrees when driving (590R), your new pressure would be 40 x 590/530 = 44.5 psig.
Sorry for the science lesson, but some people kind of like the science behind our cars.
Sorry for the science lesson, but some people kind of like the science behind our cars.
Dry air is 78% N2 and 21% O2
If saturated moisutre is 1% or
Adjust the pressures seasonally
A bit higher in winter
Make sure your source for air has a refrigerated or dessicant dryer
Preferably both
Moisture is the issue
Last edited by Ingenieur; 12-20-2014 at 03:09 PM.
#15
Member
Thread Starter
Thanks for the suggestions guys. I guess i'll bump up the PSI a bit.
The stock AMG rims are 18"? I could've swore they were 19s...
I'm not running the stock 18" anymore, but moved to 19" 245/275. That being said I now run 39/41 for cold pressures, and also always make sure that they are there when cold. All pressures go up with heat, and all pressures are listed and should be read cold. I forget what the stock 18" recommendations are, but I'd go with those. You have some flex as there are 2 recommendations, for normal or heavy loads depending on if you drive solo or with a full car most of the time.
I've passed on nitrogen myself as I think the quality is variable depending on distributor so I just stick with good old air. But if you're seeing abnormal tire wear like you report you are underinflated, which ain't good. My 2 cents
I've passed on nitrogen myself as I think the quality is variable depending on distributor so I just stick with good old air. But if you're seeing abnormal tire wear like you report you are underinflated, which ain't good. My 2 cents
#16
Super Member
Front: 235/40 ZR18 95 Y XL
8.0 J x 18 H2 offset: 45 mm
Rear: 255/35 ZR18 94 Y XL10
9.0 J x 18 H2 offset: 54 mm
#17
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W204 C63 Coupe, W166 ML350 BlueTEC, 928GT, C5 Z06 & IS300 race cars, EQE 4Matic+ on order
The OEM MB winters for the C63 (even for the 507) is a set of the old-style, pre-facelift solid five spoke 18x8 C63 fronts with 235 sized performance winter rubber (i.e. Pirelli SottoZero).