jakc

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?

chrisridebike8

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.

deadfission

Nitrogen fill. That should alleviate a lot of the changes from temperature (yes, air is 70+ percent nitrogen blah blah).

glennhl

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.

deadfission

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.

jcfay

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

glennhl

That's a new one for me, thanks for the info. But isn't the air pretty inert to rubber products?

deadfission

Not particularly. Compressed air is actually a pretty active gas in comparison to nitrogen. Hence dry rotting and cracking over long exposure to dry air.

glennhl

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.

rediesel

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

glennhl

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.

jcfay

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.

AMG6.3

Run 42-44 psi cold and u will be good.

Ingenieur

^^^ this
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

jakc

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...

jcfay

Nope, at least on my '14 C63 coupe. I think if you've got a 507 or black series you may have 19" stock, but I can't recall. My car came with these tire/wheel combinations:

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

Diabolis

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).