Tire Pressure--Stupid Question

[Garak]

Originally Posted By: edhackett
I suspect that people who claim that nitrogen doesn't leak out of their tires are actually seeing the benefit of a quality valve stem cap vs. the plain plastic, purely cosmetic caps usually found.


That's probably the most likely explanation of the whole issue in this thread.

 

[SteveSRT8]

VERY observant, ed.

 

[mechtech2]

Tires will get much warmer n summer.
Ambient temps and contact with hot roads are the causes.
Yes, there is a rise in temp/pressure when driving winter/summer.

Wild swings in pressure from leaving one setting for all year would result in uneven wear, worse life, and handling anomalies.

 

[rpn453]

Originally Posted By: TallPaul
Why do we run the same pressure winter and summer? Wouldn't it work to put the tires at say 35 psi on a 70 degree day, and assuming no leakage, if they went to 28 on a 20 degree day, why pump them up? Once you start drivng they will warm and when they warm to 70 degrees they should be at the specified 35 psi. Otherwise, by pumping them to 35 psi at 20 degrees, aren't they going to be overinflated by the time they warm to 70 degrees?


If the manufacturer recommends 35 psi cold, it will probably gain about 3 psi during normal use, and that's where they want it to run, at 38 psi. If you start with only 30 psi on a colder day, the tire will still only warm up enough to increase pressure by about 3 psi, to 33 psi, so it would still be 5 psi underinflated once it's fully warmed up.

 

[TallPaul]

Originally Posted By: rpn453

If the manufacturer recommends 35 psi cold, it will probably gain about 3 psi during normal use, and that's where they want it to run, at 38 psi. If you start with only 30 psi on a colder day, the tire will still only warm up enough to increase pressure by about 3 psi, to 33 psi, so it would still be 5 psi underinflated once it's fully warmed up.
I see. So the only way to get a 30 psi to 38 would be to run it way too high of a speed to overheat the tire and that is all very bad. Pressure change from cold to hot from use is not as great as I was assuming it to be.

 

[SteveSRT8]

Pressure gain from operation will vary wildly from car to car and even tire to tire.

Some tires have more work. Some tires are made poorly and get hotter just by use. These will gain much more pressure.

Heavy car or light car? AWD, FWD, RWD? All makes a difference to the tires.

 

[ChrisW]

Originally Posted By: edhackett
Quote:
1. Reduction of water vapor, which means a more consistent tire pressure at all temperatures, especially during landing when water wapor will quickly expand, resulting in a sudden and potentially catastrophic increase in tire pressure

True, however properly dried air will have the same benefit. When car tires are mounted a water based lubricant is used, usually in great excess. This nullifies any benefit to the dry gas. (emphasis added)


This.
To me I've known that the reason nitrogen may be a 'better' fill than plain 'shop' air is because generally speaking the process of extracting nitrogen from 'air' dries out the gas. Dried air will perform the same as nitrogen for all intents and purposes considering pressure variance. If and when I get myself setup with compressed air in my home garage, I'll consider getting an air drier and then add oil downstream for any tools.

I was at ALMS a couple years back in Monterey and asked the Pirelli guys as well as the Audi team and they said they no longer use nitrogen (it was getting expensive as they buy in industrial pre-filled tanks), they use dry air; the moisture content in the gas is what produces the most variable in pressure. And I haven't checked recently but I believe a good air drier machine is still quite a bit less expensive than a nitrogen machine/system.

 

[mechtech2]

Also, front and back tires will heat up and change pressure differently. Heavy front ends on cars is normal.

 

[rpn453]

Originally Posted By: SteveSRT8
Pressure gain from operation will vary wildly from car to car and even tire to tire.


Yeah, the numbers I threw out are just typical of what I've observed in my vehicles during moderate street use. Driving style obviously affects pressure as well.

The main point being that if it's cooler out, the tire will also run cooler. So pressure gain during use will be similar regardless of outside temperature and therefore it makes sense to start with the same pressure even in colder weather.