hrglas

You know this whole PSI thing is getting on my nerves........... I feel like after reading the numerous posts on this subject i would have it down to a science, but apparently not.

My door jam says:

Recommended "cold" pressure
Front: 33psi
rear: 32psi

I have since set mine to this....i purposely overinflated the tires, let the car sit overnight, and set them to these specs using a digital tire guage first thing in the morning before i drove the car...when i finished, the display inside the vehicle confirmed the settings. Presumably i am good to go..... or

Here is my scenario

Okay, so sunday the temperature got up to about 80-85 degrees. As i am driving into downtown chicago, the display shows the psi gradually climbing. The pressure climbed all the way up to 42psi before i decided to pull over to verify what the display was saying. when I got out to manually check, the digital guage confirmed the display. This is a difference of 9psi. Please tell me this isnt a normal increase? Isnt this a bit much even considering the outside temperature?

BLUF, what is going on here, because i am positive that i have set the tire pressure properly. in addition, what is the most your display has ever showed before you started freaking out....

pekkieboyz

What you want us to comfirm? your tire setting PSI are correct?
If so, they are, and the display doesn't lie!

chill_dog

Not sure how long of a drive Chicago is for you, but in the summer, I can go up 5 psi driving a few miles down the road. I'd say it's reasonable.

260 HP

iTrader: (3)

Personally, I think it's normal.

What's the ambient temperature when you measured the tire pressure, before you had driven it? Let's say 65 F. You said the ambient temp reached 85 F on your way to Chicago. Tire pressure increases as temperature increases, roughly 1 psi per 10 F. Even if you didn't drive the car, the tire pressure would have increased by 2 psi if the assumption above is correct.

Driving the car increases the air temperature inside the tires significantly. The tires change shape when they roll on the road, and their temperature increases. When you brake, some of the heat from your brake rotors are transmitted to the wheels and this also heats up the air inside the tires. Did you touch your wheels when you checked your tire pressure? I bet they were not just warm, but pretty hot instead. All these factors contribute in increasing the tire pressure.

I'm just curious: if it's not normal, what do you think the normal increase should be?

rwbentley

i guess you could use the thermo formula PV=NRT. You have a known (V)olume, variable (P)ressures, and variable (T)emperatures. N can be derived, and R is a constant.

http://en.wikipedia.org/wiki/Pv%3Dnrt

hrglas

Yes, on the morning that i set the tire pressure, the temperature was about 65 degrees. this was about three days prior. in the days between then and sunday, the only driving i had been doing was local neighborhood driving so the tire pressure didnt increase all that much throughout the day to raise a red flag...maybe 4psi.


i guess thats my question. but to me 9psi seems like an unusually high increase.

taking into account the factors you stated, am i to assume that as the summer months come and the temperatures get into the 90's maybe 100's, that the tire pressure will increase even more? if so, at what point will they level out and/or at what point should i worry about it?

I'm still curious to know what is the highest reading anyone has ever seen on their display after they have done a good amount of driving?


yeah, im gonna get right on that equation.......

6spMike

High humidity in the air in the tires can cause high fluctuations, too.
Hotter air can holds more water vapor = higher pressure.
Colder air causes the water vapor to condense into water, lowering the pressure.
PV=nrt won't account for that effect.
One would have to assume that the designers knew this; it's not something to worry about.
Severely overinflated tires could burst, but I think that takes quite a bit more pressure than 42. I think autocross guys run in the mid 40's all the time?
If it really bothers you, get them filled with dry nitrogen, or get an air dryer (for painting) for your compressor, and fill and deflate each tire a few times to get the moisture out.
PV=nrt, Volume is fairly constant, n is a constant, r is a constant, and t is temperature in degrees kelvin (300 kelvin = 80 F). A 10 degree change in temperature is a 3.3% change in degrees kelvin, so you can expect a 3.3% change in pressure for a 10 degree change in temperature around normal temps, or about 1 psi starting at 35 psi.
Sorry, I had a geeky moment there...

onsknth

Get this... Humidity has nothing to do with it... The air in the tires is sealed from the humidity by gobs of aluminum and rubber.

What you're experiencing is not uncommon, you've set at a much cooler and condensed temperature, that's the baseline. IT IS NOT THE NORM. By this I mean you should inflate to a consistent temperature in which the vehicle will operate, not cooler, not hotter. Air expands just as everything else when it heats, it's as simple as that... A 9psi increase is not unattainable considering the extreme differences in "baseline" temperatures in your example.

Next, I think we all need to keep in mind, we're seeing "new things" now that our cars are able to provide this "real time" information... How many times didja ever get outta any prior hoopties and check tire pressure between a 70 degree temperature difference?

6spMike

The humid air INSIDE the tire has everything to do with it.
9 psi would mean a 90 degree difference from his 65 degree baseline.
So the tires would be 155 degrees F after he drove it in order for that to explain it. Not very likely.
Get the humid air in the tires replaced with dry air or dry nitrogen= observation solved. I'm not calling it problem solved, because it isn't a problem.
Don't take my word for it, there are other threads about it.

6spMike

... I screwed this up. The absolute units equivalent to fahrenheit are rankine, and 80 F = 540 R, for a 10 degree percentage difference of 1.9%, or a 0.66 psi difference starting at 35 psi. You get the 3.3% change for a 10 degree celsius change.

hrglas

it wasnt my intention to start a mathematical debate out of this. I just think its very odd that after i set the tires to the pressure as set forth by the manufacturer, i would see an increase of 9psi...like has been said, thats equal to 90 degrees.

i would think that even if the temps reach 110, the tire pressure should only get as high as 37-38. So the other 4-5psi must be due to the heat from tire rotation and braking????? If it is this simple, fine! I'm just trying to learn this stuff and get a better understanding of all the possible factors.

6spMike

The pressure increase is not a problem. The tire is designed so if you start at the recommended cold pressure, you will be fine.

I was trying to provide an explanation of the difference you saw:
1) Simple temperature cannot account for the whole difference. 9 psi divided by .66 psi per 10 degrees equals a 136 degree increase. That means the tire would be 65+136= 201 degrees. You would notice if it was this hot.
Let's say you start at 65 degrees with 35 psi, and the tire increases in temperature to 120 degrees. That's a 55 degree increase. 55 degrees * 0.066 psi per degree is a 3.6 psi increase. So temperature only gets you to about 39 psi.

2) Water vapor in the tire accounts for the rest. Hotter air holds more water vapor, cooler air holds less. If the humidity of the air in the tire is high enough, when the tire cools, the excess water vapor precipitates out as water. It's the same process as rain. Water (a liquid) takes up much less space that water vapor (a gas), so the tire pressure drops. As a dramatic example, try this: boil about a 1/2 cup of water for a minute or two in a sealable metal can, like an old paint thinner can. Take it off the heat and immediately put the cap on. Now the whole can is filled primarily with water vapor. As the can cools, the vapor condenses back to water and the resulting pressure drop results in atmospheric pressure crushing the can. It really crushes the crap out of the can.

If the pressure difference really bothers you, then get rid of the water vapor in the tires.

Is everybody on the same page now? Sheesh.

onsknth

^^^ OK I can agree with you that the air INSIDE the tire has a relative humidity, the point I was making is the humidy of the air in the tire is unaffected by changes in outside air humidity... It simply cannot penetrate the rubber and aluminum of the wheel and the tire, otherwise it wouldn't hold air.

CleanCL

iTrader: (4)

i always do my tire pressure after 10-15 minutes of highway driving. why do you think the manufacturer has the PSI so low, that's setting it at cold. max inflation on tires are generally anywhere from 44psi to 50psi or more. just check your max inflation. i generally fill mine to 70% of max inflation leaving room for a 30% increase.

6spMike

Humidity is misleading; it is how much water vapor is in the air, as a percentage of the total the air could hold at that temperature.
Dewpoint is also a measure of how much water vapor is in the air; it is the temperature of that air at which the humidity is 100%.
If the temperature of the air in the tire is lower than the dewpoint, then the tire pressure drops, because the excess water vapor turns into liquid water.
The total amount of water in the tire never changes.