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I've been wondering.

I have about 10 minute drive to the closest gas station (for air pressure).
So by the time I get to the gas station, the air inside the tires already had a chance to be compressed and get warm, right?

That means by the time i'm near the air compressor the actual PSI readings from my tires will not be accurate?

If I drive and the tires are warm/hot (on a hot day), I set the air pressure unit in the gas station to XX psi and fill up, will that be accurate?
When are you guys filling up air and checking to make sure you have the right air pressure?
 

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it's best to check tire pressue cold, b/c when it's hot it isn't accurate b/c each tire heats up differently due to road surface, type of driving, ect.

matching "hot Psi's" to the T doesn't make much sense.

if they are within you desired range your fine.

now while autoxing i'm anal about pressures and will check them after every run to see how much they've increased and how much i need to add or subtract depending on the track condition and the heat of my tires.
 

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It depends... for the street, I just set them cold and don't concern myself with hot pressures, because it's rare that I work my tires hard enough on the street for it to matter.

At the track, all I really care about are the hot pressures. As much as 1psi off in one tire can really affect how the car handles. When I'm checking them I'll do a few hot laps, pit in, quickly jump out and check my pressures, and if they aren't where I want them I'll adjust as necessary.

The last time I was at the track I did this and I noticed my rear tires were way too high. So I lowered each rear by 2psi. This helped so much that my car rotated so nicely that I was putting even more heat in the rear, so my car gradually went back to understeer as the tires warmed up. I pulled in, and the driver's side rear tire was 1psi too high. This made sense because there were more right hand turns than left hand turns. I lowered it, went back out on the track, and everything was good. The car drove beautifully.

On a car with a mostly stock suspension tweaking the tire pressures can go a long way towards helping you get the handling balance you want. If you have a highly modified suspension, it's less important, but you still need to check it after every other change you make to ensure that they stay where you want them to. For example... you could tweak your damper settings in a way that pushes the car more towards oversteer, and as a result you put more heat into the rear tires which increases the pressures and pushes the car back to understeer, negating the damper setting adjustments you made.
 

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Funny - I just read up on that last night on tirerack.com

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=73&currentpage=1

Your tires support the weight of your vehicle, right? Well they don't! It's the air pressure inside them that actually supports the weight. Maintaining sufficient air pressure is required if your tires are to provide all of the handling, traction and durability of which they are capable.

However, you can't set tire pressure...and then forget about it! Tire pressure has to be checked periodically to assure that the influences of time; changes in ambient temperatures or that a small tread puncture has caused it to change.

The tire pressure recommended in your vehicle's owner's manual or tire information placard is the vehicle's recommended cold tire inflation pressure. This means that it should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun's radiant heat affects it.

Since air is a gas, it expands when heated and contracts when cooled. In most parts of North America, this makes fall and early winter months the most critical times to check inflation pressures...days are getting shorter...ambient temperatures are getting colder...and your tires' inflation pressure is going down!

The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

In most parts of North America, the difference between average summer and winter temperatures is about -50° Fahrenheit...which results in a potential loss of about 5 psi as winter's temperatures set in. And a 5 psi loss is enough to sacrifice handling, traction, and durability!

Additionally, the difference between cold nighttime temperatures and hot daytime temperatures in most parts of the country is about 20° Fahrenheit. This means that after setting tire pressures first thing in the morning, the vehicle's tire pressures will be almost 2 psi higher when measured in the afternoon (if the vehicle was parked in the shade). While that is expected, the problem is when you set your vehicle's tire pressures in the heat of the day, their cold pressures will probably be 2 psi low the following morning.

And finally, if the vehicle is parked in the sun, the sun's radiant heat will artificially and temporarily increase tire pressures.

We put some of these theories to the test at The Tire Rack. First, we mounted two tires on wheels. We let them sit overnight to equalize and stabilize their temperatures and pressures. The following morning we set them both to 35 psi. One tire and wheel was placed in the shade while the other was placed directly in the sun. We then monitored the ambient temperatures, tire temperatures and tire pressures through the day. As the day's temperatures went from 67° to 85° Fahrenheit, the tire that was kept in the shade went from our starting pressure of 35 psi to a high of 36.5 psi. The tire that was placed in the sun and subject to the increase in ambient temperature plus the sun's radiant heat went from our starting pressure of 35 psi to a high of 40 psi. In both cases, if we had set our tire pressures in the afternoon under the conditions of our evaluation, they would have been between 2 and 5 psi low the following morning.

Next we evaluated the affects of heat generated by the tire's flexing during use. We tried to eliminate the variable conditions we might encounter on the road by conducting this test using our "competition tire heat cycling service" that rolls the tires under load against the machine's rollers to simulate real world driving. We monitored the changes in tire pressure in 5-minute intervals. The test tires were inflated to 15 psi, 20 psi, 25 psi and 30 psi. Running them all under the same load, the air pressure in all of the tires went up about 1 psi during every 5 minutes of use for the first 20 minutes of operation. Then the air pressures stabilized, typically gaining no more than 1 psi of additional pressure during the next 20 minutes. This means that even a short drive to inflate your tires will result in tires that will probably be under-inflated by a few psi the following morning.

Add all of these together, and you can understand why the conditions in which you set your vehicle's tire pressures are almost as important as the fact that you do set it.

It's important to remember that your vehicle's recommended tire pressure is its cold tire inflation pressure. It should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun's radiant heat affects it.

And by the way, if you live in the North and park in an attached or heated garage you will lose pressure when you leave its warmth and venture into the real world outside during winter. Add 1 psi cold pressure tire pressure to compensate for each 10° Fahrenheit temperature difference between the temperature in the garage and outside.
 

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did you just copy and paste that?
 

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T3h_Clap said:
did you just copy and paste that?
Looks like it to me.

I did a DE at Putnam Park this previous weekend, and I noticed (as Mykl said) as large as a 5 psi between the right and left tires. :eek: I had no clue they would vary that much.
 

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get a pyrometer and check to see the temp diff from inside and outside of the tires.

it's scary how much difference there is on a camber challenged car.
 

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...but but.. it has less water vapor.. and it magically gets you better gas mileage. The molecules are bigger!!1
 

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Enchanter said:
This is not true. Nitrogen abides by the laws of therodynamics like everything else does.[/QUOTE


Inflating tires with nitrogen eliminates many problems you get with using compressed air.
The oxygen and water vapor in compressed air can bleed through rubber and lose pressure
over time. Expand and contract with temperature changes. Hold heat generated from
road friction, Oxidize rim and rubber surfaces ,and supports combustion.

Tires inflated with nitrogen , require less frequent top offs, operate at more stable
pressure, run cooler and hold less heat, improve tire casting life ,and reduce rim corrosion.

So if you want to elliminate the problems of compressed air put nitrogen
in your tires.

Nuff said..
 

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railroadsti said:
Inflating tires with nitrogen eliminates many problems you get with using compressed air.
The oxygen and water vapor in compressed air can bleed through rubber and lose pressure
over time. Expand and contract with temperature changes. Hold heat generated from
road friction, Oxidize rim and rubber surfaces ,and supports combustion.

Tires inflated with nitrogen , require less frequent top offs, operate at more stable
pressure, run cooler and hold less heat, improve tire casting life ,and reduce rim corrosion.

So if you want to elliminate the problems of compressed air put nitrogen
in your tires.
That's fine, but you're missing something here... nitrogen isn't exactly convenient, and many of us frequently adjust our tire pressures to fine tune the handling characteristics of our car.
 

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railroadsti said:
Enchanter said:
This is not true. Nitrogen abides by the laws of therodynamics like everything else does.

Inflating tires with nitrogen eliminates many problems you get with using compressed air.
The oxygen and water vapor in compressed air can bleed through rubber and lose pressure
over time. Expand and contract with temperature changes. Hold heat generated from
road friction, Oxidize rim and rubber surfaces ,and supports combustion.

Tires inflated with nitrogen , require less frequent top offs, operate at more stable
pressure, run cooler and hold less heat, improve tire casting life ,and reduce rim corrosion.

So if you want to elliminate the problems of compressed air put nitrogen
in your tires.

Nuff said..

Tires filled with N2 will heat up. This temperature change will cause the pressure to change, it will also cause the tire to expand and contract with these temperature changes.

Remember, compressed 'air' is 70% N2.

Testing has shown that N2 tends to leak out more slowly than 'air', but the rest of the benefits can be attained by using clean dry air.

Nitrogen isn't some magical gas, physics still apply to N2 just as it does to everything else.



PS:What does " 'Nuff said " mean anyway?
 

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Mykl , cant help you there guess its something your gonna have
to do.


Enchanter , read the first line of post #13.
 

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railroadsti said:
Mykl , cant help you there guess its something your gonna have
to do.


Enchanter , read the first line of post #13.
Just saying that nitrogen is not yet a good option for most of us.
 

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railroadsti said:
Enchanter , read the first line of post #13.
I totally agree with the first sentance. It's the rest of the paragraph that implies that N2 doesn't do the rest.

Now that I reread your post, I can twist it to see that the entire paragraph was intended to be speaking of moisture in 'air'. The grammar and punctuation you used, threw me off.
 
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