Maintaining Tire Pressure

[MaggieMay]

Chicken and egg question: I have a spiffy new tire gauge to help maintain my tire pressure which I know I should measure when the tires are cold. However, I'm roughly 2 miles to the nearest source of compressed air/gas station. So by the time I get there I imagine the tires are pretty warm - too warm? How accurate will the pressure reading be?

Do I have to have a big huge, heavy, noisey air compressor that I haul out of the basement to top off my tires every week or so? (Answer = No)

Any advice? -Thanks!

[spiderman]

Pressure does change with temp. But not that much. They have portable compressors that run out of our 12v outlet in the car.
I check my tire pressure twice a year when I swap out the winter tires.
Peter

[LRKingII]

Check pressure at home and at station and just figure the difference. 2 miles prolly wont raise it much at all.

[timberwolf]

I use a foot operated pump to add air to my tires, it doesn't make much noise, and as it doesn't require many pumps of the foot to add a few psi, I don't make much out of breath noise either.

[a priori]

Originally Posted by MaggieMay

Do I have to have a big huge, heavy, noisey air compressor that I haul out of the basement to top off my tires every week or so? (Answer = No)

No!! How about this:

Originally Posted by MaggieMay

Any advice? -Thanks!

Yep! Here's a link to the Sears site for a 12V compressor with a digital gauge. I was able to buy it on a great sale for about $20. Retail is around $34, I believe. It is a great little product, and I use it ALL the time. It also is very handy when helping people who have low/flat tires.

[wvgasguy]

General FYI, though the example does not use numbers reasonably to what we are seeing nornally. I overfill by a few psig at the gas station and then adjust in the morning for where I want them set.



Ambient temperature is the temperature of the air outside the tire.How does temperature affect inflation pressure?

First, let's define "ambient" and "contained" air temperatures. Ambient air temperature is the temperature of the air outside the tire (also known as the "outdoor" temperature) while "contained" air temperature is the temperature of the air inside the tire's air chamber.

When a truck has been parked for about 3 hours, these two become pretty much identical. We often use the term "cold" inflation pressure, but what we really mean is the pressure when the contained air temperature is the same as the ambient temperature.
"Contained" air temperature is the temperature of the air inside the tire air chamber.The air inside the tire heats up as the tire turns, right?

Absolutely. If you've driven more than about a mile, friction with the road, flexing of the tire casing and flexing of the tread begin to heat up the air inside the tire.

How hot does it get?

It varies a bit with road and ambient temperature, but if inflation is correct for the load, contained air temperature probably settles in somewhere around 50 to 60 degrees Fahrenheit above ambient.

What do you mean by "settles in"?

After you've been running for a while, the contained air temperature becomes more dependent on inflation, load and speed than on outdoor or ambient temperature.
Inflation pressure controls the shape of the tire casing, a critical factor in controlling heat and performance.So if the temperature outdoors changes, it has no effect?

It has an effect, but it's much smaller than the effect of the mechanical friction and flexing.

Then why worry about temperature at all?

It's the air inside the tire that supports the load. Besides supporting the load, the purpose of correct inflation is to keep the tire the right shape, controlling the amount of flexing.

The chart gives you an idea of how inflation pressure varies with contained air temperature. If you inflate a tire to 100 psi at 70 degrees, it might rise to 110 to 115 psi at normal operating temperature.

Isn't that high?

Not really. Tire designers take that into consideration when they design the tire.

But what if we set inflation pressure when it's hot, then drive to where it's cold?

Let's take an extreme case: Let's say you start out in Las Vegas, it's 100 degrees, and you set tires to 100 psi. Then, you drive to Reno, where the temperature is 0. After the truck has been parked for 3 hours, you might find your inflation pressure is now at about 85 psi.

What should we do then?

You should add air to bring the tires back up to 100 psi. If you do that, then drive back to Las Vegas, and if it's still 100 degrees there, checking your tires again — after a 3-hour cool-down — you may find they're at about 122 psi.

Again, adjust to the correct inflation pressure when the tires are "cold." That's because what you're adjusting with inflation pressure is the shape of the tires. That doesn't have as much to do with temperature.
While temperature fluctuations can have a significant influence on inflation pressures, frequent checking and adjustment of inflation can cancel any negative effects.Why so much emphasis on "cold" inflation pressure?

Remember, we said the contained air temperature could be around 50 to 60 degrees above ambient after you've been running for a while. If you started out at 100 psi at 70 degrees, the inflation could now be at about 106 psi.

If you stopped and adjusted inflation pressure without waiting for the tires to cool, you'd be setting inflation pressure with a contained air temperature of about 120-130 degrees.

When the tires cool down to 70 again, the pressure will go down as well, and you could easily find your tires are now almost 10 psi underinflated.

So always check and adjust inflation pressure when tires are "cold," that is, after the vehicle has been parked about 3 hours, and before it's been driven any more than a mile.

Should temperature fluctuation be a big concern for us?
Checking and adjusting cold inflation pressure frequently goes a long way toward eliminating ambient temperature and altitude effects.If you frequently travel from a very hot location to another where it's very cold, you should check and adjust inflation more frequently. If you're doing that, you'll correct for these variations automatically.

So we don't need a thermometer?

You don't. Just check and adjust inflation pressures often, making sure you do it when the tires are "cold," and you should be fine.

[jay_man2]

TMI! TMI!

[cairo94507]

I check my tire pressure monthly; I rotate my tires every 5K when I also do my oil and filter change. My car has 116K on it now and has never skipped a beat. I get almost 50K miles out of a set of tires, which when replaced are worn evenly. Tire pressure is one of the single biggest things you can monitor to keep your car handling well and safe.

[richard schumacher]

Measure it at home to determine how many pounds to add to each tire, then drive to the service station and add that much no matter what the initial readings are when you get there.

[OZ132]

Originally Posted by wvgasguy

General FYI, though the example does not use numbers reasonably to what we are seeing nornally. I overfill by a few psig at the gas station and then adjust in the morning for where I want them set.



Ambient temperature is the temperature of the air outside the tire.How does temperature affect inflation pressure?

First, let's define "ambient" and "contained" air temperatures. Ambient air temperature is the temperature of the air outside the tire (also known as the "outdoor" temperature) while "contained" air temperature is the temperature of the air inside the tire's air chamber.

When a truck has been parked for about 3 hours, these two become pretty much identical. We often use the term "cold" inflation pressure, but what we really mean is the pressure when the contained air temperature is the same as the ambient temperature.
"Contained" air temperature is the temperature of the air inside the tire air chamber.The air inside the tire heats up as the tire turns, right?

Absolutely. If you've driven more than about a mile, friction with the road, flexing of the tire casing and flexing of the tread begin to heat up the air inside the tire.

How hot does it get?

It varies a bit with road and ambient temperature, but if inflation is correct for the load, contained air temperature probably settles in somewhere around 50 to 60 degrees Fahrenheit above ambient.

What do you mean by "settles in"?

After you've been running for a while, the contained air temperature becomes more dependent on inflation, load and speed than on outdoor or ambient temperature.
Inflation pressure controls the shape of the tire casing, a critical factor in controlling heat and performance.So if the temperature outdoors changes, it has no effect?

It has an effect, but it's much smaller than the effect of the mechanical friction and flexing.

Then why worry about temperature at all?

It's the air inside the tire that supports the load. Besides supporting the load, the purpose of correct inflation is to keep the tire the right shape, controlling the amount of flexing.

The chart gives you an idea of how inflation pressure varies with contained air temperature. If you inflate a tire to 100 psi at 70 degrees, it might rise to 110 to 115 psi at normal operating temperature.

Isn't that high?

Not really. Tire designers take that into consideration when they design the tire.

But what if we set inflation pressure when it's hot, then drive to where it's cold?

Let's take an extreme case: Let's say you start out in Las Vegas, it's 100 degrees, and you set tires to 100 psi. Then, you drive to Reno, where the temperature is 0. After the truck has been parked for 3 hours, you might find your inflation pressure is now at about 85 psi.

What should we do then?

You should add air to bring the tires back up to 100 psi. If you do that, then drive back to Las Vegas, and if it's still 100 degrees there, checking your tires again — after a 3-hour cool-down — you may find they're at about 122 psi.

Again, adjust to the correct inflation pressure when the tires are "cold." That's because what you're adjusting with inflation pressure is the shape of the tires. That doesn't have as much to do with temperature.
While temperature fluctuations can have a significant influence on inflation pressures, frequent checking and adjustment of inflation can cancel any negative effects.Why so much emphasis on "cold" inflation pressure?

Remember, we said the contained air temperature could be around 50 to 60 degrees above ambient after you've been running for a while. If you started out at 100 psi at 70 degrees, the inflation could now be at about 106 psi.

If you stopped and adjusted inflation pressure without waiting for the tires to cool, you'd be setting inflation pressure with a contained air temperature of about 120-130 degrees.

When the tires cool down to 70 again, the pressure will go down as well, and you could easily find your tires are now almost 10 psi underinflated.

So always check and adjust inflation pressure when tires are "cold," that is, after the vehicle has been parked about 3 hours, and before it's been driven any more than a mile.

Should temperature fluctuation be a big concern for us?
Checking and adjusting cold inflation pressure frequently goes a long way toward eliminating ambient temperature and altitude effects.If you frequently travel from a very hot location to another where it's very cold, you should check and adjust inflation more frequently. If you're doing that, you'll correct for these variations automatically.

So we don't need a thermometer?

You don't. Just check and adjust inflation pressures often, making sure you do it when the tires are "cold," and you should be fine.

You're NOT suggesting 100 PSI as a normal baseline tire pressure--ARE you???