<< print this page >>
ask the DOCTOR
High school physics tells us the pressure of a gas inside a container varies, depending on its temperature, and, if the container’s volume can be changed by outside forces, on the outside pressure.

You may even remember the two laws, Charles’ Law and Boyle’s Law, that tell us how much pressures are affected by temperature and outside pressure.

Since we place so much importance on maintaining proper inflation pressures, we often are asked whether temperature and altitude should be considered when setting pressure. Let’s see how much effect these two factors have.


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.

Ambient temperature is the temperature of the air outside the tire.

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.

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.

Contained air temperature is the temperature of the air inside the tire air chamber.

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.

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.

UNDER


OK

OVER
Inflation pressure controls the shape of the tire casing,
a critical factor in controlling heat and performance.

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.

Inflation pressure controls the shape of the tire casing, a critical factor in controlling heat and performance.

While temperature fluctuations can have a significant influence on inflation pressures, frequent checking and adjustment of inflation can cancel any negative effects.

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?

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.

Checking and adjusting cold inflation pressure frequently goes a long way toward eliminating ambient temperature and altitude effects.

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.

Why would altitude affect inflation pressure?

Our atmosphere, the several miles of air above us, exerts a pressure on us all the time. On average, it’s about 14.7 pounds per square inch at sea level.
Why do you specify “at sea level”?

Altitude determines the height of the column of air. At sea level, that column is about as tall as it can be (except in below sea level places like Death Valley). As we go to higher and higher altitudes, the column is shorter, and the pressure on us is less.

Does this affect inflation?

Not very much. In a proposed Recommended Practice (RP233) being considered by the Technology & Maintenance Council (TMC), there’s a chart showing the effect of altitude on inflation pressures over a 5,000 foot range (roughly the rise between New York and Denver).

With 5,000 feet of altitude, inflation pressure varied only about 2 psi. Since many inflation pressure gauges aren’t much more accurate than that, you’d probably have a tough time adjusting for it anyway.

If you check and adjust your cold inflation pressures frequently, you’ll be compensating for altitude differences as well as ambient temperature differences. So no, you don’t need an altimeter either.

The earth's atmosphere, about 500 miles tall, creates an average air pressure of about 15psi at sea level.

 

Back to Table of Contents Go to next article.
<< close >>
  © 2006-2014 Bridgestone Americas Tire Operations, LLC l legal notice