How does temperature
affect inflation pressure?
First, lets 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
the tires 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 youve 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
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
After youve 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 its much smaller than the effect of
the mechanical friction and flexing.
|Inflation pressure controls
the shape of the tire casing,
a critical factor in controlling heat and performance.
Then why worry about temperature
Its 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.
Not really. Tire designers take that into consideration when they
design the tire. But what if we set inflation pressure when its
hot, then drive to where its cold?
Lets take an extreme case: Lets say you start out in
Las Vegas, its 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 its still 100
degrees there, checking your tires again after a 3-hour cool-down
you may find theyre at about 122 psi.
Again, adjust to the correct inflation pressure when the tires are
cold. Thats because what youre adjusting
with inflation pressure is the shape of the tires. That doesnt
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 youve been running for
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, youd 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
So always check and adjust inflation pressure when tires are cold,
that is, after the vehicle has been parked about 3 hours, and before
its 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
its very cold, you should check and adjust inflation more
frequently. If youre doing that, youll correct for these
|Checking and adjusting cold
inflation pressure frequently goes a long way toward eliminating
ambient temperature and altitude effects.
So we dont
need a thermometer?
You dont. 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
Our atmosphere, the several miles of air above us, exerts a pressure
on us all the time. On average, its 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), theres
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 arent much more
accurate than that, youd probably have a tough time adjusting
for it anyway.
If you check and adjust your cold inflation pressures
frequently, youll be compensating for altitude differences
as well as ambient temperature differences. So no, you dont
need an altimeter either.
|The earth's atmosphere, about
500 miles tall, creates an average air pressure of about
15psi at sea level.