B arry's T ire T ech

This is a series of articles on the technical aspects of tires, their care and usage.

My primary purpose in these articles is to help people understand tires and thereby reduce the risks we all face every day.

..........and since tires is just about the only thing I know about..........

Please drop me a note if you have a topic you want to see:

Barry@BarrysTireTech.com

Tire Inflation Pressure:


For practical purposes, when people talk about inflation pressure, they mean "cold" inflation pressure - that is: the tire and the air inside it are at the same temperature as the surrounding environment. If people want to talk about tire pressures while the vehicle is operating, they will use the words "warm", or "hot", or "build up". Keep this in mind when reading this and any other discussion on inflation pressure.


All gases (Air, Nitrogen, Oxygen, even water vapor - aka steam) closely follow what is called the Ideal Gas Law (PV=nRT) for the range of values that humans experience. And that would include tires. That's not true for vacuum of Deep Space, or inside the Sun with its enormous gravity induced pressure, but we aren't going to talk about that in this article.

For the purposes of this article, that formula says that when the ambient temperature goes up, so does the tire pressure. I talk about that further down.

Please note: Even Nitrogen follows the Ideal Gas Law, just like air does. Many people have the false notion that nitrogen behaves differently than air. It does not! I talk about that more extensively here

To the right is the current US vehicle tire placard - commonly just called the "placard". It lists the original tire size and the inflation pressure specified by the vehicle manufacturer for that size. This version has been mandatory in the US since 2008 for all light vehicles (meaning cars, and trucks under 10,000# GVW, and trailers also under 10,000# GVW). It is supposed to be on the driver's door frame, or the left front corner for a trailer.

You will, however, find some vehicles OVER 10,000# GVW that have placards as well.

Plus other countries don't necessarily use this kind of placard, but it is my understanding that they generally have one of some sort.

To the left is a version of the placard previously required (since about 1971). Its exact format and location on the vehicle was not mandated, so other placards could look a bit different than this one and could be found in a variety of locations including the glove box, driver's door, driver's door frame, other door frames, trunk lid, and the fuel filler door.

Sharp eyed readers will notice that this is the infamous placard for the Ford Explorer that many claim specfies underinflation - but I refute that here:

What if you are not using the tire size indicated on the placard?

Here's the procedure to calculate the new inflation pressure.

  1. Determine the load carrying capacity of the original tires at the specified pressure. Here's a link to a website that will be helpful: TirePressure.org The small boxes are the ones to use - such as "Metric Tire Load Inflation Chart". Pay attention to the letters in front of or behind the tire size. They are important.

  2. Find the chart for the tire size you are using (including the letters in front of or behind the numbers) and match the load carrying capacity.

  3. If the inflation pressure is larger than what's on the placard, use that.

    • If the pressure is lower than what is on the placard, use the placard pressure. (This matches the spring rate of the original tire).

    • If the load is too high for the chart, you can't use that size.

    • If the load is too low for the chart AND you don't want to use the placard pressure, use the lowest value on the chart. (Note: I think the lowest value on those charts is the lowest pressure the tire manufacturers feel will hold the bead in place on the rim - which is why I am making that recommendation.)

Ambient Temperature

Tire pressures are supposed to be set cold (meaning at the ambient temperature). That means the tire has not yet been operated or has cooled down for at least an hour.

DO NOT set tire pressures when the tires are warm or hot (meaning after they have been operated.) There is a special procedure to handle that outlined below.

If you are expecting the ambient temperature to go down significantly (more than 30°F = 17°C), adjust your starting cold pressures as follows:

  • 2% more pressure for every 10°F (6°C) change anticipated.

If the temperature is going to increase, do not follow the above procedure. Leave the pressures as is. A bit of over inflation is OK.

The effect inflation pressure has on tire wear.

Not nearly as much as people think.

In fact the image to the right is largely bad information because there are other things that have more effect. Specifically:

  • Steer tires tend to wear on the shoulders.
  • Drive tires tend to wear in the center.

This effect is stronger than inflation pressure. That means:

  • Rear Wheel Drive cars wear different parts of the tires, but at about the same rate, so more life can be obtained if you do regular tire rotation.
  • Front Wheel Drive cars, where the steer tires and the drive tires are the same, tend to wear tires evenly across the face of the tread - both front and rear - but the wear rates front to rear are significantly different (about 2 1/2 times different!). What is really interesting is that FWD cars kind of prove that the illustration to the right is false, because the rear tires on a FWD ought to wear in the center if it were true - and they don't!
  • All Wheel Drive cars depend on the torque split. Consult the vehicle manufacturer about whether the vehicle biases the split to the front or the rear.
  • 4 Wheel Drive shouldn't be an issue because the 4WD shouldn't be engaged long enough to matter.

    But just to be clear, it isn't that the image to the right is 100% wrong - indeed there is a tendency for improperly inflated tires to wear as indicated, but the image isn't complete in that other things have a stronger effect.

And this brings me to probably the most important thing on this page:

The Pressure Buildup Test

This test is a really good way to find out if the inflation pressure you are using is adequate. Please note: I did not say ideal. The test only tells you if the inflation pressure is not going to cause a structural failure due to heat of operation.

  1. Choose a time when you are going to be traveling at highway speeds: 70 mph (110 kph), not 50 mph (80 kph).
  2. Measure the inflation pressure and the ambient temperature.
    • You can skip measuring the temperature if you are going to conduct the test right away!
  3. Drive at highway speeds for an hour
  4. Measure the inflation pressure.
    • If you didn't conduct the test right away, then you need to adjust the pressure the way I outlined above: 2% for every 10°F (6°C)
    • You want less than a 10% buildup excluding the ambient temperature affect.
    • If you get more than 10%, you need to increase the inflation pressure - by the amount of the buildup. You will need to test again.
Elevation Pressure
Feet Meters psi kPa
Sea Level Sea Level 0 0
1,000 300 0.5 4
1,640 500 0.9 6
2,000 610 1.0 7
3,000 910 1.5 10
3,280 1,000 1.7 11
4,000 1,220 2.0 14
4,920 1,500 2.4 17
5,000 1,620 2.5 17
6,000 1,830 2.9 20
6,560 2,000 3.2 22
7,000 2,130 3.4 23
8,200 2,440 3.8 26
8,200 2,500 3.9 27
9,000 2,740 4.2 29
9,840 3,000 4.5 31
10,000 3.050 4.6 32

Elevation:

To the left is a chart showing the average change in atmospheric pressure by elevation. The way to interpret that chart is if you are going higher in elevation, the pressure inside the tire is going to increase by the difference in pressures listed for those 2 elevations.

For practical matters this means that going up in elevation should not require a tire pressure adjustment because a little too much pressure is OK.

And if you are going down in elevation, it's more important for low pressure tires (like cars and motorcycles), to make an adjustment BEFOREHAND, than it is for high pressure tires (trucks and trailers.)

Specifically, the difference in pressure dropping from 5,000 feet to sea level is 2.5 psi (17 kPa), so a 30 psi (205 kPa) tire would drop to 27.5 psi (190 kPa), where a 65 psi (450 kPa) tire drops to 62.5 (430 kPa, not nearly as bad.)

One way to do this is to set the pressure before the climb up, and not change it until you get back down. (except when you think there has been a leak!)

And you can tell by the chart that unless you are going to do some serious elevation change, you don't really need to change the inflation pressure.

How to add pressure to a hot (warm, builtup) tire:

If you have to drive to a place to inflate your tires, measure the pressures when they are cold (BEFORE you start). If you are like me, you'll need to write down how much more you want to add.

When you get there, measure the pressures and add what you wrote down to the tire.

If you didn't measure the pressures before you started, add enough to get to the desired cold pressure PLUS 10%. That ought to put you slightly over. Adjust later.



The Chalk Test

I am not a fan of the chalk test. I think there is too much room for error - and an error in the "Too Low" direction can be disasterous.

So allow me to describe the chalk test.

  1. Carefully set the tire inflation pressures.
  2. Rub chalk across the face of the tread. (Photo to the left shows an exaggerated version.)
  3. Drive slowly forward about 100 feet
  4. Look at the chalk mark:
    • According to the theory (that I don't agree with), a properly inflated tire will have evenly worn off the chalk across the face of the tread.
    • Too much wear in the center means the pressure is too high.
    • Too much chalk worn off in the shoulders means the inflation pressure is too low
    • (And this part I agree with) If the chalk is not evenly worn off inside to outside, there is an alignment issue.

Please note: I think the lower photo is NOT the result of a real test. I think the chalk was just erased, like a blackboard. The clue is that chalk was not erased in a more or less circimferetially straight line, but rather the inner ribs were not erased, but the outer ribs were not.

The image to the right is one of the reasons I am not a fan.

A tire with a footprint that looks like this would indicate that the pressure is too high (perhaps, way too high!)

But it isn't!!

So lowering the pressure might cause the tire to be underinflated and could fail - catastrophically. Very Dangerous!

So why is this footprint so bad?

It might have been done that way on purpose. Sometimes for cost reasons, and sometimes for rolling resistance reasons (less rubber), the tire designer will purposely design a tire that way. But sometimes, it's just something that isn't checked. Nevertheless, the inflation pressure is supposed to be set by the load on the tire, not the wear or the footprint pressure!

But if you are going to set your pressures using the chalk method anyway: Please followup with the "Pressure Buildup Test" (above). If they disagree, I am much more confident in the pressure buildup test. You should be, too.



Here's another example:

Notice how the foorptint is full width regardless of the load or inflation pressure. That means that no matter what inflation pressure you use, the chalk test will come out the same.

Tire Tread Temperature (Pyrometer) Test

This is another test I don't favor - and for pretty much the same reason.

However this test uses a pyrometer to measure the tread temperature - and you deal with the results just like in the Chalk Test.

This test is used to set up race cars (which is OK!), but shouldn't be used for street tires because race tires have a relatively short life and their heat cycles are short - where street tires can be at operating temperatures for hours. And heat is the tire killer!

Again, the Pressure Buildup Test is your friend.


Barry's Tire Tech - Main Page