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Proper Tire Air Inflation Pressure Saves Gas and Prevents Uneven Tire Wear



Does proper inflation really prevent irregular wear?
Remember those old patent medicines that claimed to cure everything from gout to gangrene? We won't claim quite as much, but proper inflation does a lot to produce long, even tire wear.

Why can't we just "fill em" and "forget em"?
Things change. For one thing, even with the best possible seal between beads and wheels and no valve or valve stem leaks, truck tires can lose as much as 2 psi per month (it's about 1 psi for passenger tires) – even if the vehicle is parked the whole time.

How does that happen?
The molecules in air are tiny, and they gradually work their way through the sidewalls of your tires, and escape.

How does this cause irregular wear?
Lots of ways. First, let's remember that tires rotate about 500 times for every mile you travel. In 100,000 miles, every part of their treads gets pressed against the pavement some 50 million times. Since uneven, irregular wear is the result of uneven abrasion, we want tires to press against the pavement the same way – every single time. Having the tire maintain a consistent shape throughout its life helps a lot.

But doesn't the tire's shape change with the load?
Absolutely. When you put load on a tire, you "squash" it against the pavement. But, there's an "ideal" shape for each tire. And, the way to get it is by adjusting inflation pressure. That's why there are load and inflation tables.
If you could see a cross-section of your tires, you'd find that with different loads, when inflation pressure is correctly adjusted for each load, cross section shapes are nearly identical, and footprints are about the same shape and size.

When we unload, are our tires over-inflated?
The cold inflation pressure needs to be correct for the actual load. Technically, you would need to change pressure when you unload, especially if you return empty. Reducing inflation pressure to compensate for reduced load could result in better tire wear and a more comfortable ride. But, you must have the correct inflation pressure for the loaded condition.
Changing inflation pressures for loaded and unloaded situations is not practical. So just choose the appropriate pressure for the loaded condition and maintain that. Never bleed pressure from a hot tire. And be sure that when you adjust pressure, the truck has been parked for 3 to 4 hours, or if it has been sitting overnight, that you've driven it less than 1 mile.

Because inflation controls tire shape, always use the recommended pressure, regardless of the ambient temperature.

That's because pressure changes with heat, isn't it?
Yes. The air inside your tires can get pretty warm. That heat causes it to expand, and pressure goes up.

How much pressure does heat add?
Our tests show that air inside your tires can get up around 160 degrees or more, depending on inflation pressure, road temperature, ambient air temperature and other factors. If the ambient temperature was 70O F when you set pressures originally, pressures could rise 10-15 percent as a result of that much heat.

Wouldn't that make them over-inflated?
Not usually. Load and inflation tables take this temperature increase into account. And besides, the volume of air inside the tire doesn't really change. The heat just makes the air molecules more active.

-2 PSI LOSS PER MONTH
Because air molecules are small enough to diffuse through rubber, inflation pressure inside a truck tire can decrease up to 2 psi per month.
    Before Measuring and Adjusting Inflation Pressures:
  • Allow Tires to Cool 3 to 4 Hours.
  • Drive Vehicle less than 1 Mile.

At what temperature should we measure and adjust pressures?
Again, load and inflation tables are set up so that you should do this when the temperature of the air inside the tires is the same as the outdoor air temperature. This is what is meant by setting pressures "cold."

Even if it's 110 in the shade?
Even if it's 110 in the shade. (And even if it's as low as 10 degrees, for that matter.) Remember, the idea is to keep the shape of the tire correct. You need a certain pressure – depending on the load – to accomplish that, regardless of the temperature.

But what happens if we travel from a cold location to someplace much hotter?
Just as with an empty backhaul, you should adjust inflation when conditions change. If you're going to be spending time in that hot climate, adjust pressures for the new location. Our studies show that a shift of about 20 to 25 degrees in outdoor temperature will result in about a 5 psi shift in inflation pressure. And again, it's probably not practical to make that small an adjustment.

How close do we have to be to the load and inflation tables?
For best results, including slowest tread wear, most uniform wear and optimum fuel economy, you should be right on the pressure specified for the load – in every tire. And matching pressures is especially crucial on dual assemblies.

Why is that?
Because an inflation mismatch greater than 5 psi means that the two tires in a dual assembly are now significantly different in circumference. But, because they're bolted together, they have to cover the same amount of road in a single revolution. So, the larger tire drags the smaller one. Very fast or irregular wear – especially on the tire with less inflation – can be the result. In one test we made, a 5 psi difference created a 5/16" difference in tire circumference.

Two tires with different diameters cannot cover the same distance in the same number of revolutions. Unless they're bolted together. In that case, the smaller tire is dragged by the larger one, which can lead to both rapid and irregular wear.

Is that enough to matter?
In a single mile, this 5/16" difference causes the smaller tire to be dragged 13 feet. In a typical year's usage of about 100,000 miles, that comes out to 246 miles. Again, it doesn't sound like much until you remember that the tire is not rolling an extra 246 miles, it's being dragged. In other words, it's as though you spun the tire against the pavement for 246 miles! At 55 mph, that would be about 4-1/2 hours of wheel-spinning.

Ouch!
You bet! But it makes a good point It's not big things that cause irregular wear, but little things that happen over and over. And incidentally, the same thing applies to matching tire diameters on dual assemblies. Even if the tires are of identical age and model, be sure to match tread depths within 4/32" (equivalent to 8/32" in diameter and 3/4" in circumference). Next time, we'll continue looking at maintenance issues in irregular wear, including more information on mounting, lubrication and tire and wheel balance.

This documentation was provided by Bridgestone Firestone North American Tire, LLC.


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