Bridgestone South Africa

How Can We Blow Away Irregular Wear?

Good maintenance can go a long way to reducing the incidence of irregular wear.
The easiest, least expensive and most cost effective maintenance you can perform on your tyres is to keep them properly inflated. We will analyse the "whys" and "hows" in this section.

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 tyre 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 tyres can lose as much as 30 kpa per month (its about 7 kpa for passenger tyres) - 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 tyres, and escape.

How does this cause irregular wear?

Lots of ways. First, lets remember that tyres rotate roughly 300 times for every kilometre you travel. In 100,000 kilometres, every part of their tread gets pressed against the road surface some 30 million times.

Since uneven, irregular wear is the result of uneven abrasion, we want tyres to press against the road surface the same way - every single time. Having the tyre maintain a consistent shape throughout its life helps a lot.

But doesn't the tyres shape change with the load?

Absolutely. When you put load on a tyre, you "squash" it against the road surface. But, there's an "ideal" shape for each tyre. 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 tyres, 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.

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Proper inflation gives tyres the right shape for slow, even wear and optimum fuel economy

When we unload, are our tyres 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 tyre 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 tyre. 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 2 kilometres.

That's because pressure changes with heat, isn't it

Yes. The air inside your tyres can get pretty warm. That heat causes it to expand, and pressure goes up.

How much?

Our tests show that air inside your tyres can get up around 70 degrees or more, depending on inflation pressure, road temperature, ambient air temperature and other factors. If the ambient temperature was 21 degrees Celsius when you set pressures originally, pressures could rise 10-15 percent as a result of that much heat.

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

Wouldn't that make them over-inflated?

Not usually. Load and inflation tablestake this temperature increase into account. And besides, the volume of air inside the tyre doesn't really change. The heat just makes the air molecules more active.

At what temperature should we measure and adjust pressures?

Again, load and inflation tablesare set up so that you should do this when the temperature of the air inside the tyres is the same as the outdoor air temperature. This is what is meant by setting pressures "cold".

Even if it's 43 degrees in the shade?

Even if it's 43 in the shade. (And even if it's as low as -5 degrees, for that matter). Remember, the idea is to keep the shape of the tyre correct. You need a certain pressure - depending on the load - to accomplish that, regardless of the temperature.

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

What happens if we travel from a cold location to someplace much hotter?

Just as with an empty return trip, 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 3 to 5 degrees in outdoor temperature will result in about a 30kpa 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 tyre.

And matching pressures is especially crucial on dual assemblies.

Why is that?

Because an inflation mismatch greater than 30kpa means that the two tyres 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 tyre drags the smaller one. Very fast or irregular wear - especially on the tyre with less inflation - can be the result. In one test we conducted, a 30kpa difference in tyre circumference.

Two tyres with different diameters cannot cover the same distance in the same number of
revolutions, unless they're bolted together In that case, the smaller tyre is dragged along by the larger one, greatly accelerating the wear on the smaller tyre.

Is that enough to matter?

In a single kilometre, this 21mm difference causes the smaller tyre to be dragged 3.9 metres. In a typical year's usage of about 100 000 kilometres, that comes out to 390 kilometres. Again, it doesn't sound like much until you remember that the tyre is not rolling an extra 390 kilometres, it's being dragged. In other words, it's as though you spun the tyre against the road surface for 390 kilometres! At 80 km/h, that would be nearly 5 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 tyre diameters on dual assemblies. Even, if the tyres are of identical age and model, be sure to match tread depths within.

Bridgestone South Africa

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