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Real Answers readers tell us one of
the biggest challenges they face is fighting premature tire removal due to irregular wear.
As we continue to explore how tire selection can promote even wear, we look at some tread
design tactics that can help prevent the initiation and spread of irregular wear,
especially on steer radials.
The
Side Groove™ feature on the new Bridgestone R227 represents
the latest advance in preventing irregular shoulder edge
wear. |
| Why does irregular wear so often start on rib edges?
Irregular wear is caused by irregular abrasion, and irregular
abrasion is caused by uneven pressure against the pavement
in different areas of the tread surface. So the pressure on rib edges is uneven?
It can be. And once you understand why uneven pressure can cause irregular
wear, you’ll see that many different kinds of irregular
wear result from this single cause. What causes uneven pavement pressure?
Let’s start with a very simple example. Imagine a dual assembly,
but that one tire has a bigger diameter than the other.
Clearly, the smaller tire doesn’t press against the
pavement as hard. Does that mean it wears slower?
Quite the contrary. The smaller tire, the one not pressing against
the road as hard, actually wears faster — often much
faster — than the larger tire. Why is that?
Because the smaller tire has a smaller circumference, it should take
more revolutions to cover the same distance as the larger
tire. But because both tires are bolted together, they have
to make an identical number of revolutions to cover a given
stretch of road.
The only way the smaller tire can do that is to slip and skid as it is dragged along by
the larger tire. The big tire wears out the smaller tire, while the smaller tire acts as a
brake on the larger tire. This is one reason dual matching is so important. |
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 along
by the larger one, greatly accelerating the wear on the
smaller tire.
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| What does this have to do with a single steer tire?
A rib tire is like several tires bolted together. If one
or more ribs are smaller than the others, they may wear
much more rapidly than the larger ones, for the same reason
that the smaller tire in the dual assembly did.
The result can be a wear pattern called "rib punch wear," sometimes seen on
multi-rib tires. |
A
multi-rib tire can behave much like several tires bolted
together. Smaller ribs can wear very rapidly, resulting
in a condition called "rib punch wear."
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| How does this relate to rib and shoulder edge wear?
What can happen to a multi-tire assembly or multi-rib tire
can happen to a single rib too. Just as a tire or rib may
not press as hard against the pavement as another, different
parts of ribs may not press against the pavement as hard
as other parts.
The closer you are to the edge of a rib, or to the shoulder of a tire, the less
pressure there tends to be. That can result in more rapid wear at those edges. The
pressure differences may be tiny, but over tens of thousands of miles, tiny differences in
abrasion can cause shoulder edge wear or "river" or "wavy" wear.
Is this where those little ribs and grooves come in?
Yes. One of the first irregular wear-reducing designs is what is called
a Defense Groove™ feature. This groove creates a narrow
rib along the shoulder of the tire. The pressure is less
here, and the rib wears more rapidly than the shoulder rib
beside it.
This increases the pavement pressure at the edge of the shoulder rib, helping to
equalize pressure across the entire rib — just what we need to promote even wear.
And, as the tire wears, the defense "rib" continues to wear more rapidly than
the shoulder rib, keeping the pressure up on the shoulder rib edge for continued irregular
wear protection. In a sense, it "sacrifices" itself to protect the adjacent rib. |
The
Defense Groove™ feature on some Bridgestone steer radials
creates a small rib that wears more rapidly than the shoulder
rib beside it, helping to equalize pressure on the shoulder
rib and thereby prevent shoulder edge wear.
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| The Equalizer RibTM design on the R227 looks similar. Does it do the
same job? Basically, yes. But instead of protecting the
shoulder ribs, the R227 Equalizer Ribs were created to equalize pressure and wear on the
main ribs of the R227 tread design. |
The
Equalizer Rib™ feature of the Bridgestone R227 works
much like Defense Groove designs, but helps protect the
main ribs rather than the shoulder ribs.
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| The R227 doesn’t have the Defense Groove feature. How does it
resist shoulder edge wear?
With a new irregular wear-fighting technique. The shoulder
ribs of the R227 feature Bridgestone’s new Side Groove™
design. This groove is in the side of the shoulder, instead
of in the tread itself.
It helps equalize pressure over the surface of the shoulder rib, but in a different
way: Because it can open and close in response to changing shoulder edge forces, you might
say it acts almost like a shock absorber or load leveler for the shoulder edge.
Is this better?
For some applications. Because the Side Groove structure is positioned
away from the road surface, it doesn’t pick up and
hold small stones—which can cause irregular wear to
start. And, it is far less likely to be torn or damaged
during turns.
Because of its ability to open and close, the Side Groove design does an excellent job
of equalizing shoulder rib pavement pressure, even under some fairly severe side forces.
Will all new tires incorporate this design?
No. As we’ve said before, no single design is best for every application.
Very long distance line haul operators will find that either
a Defense Groove or Side Groove design will probably provide
excellent irregular wear resistance.
Haulers who make some shorter runs may get better results with the Side Groove design
because it copes so well with turning and side forces.
And, operators who make mostly short hauls may find that neither feature is
particularly helpful, and that a tread design like that of the R250 or R250F works best.
And that, after all, is the point of the first letter in our "S.M.A.R.T."
acronym, "S." For even wear, you have to "Select the right tire for the
job." Next time, we’ll look at selecting the right drive tire. |
The
Side Groove™ design of the Bridgestone R227 equalizes
shoulder rib pavement pressure under a wide variety of conditions,
to help prevent shoulder edge wear. Its position on the
shoulder also prevents it from picking up and retaining
stones.
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