technically SPEAKING
Why does
“m-i-s-m-a-t-c-h”
spell
“t-r-o-u-b-l-e”
We’ve always preached correct
inflation pressure as a way to
prevent many tire-related problems.
And now, new studies done by
Bridgestone engineers suggest that
matching inflation pressure on duals
is equally important.
Big mismatches and small mismatches
were studied, and as we’ll see, each
condition caused serious problems.
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technically SPEAKING
Why is inflation pressure mismatch on duals
so bad for them?
The tires in a dual pair share the load. And, the load
each carries depends largely on the tire’s diameter. If both
have the same remaining tread depth, diameter depends
on inflation pressure.
If one tire has a lower inflation pressure than the
other, its diameter is smaller. It carries less of the load,
and the other tire carries more.
Is there a bad effect on the larger tire?
Just as the bigger tire tends to drag the smaller one,
there’s a sense in which the smaller tire acts as a slight
“brake” on the bigger one, which could make it wear
faster too.
Just as the larger tire “drags” the smaller one
to match the distance covered by the larger tire, the smaller tire
also exerts a “braking” force on the larger one.
If two otherwise identical tires have different inflation pressures,
they cannot cover the same distance in a single revolution, unless
they’re bolted together. In that case, the bigger tire drags the smaller one,
causing it to have both rapid and irregular wear.
What’s the result?
It depends on how much mismatch there is. Let’s start
with just 5 psi. In one test, a 5-psi mismatch made a
5/16-inch difference in tire circumference.
Doesn’t sound like much, but remember that dual
tires are bolted together, so when one makes a revolution,
the other must also make a revolution.
But, if the circumference of the smaller tire is 5/16inch short, its tread is dragged against the road surface
that amount.
That’s only a little over a quarter of an inch.
Yes, and a tire makes about 500 revolutions a mile. In
100,000 miles, that’s 50 million of those 5/16-inch drags.
And that works out to over 246 miles.
So, it’s as though you spun the smaller tire against
the pavement at highway speed for 246 miles! That creates some seriously rapid wear, and most likely, some very
irregular wear.
Of course, rapid wear on that smaller tire just makes
the mismatch worse, because now the tires will be mismatched in both psi and tread depth.
Plus, irregular wear, once it starts, is rarely “scrubbed
out.” Once it gets a foothold, it tends to continue, even if
someone corrects the inflation.
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So if a small mismatch is bad, a big one is worse?
Maybe much worse. In another test conducted by
Bridgestone engineers, drive and trailer tire pairs were
deliberately grossly mismatched in inflation
pressure. The actual loads on
each tire were measured with
scales, and the mismatched
pairs were run from 770 to
over 2500 miles.
In tests conducted by Bridgestone,
measurements were made to
determine the effect of inflation
pressure on footprint pressure.
While one tire was
inflated to 95 psi, the other
was inflated to just 5, 10
or 20 psi. The inflation
pressures and loads are
summarized in the chart.
LOW TIRE LOW TIRE MATE TIRE MATE TIRE MATE TIRE
PSI
LOAD
PSI
LOAD
OVERLOAD %
5
850
95
10 1050-1350 95
20 1550-1950 95
7650
151
7250-7450 143-147
6550-6950 129-137
©2006, Bridgestone / Firestone North American Tire, LLC • Real Answers, Volume 10, Issue 2
Since the normal load for a 295/75R22.5 tire at
95 psi is 5070 lb, having a grossly underinflated tire
alongside one that normally would be correctly inflated
results in loading that tire to nearly 130 to over 150
percent of its
proper load for
that inflation.
This tire, inflated
to just 10 psi
(by comparison to
95 psi for its mate),
shows extreme
irregular wear, and
is completely worn out,
after only 2504 miles.
What’s the result?
The low tire suffers rapid and, probably, severely irregular
wear. And, that irregular wear doesn’t go away if the
inflation is corrected. Chances are it will only get worse.
On the other side, the tire with the higher inflation
pressure has been severely overloaded, but it may show
no ill effects. Wear may not appear excessive or uneven.
But there may be big problems inside.
What kind of problems?
Running overloaded causes massive heat build-up in
tires. And that leads to all sorts of problems. For example, the belts can actually detach from one another.
And that’s just what happened in one of the tests.
The tire with 95 psi experienced a detachment between
the #2 and #3 belts. That eventually caused it to “throw”
its tread. Remarkably, the tire held air pressure, even
after losing the tread and top two belts.
Such a belt detachment is
usually invisible at first, but the
results are a ruined tire that
can’t even be retreaded.
Heat from excessive overloading caused
this tire to separate between the #2 and
#3 belts. Remarkably, the casing is still
holding pressure. It was mated for just
1527 miles with a tire that ran at 5 psi.
What’s the lesson to be learned from all this?
First, mismatches of dual pairs are bad, regardless of
whether they’re large or small.
Second, “limping in” to a dealer or truckstop with
one “flat” tire can cause big trouble. The flat tire may
suffer irregular wear from which it can never recover.
And, the other tire may be so overloaded that it may
very well suffer internal damage that can cause it to fail
at a later date.
So, if a driver has a flat on one tire of a dual pair,
then limps in, you may very well end up with not one,
but two ruined tires.
And the real trouble is that the second tire may be
a potential flat that looks good now, but is going to fail
catastrophically down the road.
So what do you recommend?
Always keep dual pairs properly inflated and match
inflations as closely as possible.
Then, if a driver experiences a flat in a dual pair, the
best strategy is to pull off the road at once and call for
help. While we recognize that many fleets – and even
some law enforcement people – would rather see the
driver limp in, that’s not going to be good for the tires.
If drivers must continue to a dealer or truckstop,
they should note their mileage when the flat occurred
and when they get the tire fixed.
Then, a skilled tire technician should carefully
examine both tires – inside and out – to make sure
they’re OK for continued service.
DUAL MATCHING
RECOMMENDATIONS:
Match psi
Match RTD within 8/32 or less
In case of a flat, pull over
and call for help.
If a tire is driven underinflated or
overloaded for any distance,
remove from wheel and
inspect thoroughly inside and out
before returning to service.
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