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All
axle positions are not equal
It might be surprising to learn that tires on different axles
make different contributions to overall fuel economy. In one early
Bridgestone test, engineers discovered that changing only trailer
tires to more fuel-efficient designs had a huge effect.
The eight trailer tires represented just 44 percent of the 18
wheel positions, but produced 67 percent of the fuel economy that
was found when all tires were changed to fuel-efficient types.
This would have been startling in itself, but it was even more
surprising when the loads on the various tires were considered.
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Bridgestone
tests used both single trailers-with tandem drive axle
trailers-with sigle drive axle trailers.
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If
you base your calculations solely on axle weight, you’d expect
only about 42 percent of the fuel economy to be attributed to
trailer tires, not the 67 percent found in tests.
Amazingly, many fuel economy calculations continue to use weight
distribution to determine the contribution of axle positions to
tire fuel consumption. In fact, repeated testing shows that the
distribution of fuel efficiency effects is much more dependent
on vehicle configuration, loads and speeds.
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Changing
trailer tires to fuel-efficient types produces a larger
effect than changing tractor tires to fuel-efficient
types. |
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The
actual contribution of individual axle positions to
fuel economy varies with vehicle configuration. |
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Drive
tires contribute far less to fuel economy than their
number or loads would lead you to believe. |
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Testing
modern configurations
More recently, Bridgestone engineers tested position effects on
fuel economy using tractors with setback axles, and with both
single trailers and
“pups.”
Single trailers
With modern tandem drive tractors pulling single trailers, tests
showed that about 52 percent of tire fuel economy was attributable
to trailer tires and about 48 percent to tractor tires.
Again, weight distribution predicted trailer tires would contribute
only 42 percent of the fuel economy effect, drive tires 42 percent,
and steer tires 16 percent. That’s very different from the
52 percent, 31 percent and 17 percent actually found.
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Tractor
and trailer configuration affects both the contribution
of tire rolling resistance to fuel economy and the distribution
of fuel economy effects at the various axle positions.
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And,
for each three percent change in rolling resistance, fuel economy
changed by about one percent.
“Pups”
When a tractor with a single drive axle was pulling two pups,
the trailer tires accounted for fully 75 percent of the fuel economy
effect, with the tractor tires contributing only 25 percent.
Again, axle weight distribution predicted very different results.
In addition, the effect of rolling resistance changes was different.
With pups, it took about a four percent change in tire rolling
resistance to produce a one percent change in overall fuel economy.
Part of the reason for this is that with pups there are two trailer
air gaps and therefore air resistance has a bigger effect on fuel
economy than it does with a single trailer.
Other configurations
Bridgestone also tested a variety of other tractor and trailer
combinations, including triples. Loads and speeds were also varied.
As in other tests, the largest contribution to fuel economy continued
to be made by trailer tires.
Drive axle effects
In all the configurations tested, the contribution of drive tires
to tire fuel economy was significantly less than predicted. Again,
weight distribution did a poor job of predicting effects.
Bridgestone engineers believe that the dynamics of drive axles
are very different from those of essentially free-rolling axles,
like steer and trailer axles, and this may account for part of
the discrepancy.
New tractor
tires or new trailer tires?
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Although
trailer tires tend to produce bigger fuel efficiency
effects than other tires, operational considerations
and return on investment should determine your tire
choices. |
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Only
about half the savings measured in controlled tests
will actually appear in your real world fuel economy
records. |
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In
all the tests, one thing was clear: much of the fuel economy produced
by tires came from trailer tires—and the amount varied considerably,
depending on configuration, load and speed.
Therefore, changing trailer tires to fuel-efficient types can
be expected to produce more benefit for less cost than replacing
all tires. Since trailer tires are generally less expensive than
drives or steers, this makes changing trailer tires an attractive
investment option.
Another way to look at it is to say that if you change only your
tractor tires, you’ll get less than half the benefit of fuel-efficient
tires, even though you’ve replaced more than half of your
tires.
Nevertheless, since tractor tires usually wear out faster than
trailer tires, changing tractor tires to more fuel-efficient types
may produce fuel economy improvements more quickly.
And choosing tires on the basis of fuel efficiency alone
may not be operationally or economically practical. Using less
expensive retreads on drive and trailer positions may outweigh
fuel economy considerations.
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