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The Tire Doctor Responds: Some of
today's truck tires are marked as being unidirectional. An arrow may appear on the
sidewall- as with the Bridgestone R226 and R227 radials-indicating which direction the
tire should normally turn. In fact, it's the tread pattern that is unidirectional, not the
casing.
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Why Unidirectional Anyway?
Unidirectional tread patterns have been common on high performance passenger car tires for
years. Recently, Bridgestone originated the unidirectional concept for truck steer radials
as well.
The idea is that by designing the tread to rotate in just one
direction, certain characteristics, like resistance to irregular wear, wet weather
handling or road feel can be enhanced.
What Happens If I Change
Directions?
An arrow is molded on the sidewall of the R227 and R226 unidirectional radial truck tires
and it indicates Bridgestone's recommended direction of rotation.
When that arrow is at the top of the tire, it should point toward
the front of the truck, as long as the original tread is being used. If you discover that
you have a unidirectional radial that is rotating in the wrong direction, it should be
turned around so that the arrow is pointing in the correct direction.
Improved irregular wear resistance is the primary reason for
unidirectional tread designs. Running them in the opposite direction could result in some
reduction in irregular wear resistance.
If you rotate steer tires from side to side-in order to equalize
wear-you should demount, turn around, and remount the tires before swapping sides.
This will keep the tires rotating in the proper direction for
optimum performance. Simply swapping wheel positions without demounting could result in a
reduction in the tire's resistance to irregular wear.
Beneath the unidirectional tread pattern, the casing
-like all Bridgestone truck radial casings-is not uni-
directional. It is designed to perform equally well regardless of the direction of
rotation.
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Belts are Non-Directional.
The belts of modern truck tires are designed to be at
different angles for strength, to provide cornering power and for casing protection. If
you could view the belt package from above (looking through the tread rubber) you would
see one of the belts angled sharply up to the right. Turn the tire around 180 degrees and
that belt will still be angled sharply up to the right. In this sense the belt package is
directionally symmetrical.
Once The Tread Is Gone,
Who Knows?
We also know that when a tire is retreaded, no one attempts to find out which direction
the tire had been rotated. Nor do your retreads come back with marks
indicating which way they should turn.
Of course, that's because, as we've said, the casing, even on a
unidirectional radial, is not unidirectional.
If It Doesn't Hurt, Is It
Ever Good?
Sometimes, during the life of a tire tread, it is an excellent idea to change the
direction it rotates. With certain types of irregular wear, changing the direction of
rotation can counteract wear patterns that have developed.
This is especially true on open shoulder design drive axle tires.
They can develop what is called "heel and toe wear", resulting from the way the
blocks make contact, then leave the surface of the road. In this type of irregular wear,
the tread elements wear at an angle which is dependent on the direction of rotation.
By reversing the direction of the tire's rotation, the tread
elements are worn at the opposite angle, which tends to equalize the overall wear.
Look for more in-depth information on "heel and toe
wear" in future issues. |
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