ROCHESTER HILLS, Mich. (Jan. 19, 2004) — Misalignment is the second leading cause of lost tread mileage after improper inflation.
This is probably because most truckers don't understand the importance of total vehicle alignment (steer as well as drive and trailer axle alignment), nor do they know how to read their tires' unusual wear.
Most people focus on only the steer axle when steer tires are wearing funny. But a large percentage of the time, the misalignment problem can be located in the tractor drive axles as well as the trailer axles.
The science of tire wear
Now, understanding irregular tire wear is not rocket science. But it is tire science, which takes a person with above-average intelligence, skills and tire knowledge to understand. That's why we in the tire business are very special people. We hold the key to unlocking tire mysteries for the average commercial tire user.
Irregular wear patterns are caused by abrasion that is concentrated in one area of the tire tread instead of across the entire footprint. Side forces created by turning, off tracking, incorrect toe, etc. can drag or slip the tire across the pavement and concentrate wear on one side of the tire or one side of the ribs.
Steer tires tell most of the story when it comes to alignment. Steer tires are always affected when the steer axle is out of alignment as well as when the rear axles are pushing the vehicle in a direction other than straight.
You can diagnose the cause of misalignment-induced, irregular wear by feeling the tread with your palm and fingertips. To do this, lightly move the palm of your hand around the tire tread circumferentially. Then move across the tread laterally, sensing feather edges and differences in tread element depths.
Circumferential readings allow you to determine heel/toe wear and cupping/scallop wear, which are not alignment-related conditions. Lateral readings tell you which way the tread is feather edged. If the feathered edges are in the same direction, the misalignment is caused by torque thrust and if they are in the opposite direction, either toe or camber causes the condition.
Toe setting is the first and foremost factor that affects steer tire wear. Toe refers to the inward or outward pointing of the wheels when viewed from the top of the vehicle. It is the difference in distance between the centers of the front and the rear of the wheels as seen in a top view of the truck.
Toe-in exists when the wheels are closer together in the front than in the rear. A little toe-in is necessary to compensate for the tendency of the wheels to deflect rearward as the vehicle is loaded and while it is in motion. Due to this tendency, the wheels of a vehicle with properly adjusted toe-in are traveling straight ahead when the vehicle itself is traveling straight ahead. This results in directional stability and minimum tire wear.
With toe-out, the wheels are closer together in the rear. While a little positive toe is necessary for improved tread life, negative toe accelerates tread wear.
Toe is the most critical alignment setting. An eighth-inch of toe adjustment error results in 11.5 feet per mile of sideslip. Excessive toe-in results in full shoulder wear—excessive wear across one shoulder—on the outside shoulders on both tires. Excessive toe-out results in full shoulder wear on the inside shoulders of both tires.
If feather wear is evident where the tread appears to be worn high to low on each rib across the tread face, a severe toe condition or worn steering components are to blame. When running your hand across the tread, an easy rhyme to remember is, “Smooth in-Toe-in” and “Smooth out-Toe-out.” (You won't hear rocket scientists singing this rhyme, but if it helps you, go with it.)
The second most common cause of irregular wear is rear axle alignment. On a properly aligned vehicle, all the axles should be perpendicular to the frame and parallel to each other, and all the wheels should track the front wheels.
Tandem drive axles that are not parallel to each other and axles that are not perpendicular to the chassis centerline are the two primary misalignment conditions that affect vehicle tracking and have a definite effect on steer tire wear.
Rear tandem axles that are not perpendicular or “square” to the frame but are parallel to each other create a “thrust angle,” which tends to push the vehicle off course. The driver feels the vehicle pulling in the direction the drive axles are angled and must steer in the opposite direction to keep the truck traveling in a straight line.
The steer axle tires are constantly subjected to “scrubbing” or side forces as they continually correct the direction of travel of the truck, which results in fast and irregular wear.
This condition on trailers is known as “dog tracking” and is quite visible as you follow the vehicle down the highway since the trailer appears to be traveling at an angle to the tractor. The driver feels the vehicle “wandering” and must make constant steering corrections to keep driving straight ahead.
Tires on a trailer with this condition are dragged sideways a few feet for every mile of operation. In severe conditions, this can result in dragging the trailer tires sideways for thousands of miles for every 100,000 miles the vehicle runs.
Tandem skew, or scrub angle, occurs when tandem axles are not parallel to one another. Non-parallel axles cause all the axles to work against each other. The steer axle must be turned to compensate for the push of the rear axles to keep the vehicle moving in a straight path. Drive axles will fight each other to determine the direction the vehicle will go. Trailer axles with tandem skew will be dragged in different directions.
Drive axles rarely ever exhibit irregular tread caused by misaligned axles. More often than not, tires on the steer axle fall victim to irregular wear as a result of misalignment in other parts of the tractor-trailer combination. One early indication of tandem thrust is having both steer tires feather in the same direction, as opposed to toe which causes feathering in opposite directions.
One-sided wear—which is excessive wear on one side of the tire extending from the shoulder towards the center of the tread—is a more severe wear pattern caused by drive axle thrust angle. In both cases the drive axle pushes the vehicle either to the right or the left, and the driver has to compensate by steering in the opposite direction to keep the vehicle traveling straight.
This results in abrasion on the outside shoulder of one steer tire and the inside shoulder of the other steer tire.
Trailer tires, however, do exhibit irregular wear as a result of poor alignment. Rapid shoulder wear on one shoulder of all the tires on an axle can be caused by an axle that is not “square” to the frame. Tires are worn on the edge of one shoulder, and wear can sometimes extend into the inner ribs.
In this condition, both dual tires on one side of the axle are worn on the outside shoulder and the inside shoulder of both of the tires on the other side of the axle. One-sided wear on trailer tires, which appears as excessive wear on one side of the tread, can be caused by non-parallel axles as well as excessive toe.
The problem is that toe cannot usually be adjusted on drive and trailer axles—only on steer axles. So you'd better hope that the axle is not perpendicular to the frame, since that situation cannot be corrected.
Camber's role in tire wear
Camber is the third leading misalignment cause of irregular wear. Camber is the inward or outward tilt of the top of the tires when viewed from the front of the vehicle.
The tops of tires on vehicles with positive camber are farther apart than the bottoms of the tires. Negative camber results in the bottoms of the tires being farther apart than the tops of the tires. Excessive positive camber will wear the tread on the outer half of the tire while negative camber will wear the tread on the inner half of the tire.
Excessive camber can cause steer axle tires to exhibit one-sided wear and trailer tires to develop rapid shoulder wear—one shoulder and one-sided wear in more extreme cases. In rare cases, drive axles that have been bent and have way too much camber can wear the outside shoulders of the drive tires, too.
Know when to align
As you can see, checking alignment requires that the entire vehicle be checked, not just the steer axle. So the next time you get a trucker complaining that his steer tires must be bad because steer axle alignment checked out OK, tell him to have his drive and trailer axles checked as well. That is probably where the problem is.
If you have fleets asking you when they should have their vehicles aligned, the Technology and Maintenance Council (TMC) recommends the first alignment be done at 15,000-30,0000 miles but no longer than 90 days after the vehicle is put into service.
Subsequent alignments should be done at 80,000- to 100,000-mile intervals or intervals of 12 to 18 months—whichever comes first. Alignment also should be checked any time a component that affects alignment is replaced.
Reading irregular tire wear and diagnosing misalignment conditions is not rocket science as you can see, but enough money is worn off on the road each year in lost tread mileage because of it to put a man on the moon.
Take every opportunity you can to impress your commercial accounts with your understanding of this science. You'll save them money and impress them with your wizardry.