Believe it or not, after improper inflation maintenance, vehicle misalignment is probably the chief cause of poor tire performance.
A properly aligned and maintained vehicle can extend tire mileage significantly compared with a vehicle whose alignment and maintenance is incorrect and sloppy.
In tests performed over the last few years, it was found that properly aligned vehicles could virtually eliminate irregular wear and double tire mileage. These facts alone should inspire all of you commercial tire service and maintenance providers to take a closer look at alignment conditions and how they affect your customer's tire wear. Chances are, your customers are losing mileage to this silent killer of tread performance.
By definition, alignment is "the process whereby the vehicle and all the tires are traveling in the same direction." The significant components of vehicle alignment are toe, camber, caster and axle parallelism, which includes thrust angle and scrub angle. The most common misalignment conditions that eat your customers' tires are toe and axle parallelism. Let's look at these two factors.
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.
The purpose of adjusting the toe is to allow the tires to run straight during normal operating conditions. The toe measurement will change from unloaded to loaded conditions.
Toe-in exists when the wheels are closer together in the front than in the rear and 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.
Incorrect toe settings generally have the greatest effect on steer tire wear and are probably the easiest to correct. Incorrect toe settings will be indicated by "feathering" of the treads of the tires on both ends of an axle. Both tires will be feathered toward the inside in the case of excess toe-in, or toward the outside in the case of excessive toe-out.
Tractors and trucks should have their toe set at zero to slightly positive (toe-in). Excessive toe-in, however, would be detrimental to tire wear. Toe-out on the steering axle will result in the vehicle "darting," excessive tire wear and driver complaints.
In 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, or axles that are not perpendicular to the chassis centerline, have a definite effect on steer tire wear, and also on drive-axle tire wear.
Rear tandem axles that are not perpendicular to the frame, but are parallel to each other, create a "thrust angle" that 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 correct the direction of travel of the truck, which results in fast and irregular wear.
One indication of tandem thrust is having both steer tires feather in the same direction, as opposed to toe which causes feathering in opposite directions. Excessive thrust angle will wear the shoulders on the same sides of both tires. Irregular wear will also be apparent on the drive tires.
An out-of-parallel 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. This is called "tire scrub," and it results in fast and irregular trailer tire wear, as well as steer- and drive-axle tire wear.
Tandem skew, or scrub angle, occurs when tandem axles are not parallel to one another. Drive axles will fight each other, as well as the driver, to determine the direction the vehicle will go. Trailer axles with tandem skew will be dragged in different directions. This condition will cause more excessive and rapid tread wear on all tires of a vehicle.
Drive axle misalignment will create irregular wear on drive tires, as well as steer tires, and is the most common cause of alignment-related irregular wear.
Trailer axle misalignment will produce irregular wear on trailer, drive and steer tires, and is the second leading cause of alignment-related irregular wear. Steer axles have the fewest problems with alignment-related irregular wear.
Tandem axles should be parallel to each other within 1/8 inch or less and all axles should be perpendicular to the chassis centerline within 1/8 inch or less.
Axle parallelism, otherwise known as tracking, is one of the most common causes of rapid tire wear and also one of the easiest to correct. Failure of the wheels to track is usually due to lack of parallelism between the axles, a broken spring leaf, worn springs, loose "U" bolts, bent frame, improperly adjusted torque or locating rods, or excessively worn torque or locating rod bushings. Fifth wheels also can bind up and become susceptible to side forces as can worn pintle hooks.
While you may not be able to simply take your customers' vehicles and have them aligned, you should be able to identify these misalignment conditions and advise your fleet accounts of the problems that are eating their lunch and preventing them from obtaining the low tire, cost-per-mile they desire.