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Get This Straight
Everything that happens while your truck is running consumes energy. Much of it is necessary and normal, and tire wear is no exception. When molecules of rubber are abraded from a tire, fuel is consumed to provide the force. That is a regular part of rolling resistance. But picture the force it takes to drag a truck sideways for several feet every time it moves a mile down the road. That’s what happens when tractor and trailer wheels are out of alignment.
That misalignment uses extra energy that comes from only one place, the fuel that your engine burns. If that side scrub weren’t enough, it takes a great deal of force to abrade each tire. And once worn excessively, it takes a great deal of money to replace the tires. Even if you bobtail and pull only others’ trailers, you’re still replacing up to 10 tires. At today’s prices, that could easily approach $5,000. By maintaining your tire alignment, you can probably save one or two sets over the life of the truck. Don’t forget to add in the 3 percent or so of fuel that vehicle misalignment can cost, increasing annual savings by $1,500 to $1,600.
When thinking of alignment, we often put our focus on the steer axle. It’s the first — and often the only — place we look when we feel the truck pulling to one side or the other. But every axle on a tractor or trailer can, and often will, go out of line.
Being “aligned,” or in line, means that when you steer straight ahead, every wheel and tire rolls in a straight line in that same direction. It also means that when you steer into a turn, the lines extended from the axles of each wheel on the steer axle intersect along a line perpendicular to the center line of the truck and halfway between the drive axles on a tandem drive. On a single axle truck or tractor, that line is the extension of the axle, also perpendicular to truck’s center line. This geometry lets all wheels track evenly and rotate about the same center point when turning.
Misalignment occurs when different wheels point in different directions when rolling, and rotate around different points when turning. The result is uneven tire wear. There are different wear patterns that indicate what type of alignment problem is present. The Technology and Maintenance Council (TMC) issued a Recommended Practice, RP642, on Total Vehicle Alignment in 2000, and updated it in 2004. It is sub-titled “Recommendations for Maximizing Tire and Alignment-Related Component Life.”
Tire problems can be revealed by running your hand across the tread of each tire and around the tire’s circumference. Smooth, even wear indicates proper alignment. Feather wear is described in the RP as “tread ribs or blocks worn so that one side is higher than the other resulting in step-offs across the tread face.” If you feel edges in one direction while the tread feels smooth in the other direction, you have feather wear.
If wear is greater to the outside of both steer tires, with the edges on the inside of each tire, there is a toe-in condition. The tires are pointed inward as the truck rolls. The opposite wear, more on the inside, indicates toe-out. “If wear on both steer tires is on the same direction, drive axle or other chassis misalignment is indicated,” according to RP642. A combination of even and feather wear indicates a combination of drive axle misalignment and either toe-in, or toe-out.
Drive axles and trailer tandems can be knocked out of alignment by strong and repeated impacts, such as by driving over potholes and poorly maintained railroad tracks, by repeated curbing at speed or by impacting loading docks, especially at an angle. All alignment can be affected by component wear such as bearings and hubs, bushings, torque rods and links.
Diagonal wear is described as a series of flat spots worn across the tread at about 25 to 30 degrees. It is attributed to any of, or a combination of, the following: bad wheel bearings, toe-out, mismounting wheels and tires and mismatched dual tires. Rapid wear on just one shoulder is often due to improper camber settings, but can be due to bent or misaligned axles and loose or worn wheel bearings.
TMC recommends inspecting all tire and wheel assemblies for irregular tread wear at each PM or 45 days, whichever is sooner. Record observations and compare them with previous conditions as outlined in RP642. Every 80,000 to 100,000 miles, have a qualified provider do a full vehicle alignment. Again, record all readings, before and after. Compare them with each other and with the tire measurements to identify any problems needing maintenance.
Total vehicle alignment is a complex subject, but one that is important to any truck operator’s financial health. For a copy of RP642, or better yet, an owner-operator TMC membership that includes the entire RP Manual, call TMC at (703) 838-1763.