Studies have shown that 80 percent of Class 8 tractors operate misaligned, and over 90 percent of the trailers they pull have serious alignment problems. Even new trucks aren’t immune. The Technology and Maintenance Council (TMC) of the American Trucking Associations recommends re-alignment after 90 days of service, or between 15,000 and 30,000 miles.

New axles and suspensions have to seat themselves, TMC suggests.

What are the consequences of ignoring things? According to Hunter Alignment, improper vehicle alignment can lead to excessive tire wear, increased fuel consumption caused by increased rolling resistance, unsafe vehicle handling characteristics, driver fatigue and driver retention problems, and premature suspension component wear.

In a typical highway operation, it’s reasonable to expect that steering tires will last up to 100,000 miles and drive tires considerably more. But if the tires are being forced sideways, even slightly, they’ll overheat and wear down so fast that even bumping them to a trailer position won’t save them. Once abnormal wear gets out of hand, replacement is the only solution. For example, if the overall thrust angle on a tandem drive axle deviates from the chassis centerline by even a 1/4 in., the truck must attempt to correct the direction of travel by as much as 55 ft for every mile driven, or almost 1 percent of its overall distance traveled.

Misalignment could also affect fuel consumption by as much as 4 percent, not just in extra distance traveled but also by trying to drag a tire sideways mile after mile. If the truck travels 100,000 miles a year and normally gets, for example, 7.1 mpg, the additional fuel used at today’s prices would amount to almost $1,900. When replacement costs for tires are factored in, the price for failure to stay ahead of alignment problems can quickly multiply.

Alignment expert Doug Peters of Kal Tire in Kamloops, B.C., says beyond the mechanical difficulties, driver issues are as good a reason as any for having the vehicle properly aligned. “Tired drivers are accidents looking for a place to happen,” he notes. “You’re not in the best shape to drive when you’re constantly having to fight the truck’s unpredictable handling.”

Self-Examination

Before attempting any adjustments, check that worn or damaged parts aren’t causing (or being caused by) problems that an alignment could cure. Examine the kingpins and tie-rod ends for excessive wear, and the wheel bearings for correct endplay. The steering box could be worn or loose, too, the brake drums could be out-of-round, or the axle spindles damaged. Don’t neglect the ride height. A simple adjustment here can also affect the front-end geometry.

Next, examine the tires themselves starting with the steering axle. Are the ribs wearing evenly across the entire face? If not, is there evidence of ‘feathering’?
Al Kohn, the manager of training and technology at Goodyear, calls this “fingertip diagnostics.”

“Run your hand over the tread from side to side. If the ribs are rough in one direction and smooth in the other, then the tires are being scrubbed sideways,” Kohn suggests.

Among the more common alignment problems, toe-in is the most critical steering adjustment when it comes to abnormal tire wear. The others are caster, and rear thrust.

Even if you have to go outside your own shop to have this work done, it’s worth it. A typical checkover at an alignment shop will cost about $75. Setting the toe-in will cost about $125. Adjusting drive axles will be about $150 for a tandem group (with an additional $100 for each cut and weld if you are running Nuway rear suspension). A caster adjustment will cost around $65, with an additional $50 for each shim. Rear-axle thrust adjustments are an additional $100. So, the worst case will see you spending from $400 to $600 depending on the severity of the work. Think of it as the cost of one tire and a half a tank of fuel. Then balance that against the cost of extra fuel over one year and the continuing damage being done to tires and other components. It’s not hard to see the upsideThe Technology and Maintenance Council Recommended Practices Manual (TMC RP#642) is the recognized source for recommended alignment settings. The published settings are intended only as a guideline, however. Actual specs will vary between manufacturers and chassis styles. Wheel alignment is as much an art as a science, so compromise will be part of every adjustment.

Factory-specified steering axle toe-in (1/16 in per wheel +/- 1/32 in. up to 1/8 in. total overall) accounts for suspension dynamics when the vehicle is under load at highway speed. The small amount of toe-in allows for a neutral toe position when the truck is up to speed.
Typical caster angles are between 3 and 5 degrees– sometimes more. To counter the crown of the road and the tendency for the truck to want to take to the right, alignment shops have found a working balance at 3 degrees for the left and 4 for the right. Shops use metal shims to bias the whole axle toward the center of the road. Ideally, the forces cancel each other out.

Camber adjustments require axle bending and are not usually part of the alignment process. However, a fully equipped shop can make these adjustments as well. Accurate tire pressure helps stabilize camber by maintaining proper front axle height.

For drive axles, TMC recommends tandem axles be perpendicular to within 1/8 in. when measured from the axle end to the chassis centerline. In addition, the axles must be parallel with one another to within 1/8 in. when measured between axle ends. For more information on the TMC Recommended Practices manual, see www.truckline.com/aboutata/
councils/tmc/rp.