When the wheels on a vehicle don’t all agree on which direction to travel, they fight each other, removing a little rubber with every revolution. Your customers will have tires that wear more quickly, compromised handling and stability, and lower fuel economy because it takes extra energy to erase tire tread.
The wheels on a vehicle get out of alignment from shock damage, which results in some degree of bending in suspension components and/or the frame, and from gradual wear of bushings, ball joints, kingpins, tie-rod ends – anywhere mating components move relative to each other and produce friction and wear.
Minor misalignment usually can be corrected by means of mechanical adjustments. Sometimes, however, components must be replaced to restore alignment. Any alignment procedure, therefore, should start with a thorough inspection of all steering and suspension components.
FRONT-END ALIGNMENT
Three major components of front-end alignment are caster, camber and toe. These values can be measured by a variety of means, from string, plum-bobs, protractors and measuring tapes, to sophisticated, computerized, laser-measurement devices offered by a variety of manufacturers.
A vehicle’s service manuals should provide proper specs for caster, camber and toe, and will tell you where and how to measure and make adjustments.
CASTER
Caster is the forward or backward tilt of the kingpin or support arm at the top of a wheel-suspension assembly, and it is measured in degrees. Forward tilt is called negative caster, and backward is positive.
From tire-wear and fuel-economy standpoints, caster is the least critical element of alignment. But it has a profound impact on vehicle stability and handling.
Modern vehicles are designed to include some degree of positive caster. This causes the front tires to tilt on their left edges when turning left, and on their right edges when turning right. In either case, this slightly lifts the vehicle’s front end. Thus, positive caster allows gravity to help keep the front wheels pointing straight ahead as the vehicle is going down the road.
While it doesn’t usually cause abnormal tire wear, improper caster is frequently at the root of driver complaints of wandering and pulling. Wandering often is caused by improper caster at both front wheels, while pulling can result from one front wheel having more or less caster than the other.
On some vehicles, caster is not adjustable, and an out-of-spec condition often is traced to a worn or bent steering or suspension component(s).
CAMBER
Camber is the departure, from vertical, of a wheel/tire assembly. Also measured in degrees, camber is said to be positive when the top of the wheel tilts outward from a vehicle, and negative when the top of the wheel tilts inward.
Most vehicle designs incorporate some positive camber, especially at the right front wheel, mainly to compensate for normal road crown. This keeps a greater percentage of the tire’s tread area in contact with the road.
Improper camber can cause a vehicle to pull, much as a bicycle will want to go in the direction it is leaning. It also can decrease braking traction, since the contact patch at the tire/road interface is reduced.
Improper camber frequently leaves circumferential wear on just one shoulder of a tire or tires. Camber wear is apt to be beveled, or even stepped, if combined with underinflation, and camber wear often is found on one steer tire only.
Like caster, camber often is unadjustable by conventional means. And, while out-of-spec camber usually is caused by a loose wheel bearing or worn kingpin or bushing, it also can be the result of a bent axle.
TOE-IN/OUT
Toe can be described as the deviation, from parallel, of the longitudinal planes of the two front tires. If the leading edges of the tires are closer together than the trailing edges, the wheels are toed-in. If the leading edges are farther apart than the trailing edges, the wheels are toed-out.
Toe can be measured and expressed in inches or millimeters difference between leading and trailing tire edges, or in degrees deviation from parallel.
Toe has the greatest effect on tire wear. If both front tires are evenly worn on their outside shoulders, it’s a good bet that there’s excessive toe-in. If the wear is on the inside shoulders, the wheels are likely toed-out. In either case, the tires simply aren’t pointing in the same direction, and each one experiences some road scrub as it argues with the other about where the vehicle’s going to go. Also, since the tires aren’t in agreement about the direction of the vehicle, an out-of-spec toe setting also can contribute to vehicle wandering.
Toe-related wear, in early, less-severe stages, can show up as feathering, which usually can be felt by hand before it can be seen. Feathering is a condition where the individual tread ribs wear more on one side than the other.
If the direction of feathering is on opposite sides on the right and left tires (e.g., left on the left and right on the right), a toe problem is a strong possibility. If it’s on the same side (e.g., on the left side of both tires), you’re probably looking at a camber problem.
A small amount of toe-in is built into most steering/suspension systems since, with a vehicle loaded and going down the road, the front wheels have a natural tendency to “run away” from each other. A little toe-in helps keep them parallel while in motion.
Toe can be knocked out of spec easily by a worn idler bushing, worn tie rod or drag-link joints, or bent tie rods. Fortunately, toe is adjusted easily by unlocking and turning threaded sleeves that lengthen or shorten the tie rods – sort of like a turnbuckle between two cables.
REAR-AXLE CONSIDERATIONS
Rear-axle misalignment on a two-axle vehicle is easy to visualize. When the thrustline, which runs perpendicular to the center of the rear axle, departs from the geometric centerline of the truck, misalignment exists.
The amount of misalignment is expressed in degrees of the angle formed by those two lines. Positive thrust angle means the rear axle is tracking to the right, and will tend to steer the truck to the left. The driver will have to compensate by steering to the right, and the truck will “dog-track” down the road.
Also, with front wheels cocked to the right, positive caster will effectively be turned into negative camber at the left front wheel, and positive at the right, as the spindles rotate about the kingpins.
So, even if the front end were aligned to proper specs, the dynamic condition would be out, resulting in tire edge scuffing or feathering, and reduced fuel economy. Misalignment between two trailer axles is even more detrimental because the front member of the tandem can’t be steered to compensate.
On a three-axle truck or tractor, the situation gets a little more complex. Each rear axle may produce its own thrust angle. If both angles are identical in direction and degree, the result is similar to single-drive-axle misalignment. But if the two rear axles’ thrust-lines point in different directions, the result is tandem scrub.
Tandem scrub is expressed in degrees of the angle formed by the two axles. This value should be as close to zero as possible, since even a small scrub angle involves all the tires on the vehicle, and results in a marked decrease in tread life and fuel economy.
While some rear-axle suspensions are adjustable for axle location and squareness to the frame, most rear-axle misalignment is the result of a bent, broken or loose suspension component, or a worn-out bushing.
Significant deviations in drive-axle toe and camber are extremely difficult to correct, since they’re often caused by a major impact that bent the solid axle housing.
One final note: Some alignment experts advise making sure all wheels are properly aligned with respect to a vehicle’s frame, while others insist that wheels be aligned with respect to each other, since a vehicle’s frame may be distorted.
The best scenario, of course, is when the wheels are all within specification with respect to each other, and to the frame.
