Irregular and premature tire wear has many causes. Some of them are incorrect inflation pressures, lack of regular rotation, poor driving habits, or improper wheel alignment. If the wheel alignment is reset because of tire wear, always reset the toe as close to zero degrees as the specification allows. Refer to "Rear Toe Adjustment" in this section.

Rotate the tires if

Check wheel alignment if

The original equipment tires have built-in tread wear indicators to show when the tires need replacement. These indicators appear as bands when the tire tread depth becomes shallow. Tire replacement is recommended when the indicators appear in three or more grooves at six locations.

Waddle is side-to-side movement at the front or rear ofthe vehicle. It is caused by the steel belt not being straight within the tire, or by excessive lateral runout of the tire or wheel. It is most noticeable at low speeds, 8 to 48 km/h (5 to 30 mph), but may appear as ride roughness at 80 to 113 km/h (50 to 70 mph).

The vehicle must be road tested to determine which end of the vehicle has the faulty tire. The rear end of the vehicle will shake from side to side or "waddle" if the waddle tire is on the rear of the vehicle. From the driver's seat, it feels as though someone is pushing on the side of the vehicle. If the faulty tire is on the front of the vehicle, the waddle is more visual. The front sheet metal appears to be moving back and forth, and the driver's seat feels like the pivot point in the vehicle.

Waddle can be diagnosed using the method of substitutingknown good tire and wheel assemblies on the problemvehicle.

Lead/pull is the deviation of the vehicle from a straight path on a level road with no pressure on the steering wheel. Lead is usually caused by

The way in which a tire is built can produce lead/pull in the vehicle. Off-center belts on radial tires can cause the tire to develop a side force while the vehicle rolls straight down the road. If one side of the tire has even a little larger diameter than the diameter of the other side, the tire will tend to roll to one side. Unequal diameters will cause the tire to develop a side force which can produce vehicle lead/pull.

The radial lead/pull diagnosis chart should be used to determine whether the problem originates from an alignmentproblem or from the tires. Part of the lead diagnosis procedure calls for tire rotation that is different from the proper tire rotation pattern. If a medium- to highmileage tire is moved to the other side of the vehicle, be sure to check for ride roughness. Rear tires will not cause lead/pull.

Wheel imbalance causes most highway speed vibrationproblems. A vibration can remain after dynamic balancingbecause

Measuring tire and wheel free runout will uncover only part of the problem. All three causes, known as loaded radial runout, must be checked using the method of substitutinga known good tire and wheel assembly on the problem vehicle.

Low-speed vibrations, which occur below 64 km/h (40 mph), are usually caused by runout. High-speed vibrations, which occur above 64 km/h (40 mph), can be caused by either imbalance or runout.

Prior to performing any work, always road test the car and perform a careful visual inspection for

Balancing is the easiest procedure to perform and should be done first if the vibration occurs at high speeds. Do an off-vehicle, two-plane dynamic balance first to correct any imbalance in the tire and wheel assembly.

An on-vehicle finish balance will correct any brake drum, rotor, or wheel cover imbalance. If balancing does not correct the high-speed vibration, or if the vibration occurs at low speeds, runout is the probable cause.

Runout can be caused by the tire, the wheel, or the waythe wheel is attached to the vehicle. To investigate the possibility of wheel runout, refer to the following proceduresas well as the wheel runout diagnosis chart in this section.

Whenever a tire is rotated on the wheel, or whenever atire or wheel is replaced, rebalance the assembly.

The front caster is not adjustable. If the front caster angle is not within specifications, inspect for suspension support misalignment or front suspensiondamage. Replace any damaged suspension components as necessary.

The first responsibility of engineering is to design safe steering and suspension systems. Each component must be strong enough to withstand and absorb extreme punishment. Both the steering system and the front andthe rear suspension must function geometrically with thebody mass.

The steering and the suspension systems require that the front wheels self-return and that the tire rolling effort and the road friction be held to a negligible force in order to allow the customer to direct the vehicle with the least effort and the most comfort.

A complete wheel alignment check should include measurements of the rear toe and camber.

Four-wheel alignment assures that all four wheels will berunning in precisely the same direction.

When the vehicle is geometrically aligned, fuel economyand tire life are at their peak, and steering and performanceare maximized.

Toe-in is the turning in of the tires, while toe-out is theturning out of the tires from the geometric centerline orthrust line. The toe ensures parallel rolling of the wheels.

The toe serves to offset the small deflections of the wheel support system which occur when the vehicle is rolling forward. The specified toe angle is the setting which achieves 0 degrees of toe when the vehicle is moving.

Incorrect toe-in or toe-out will cause tire wear and reducedfuel economy. As the individual steering and suspensioncomponents wear from vehicle mileage, additional toe will be needed to compensate for the wear.

Always correct the toe dimension last.

Caster is the tilting of the uppermost point of the steeringaxis either forward or backward from the vertical when viewed from the side of the vehicle. A backward tilt is positive, and a forward tilt is negative. Caster influencesdirectional control of the steering but does not affect tirewear. Weak springs or overloading a vehicle will affect caster. One wheel with more positive caster will pull towardthe center of the car. This condition will cause the car to move or lean toward the side with the least amount of positive caster. Caster is measured in degrees and is not adjustable.

Camber is the tilting of the top of the tire from the verticalwhen viewed from the front of the vehicle. When the tires tilt outward, the camber is positive. When the tires tilt inward, the camber is negative. The camber angle is measured in degrees from the vertical. Camber influencesboth directional control and tire wear.

If the vehicle has too much positive camber, the outsideshoulder of the tire will wear. If the vehicle has too muchnegative camber, the inside shoulder of the tire will wear.

Steering axis inclination (SAI) is the tilt at the top of thesteering knuckle from the vertical. Measure the SAI angle from the true vertical to a line through the center of the strut and the lower ball joint as viewed from the front of the vehicle.

SAI helps the vehicle track straight down the road and assists the wheel back into the straight ahead position. SAI on front wheel drive vehicles should be negative.

The included angle is the angle measured from the camber angle to the line through the center of the strut and the lower ball joint as viewed from the front of the vehicle.

The included angle is calculated in degrees. Most alignment racks will not measure the included angle directly. To determine the included angle, subtract the negative or add the positive camber readings to the steering axis inclination (SAI).

The scrub radius is the distance between true vertical and the line through the center of the strut and lower ball joint to the road surface. Scrub radius is built into the design of the vehicle. Scrub radius is not adjustable.

The setback is the distance in which one front hub andbearing assembly may be rearward of the other front hub and bearing assembly. Setback is primarily caused by a road hazard or vehicle collision.

The turning angle is the angle of each front wheel to thevertical when the vehicle is making a turn.