Learn Wheel Alignment Explained – Types of Alignment, Caster, Camber, Toe, Thrust Line, SAI, Included Angle, Scrub Radius, Turning Radius, Load Distribution, Prealignment Inspection and Ride Height

Introduction:

A vehicle’s wheels, tires, suspension system and steering system are all engineered to work together to provide safe, stable, and reliable handling a condition achieved through proper wheel alignment. During this process, the angles of the wheels are measured and adjusted to place the tires perpendicular to the ground, parallel to each other and aligned to the geometric centerline of the vehicle. These adjustments are made by changing the position of various steering and suspension parts according to the angles set by the vehicle’s manufacturer. Correct wheel alignment allows the wheels to roll without scuffing, dragging or slipping on different road surfaces. Proper alignment of both the front and rear wheels ensures greater safety, easier steering, longer tire life, reduced fuel consumption, and less strain on the steering and suspension systems. Alignment restores the geometry of the suspension to the angles originally determined to properly locate the vehicle’s weight on the tires and to facilitate steering. The wheels should be checked for alignment whenever new tires or steering and suspension parts are installed, as well as whenever the tires show abnormal wear. Misalignment typically occurs due to worn suspension parts, a change in ride height or driving hard into a pothole or curb all of which directly affect the wheel angles.

Types of Wheel Alignment:

There are two basic types of alignment performed today: two wheel and four wheel. In a two-wheel alignment, only the angles of the front wheels are measured and adjusted. This method does not account for the relationship between the front and rear axles and was more common before suspension and steering systems became complex.

Four wheel alignment measures the angles at all four wheels. On some vehicles, adjustments are made only to the front wheels because there is no provision to adjust the rear. However, by adjusting the front wheels so they rotate in the same direction as the rear wheels, the vehicle will tend to move straight. Many vehicles do offer rear-wheel adjustments, in those cases, the rear wheels are adjusted first, followed by the fronts, which are then aligned to the vehicle’s centerline.

Road Crown:

Most roads are not perfectly flat. They are paved at a slight angle to allow water to run off rather than accumulate. This angle, called road crown, can cause a vehicle to pull toward the right side of the road. To compensate, different alignment angles are set on each side of the vehicle.

Alignment Geometry:

Several specific angles determine proper suspension and steering geometry.

Caster:

It is the angle of the steering axis of a wheel from vertical, as viewed from the side of the vehicle. The forward or rearward tilt from the vertical line defines caster. Tilting forward is negative caster; tilting backward is positive caster. Caster is often the first angle adjusted during an alignment, as it is designed to provide steering stability. The caster angle for each wheel on an axle should be equal. Unequal caster angles cause the vehicle to steer toward the side with less caster. Excessive negative caster can cause sensitive steering at high speeds, leading to wandering. Caster is not considered a tire-wearing angle. It is affected by worn or loose ball joints, strut rods, and control arm bushings. On some strut suspension systems, caster adjustments are not possible without an aftermarket service kit. In such cases, a caster problem usually indicates a worn or bent component that must be replaced or repaired. Where adjustment points are provided, they are typically made at the top or bottom mount of the strut assembly.

Camber:

It is the tilt of the front or rear wheels inward or outward from vertical, as viewed from the front of the car. This angle is designed to compensate for road crown, passenger weight, and vehicle weight. Camber is usually set equally for each wheel. Equal camber means each wheel tilts outward or inward the same amount. Unequal camber causes tire wear and pulls the vehicle toward the side with more positive camber. Camber angle changes due to suspension travel are controlled by the suspension’s pivots. It is affected by weak or broken springs, worn or loose ball joints, control arm bushings, and wheel bearings. Anything that changes ride height also affects camber. Adjustments are made at the control arms on most vehicles. Some strut suspensions include a camber adjustment at the spindle assembly or at the top mounting of the strut. Very little camber or caster adjustment is required on strut suspensions if the tower and lower control arm locations are correct. A serious camber error without damage to mounting positions indicates bent suspension parts, which should be replaced.

Toe:

It is the distance comparison between the leading edge and trailing edge of the tires. If the leading edge distance is less, there is toe-in, if greater, toe-out. Toe is critical as a tire wearing angle. Wheels that do not track straight ahead drag as they travel forward. Excessive toe-in or toe-out causes a saw tooth edge on the tread surface from sideways dragging. Excessive toe-in specifically causes wear on the outside edge of the tire, while toe-out wears the inside edge. Toe adjustments are made at the tie-rod and must be set equally on both sides of the car. If toe settings are unequal, the vehicle may pull due to an off center steering wheel. This condition should be corrected by equalizing the toe on both sides with the steering wheel centered. Toe changes with vehicle speed. As the vehicle moves, friction forces the tires toward zero toe; however, aerodynamic forces alter ride height, which also changes toe and camber. Therefore, most toe specifications anticipate these changes and are set to provide zero toe at highway speeds.

Thrust Line and Thrust Angle:

A primary consideration in any alignment is ensuring the vehicle runs straight with the rear tires tracking directly behind the front tires when the steering wheel is straight ahead. The geometric centerline should parallel the road direction when rear toe is parallel to that centerline. If rear toe does not parallel the centerline, a thrust direction to the left or right is created. This difference the thrust angle causes handling issues. Any time the front axle centerline is not parallel to the rear axle, the vehicle tends to travel according to the rear axle angle, pulling in the opposite direction of the thrust line. A thrust line to the right causes a pull to the left, a thrust line to the left causes a pull to the right. This problem can cause tire wear, poor directional stability on ice, snow or wet pavement, and can make the vehicle pull during braking or hard acceleration. A non zero thrust angle also leaves the steering wheel off center. Nonparallel axles are usually caused by a shifted rear axle on its spring supports, rear wheel misalignment or accident damage.

All vehicles are built around a geometric centerline running through the chassis from back to front. The thrust line is the direction the rear axle would travel if unaffected by the front wheels a condition also called tracking. Correct tracking requires all suspension parts to be in good condition and correctly located, with axles and wheels parallel to each other and the centerlines through the axles and spindles at 90 degree angles to the vehicle’s centerline. Simply put, all four wheels should form a perfect rectangle. An offset thrust line causes a pull away from the line and produces tire wear similar to incorrect toe settings. Minor variations between the thrust line and centerline are generally not noticeable and do not cause handling problems, provided the front wheels are aligned parallel with the thrust line.

Steering Axis Inclination (SAI):

Steering axis inclination (SAI) locates the vehicle weight to the inside or outside of the tire’s vertical centerline. It is the angle between true vertical and a line drawn between the steering pivots as viewed from the front. This engineering angle projects the vehicle’s weight to the road surface for stability and helps the steering system return to straight ahead after a turn. If SAI is zero, the upper and lower ball joints or strut pivot points would be directly over one another, leading to tire scrub in turns, lack of control, and increased effort during turn recovery. With a tilted SAI, a triangle forms between the ball joints and spindle, creating an arc when turning with a high point straight ahead and a drop when turning to each side. This motion transmits through the control arms to the springs and vehicle weight. The forces in a turn attempt to lift the vehicle; the tilting effect of SAI offsets these lifting forces and helps pull the tires back to straight-ahead after the turn.

Front wheel drive vehicles with strut suspensions typically have a higher SAI angle 12 to 18 degrees compared to short long arm rear wheel drive suspensions, which run 6 to 8 degrees, because the extra leverage improves directional stability. If SAI angles are unequal side to side, torque steer, brake pull, and bump steer (jerking from side to side) can occur even if static camber angles are within specifications. Checking the SAI angle helps locate alignment problems. A side to side variance may indicate an out of position upper strut tower, a bowed lower control arm, or a shifted center crossmember. On a short long arm suspension, SAI is the angle between true vertical and a line from the upper ball joint through the lower ball joint. On a strut-equipped vehicle, this line runs from the center of the strut’s upper mount down through the center of the lower ball joint.

Included Angle:

When the camber angle is added to the SAI angle, the sum is called the included angle. This angle is not directly measured by an alignment machine; it is calculated by adding the camber and SAI for each side. The included angle must be the same on each side of the vehicle, even if camber differs side to side, if not, the vehicle will pull. Comparing SAI, included angle and camber can identify damaged or worn components. For example, if the SAI reading is correct but the camber and included angles are less than specifications, the steering knuckle or strut tower may be bent.

Scrub Radius:

Scrub radius is the distance between the center of the tire and the point where the SAI angle intersects the ground. It must be equal on both sides of the vehicle; otherwise, the vehicle will pull. Scrub radius is not adjustable or measured it is observed, as it is part of the suspension’s design. Positive scrub occurs when the tire’s contact patch is outside the SAI angle negative scrub occurs when the patch is inside the angle. Most front wheel drive vehicles use a negative scrub radius to reduce torque steer, while most short-long arm suspensions use positive scrub. If a properly aligned vehicle continues to pull, inspect for offset wheels or any issue affecting the SAI.

Turning Radius:

Turning radius also called toe-out on turns or turning angle relates to the amount of toe out present when cornering. As a car goes around a corner, the inside tire must travel in a smaller radius circle than the outside tire. The steering geometry accomplishes this by turning the inside wheel sharper than the outside wheel, resulting in toe-out during turns. This design eliminates tire scrubbing by keeping the tires pointed in their required direction of travel. Turning radius is not an adjustable angle, if incorrect, the tie-rods, steering arm, or steering knuckle are likely damaged and must be replaced.

Load Distribution:

Load distribution refers to the weight placed on each wheel. Every vehicle is engineered to operate at a designed curb height also called trim height. At this height, each wheel must carry the correct amount of weight. Excessive loading to the front, rear or one side changes the curb height, upsetting vehicle balance and steering geometry. Incorrect alignment, sagging springs and bent suspension parts can also alter load distribution, placing excessive load on only one or two wheels. Springs, shocks, suspension components and geometry are all engineered to work together as a balanced team. If one wheel operates under a different weight load or steering geometry, the vehicle will not ride or handle as designed.

Prealignment Inspection:

Before beginning an alignment, it is essential to determine why the vehicle needs one. Symptoms often lead to the root cause. A customer interview should be followed by a test drive, during which the technician checks that the steering wheel is straight, feels for vibrations in the steering wheel, floor or seats, and notices any pulling or abnormal handling such as hard steering, tire squeal while cornering or mechanical pops and clunks. This test drive helps identify problems that must be corrected before proceeding. The prealignment inspection itself is extremely important. If any parts are found defective, they must be replaced before alignment. The inspection should include a careful review of the tires, wheels, suspension system, and steering system. All abnormal loads should be removed before taking measurements, since added weight affects ride height and, consequently, alignment angles. However, if the vehicle is normally used to carry heavy objects such as toolboxes those should remain in place.

Ride Height:

Finally, the vehicle’s ride height must be checked. Every vehicle is designed to ride at a specific curb height, with specifications and measuring points provided in service manuals. Proper alignment is impossible if the ride height is incorrect, particularly for camber, which changes as the vehicle’s height changes.