AUTO DIMENSIONS INC.

Wheel Alignment Explained

Wheel Alignment should be checked whenever new tires are installed, suspension components installed, when the vehicle has encountered a major road hazard or curb and any time unusual tire wear patterns appear.

Wheel Alignment is the Measurement of complex suspension angles and the adjustment of a variety of suspension components. It is a suspension-tuning tool which greatly influences the vehicle's handling and tire wear.

Wheel alignment consists of adjusting the angles of the wheels so that they are parallel to each other and perpendicular to the ground, thus maximizing tire life and ensures straight and true tracking along a straight and level road.

The primary static suspension angles that need to be measured and adjusted are caster, camber, toe and thrust angle.

The following are definitions
Conditions and Possible Causes of each angle and its influence on a vehicle and its tires.

 

Camber

 

                        

 

Camber is the angle of the wheel, measured in degrees, if the top of the wheel is tilted out then the camber is positive, if it's tilted in, then the camber is negative.

 

If the camber is out of adjustment, it will cause premature tire wear on one side of the tire's thread. When the camber is out of adjustment it can cause a pulling problem to the side with the more positive camber.
This usually happens when the vehicle has been involved in an accident which has caused structural damage or damage to the strut and / or spindle assembly. Camber also goes out of adjustment when the springs sag and causes ride height to change, or when ball joints and or other attached parts are worn or defective. It also varies depending on speed as aerodynamic forces changes riding height.

 

After repair and alignment, pulling problem could persist due to the insufficient and or uneven tire to road contact. If a tire shows camber wear pattern, moving it to the rear might be effective but replacement might be best.

 

Whenever camber changes, it directly affects toe.

 

On most front-wheel-drive vehicles, camber is not adjustable, however there are aftermarket kits that allow sufficient adjustment to compensate for accident damage or the change in alignment due to the installation of lowering springs.

 

Caster

                                          

 

Caster is the angle of the steering pivot, measured in degrees.

Viewed from the side, the caster is the tilt of the steering axis. When the wheel is in front of the load the caster is positive. Three to five degrees of positive caster is the typical range of settings, with lower angles are being used on heavier vehicles to reduce steering effort.

 

If the caster is out of adjustment, it can cause problems in straight-line tracking. If the caster is different from side to side, the vehicle will pull to the side with the less positive caster. If the caster is equal but too negative, the steering will be light and the vehicle will wander and be difficult to keep in a straight line. If the caster is equal but too positive, the steering will be heavy and the steering wheel may kick when you hit a bump.

 

Caster has little or no effect on tire wear.


One of the best ways to visualize caster is to picture the caster on a shopping cart. The pivot while not at an angle intersects the ground ahead of the wheel contact patch. When the wheel is behind the pivot at the point where it contacts the ground, it is in positive caster.

 

Like camber, on many front-wheel-drive vehicles, caster is not adjustable. If the caster is out of adjustment on these vehicles, it indicates that something is possibly bent from an accident, and must be repaired or replaced.

 

Toe

                    

The vehicle's toe is the most critical alignment settings relative to tire wear. if the toe setting is just 1/32-inch off of its appropriate setting, each tire on that axle will scrub almost 3 1/2 feet sideways every mile, therefore reducing tire life.

Like camber, toe will change depending on vehicle speed, as aerodynamic forces changes the riding height hence affecting camber and toe due to the geometry of the steering linkage in relation to the geometry of the suspension. 

The toe angle identifies the direction of the tires compared to the centerline of the vehicle. Rear-wheel drive vehicle "pushes" the front tires, as they roll along the road, resistance causes some drag resulting in rearward movement of the suspension arms against their bushings. Most rear-wheel drive vehicles use positive toe to compensate for suspension movement.

Front-wheel drive vehicle "pulls" the vehicle, resulting in forward movement of the suspension arms against their bushings. Most front-wheel drive vehicles use negative toe to compensate for suspension movement.

Toe can also be used to alter a vehicle's handling traits. Increased toe-in will reduce oversteer, steady the car and enhance high-speed stability.

Increased toe-out will reduce understeer, free up the car, especially during initial turn-in while entering a corner.

Before adjusting toe outside the vehicle manufacturer's specification to manipulate handling, be aware that toe setting influences tire wear. Excessive toe settings often causes drivability problems, especially during heavy rain. This is because most highways have tire groves from the daily use by loaded tractor trailers. These heavy vehicles leave groves that fill with water. When one of the vehicles front tire encounters a puddle, it loses some of its grip, the other tire's toe setting will push causing excessive toe-in, or pull causing excessive toe-out. This may cause the vehicle to feel unstable.

 

   Steering Axis Inclination (SAI)

 
 

Steering Axis Inclination (SAI) is the measurement in degrees of the steering pivot line when viewed from the front of the vehicle. On a SHORT-LONG ARM (SLA) SUSPENSION the line runs through the upper and lower ball joints.

 

On a MacPherson strut suspension; the line runs through the lower ball joint and upper strut mount or bearing plate. This angle (SAI), when added to the camber to forms the included angle and causes the vehicle to lift slightly when the wheel is turned from a straight position. The vehicles weight pushes down and causes the steering wheel to return to the center when you let go of it after making a turn.

 

Like caster, it provides directional stability and also reduces steering effort by reducing the scrub radius.

 

If the Steering Axis Inclination (SAI) is different from side to side, it will cause a pull at very slow speeds. SAI is a  nonadjustable angle, it is used with camber and the included angle to diagnose bent spindles, struts and mislocated crossmembers.

 

The most likely cause for Steering Axis Inclination (SAI) being out of specification is bent parts, which has to be replaced to correct the condition. On older vehicles and trucks with king pins instead of ball joints, Steering Axis Inclination (SAI) is referred to as (KPI) King Pin Inclination.

 

 

Included Angle


 

Included angle is the sum of the Camber and Steering Axis Inclination (SAI) angles Included angle is not directly measurable. It is used primarily to diagnose bent suspension parts.

 

If the camber is negative, then the included angle will be less than the Steering Axis Inclination (SAI), if the camber is positive, it will be greater.

 

The included angle must be the same from side to side even if the camber is different. If there is a difference, then something is bent, possibly the steering knuckle.

 

 

Scrub Radius

 

Scrub Radius is the distance between the extended centerline of the steering axis and the centerline of the tire where the tread contacts the road. This distance must be exactly the same from side to side or the vehicle will pull strongly.

If the steering centerline is inboard of the tire centerline, the scrub radius is positive. If the steering centerline is outboard of the tire centerline, the scrub radius is negative.

Rear-wheel drive cars and trucks generally have a positive scrub radius while FWD cars usually have zero or a negative scrub radius because they have a higher Steering Axis Inclination (SAI), angle.

Using different wheels other than stock can alter the scrub radius.

 

Riding Height

Riding height is usually measured in inches, from the rocker panel to the ground. A good wheel alignment charts should provide specs, but the main thing is that the measurements should be within one inch from side to side and front to rear.

 

Riding height is not usually adjustable except on vehicles with torsion bar type springs, coil-over and some air suspensions. 

 

On a nonadjustable type suspensions, springs replacement is best way to fix this problem.

 

Note: Springs should only be replaced in pairs. Changes in riding height affect camber and toe, so if springs are replaced or torsion bars are adjusted, the wheel must be aligned to avoid tire wear.  

 

Set Back

Set back is when one front wheel is set further back than the other. With alignment equipment that measures toe by using only the front instruments, any setback will cause an uncentered steering wheel. Any good 4-wheel aligner will reference the rear wheels when setting toe in order to eliminate this problem.

 

Some good alignment equipment will measure set back and give you a reading in inches or millimeters.
Some manufacturers consider a set back of less than 1/4-inch normal tolerance. More than that and there is a good chance that something is bent.
 

Setback is Caused By: Manufacture or Collision.


Thrust Angle


 

Thrust angle is the direction that the rear wheels are pointing in relation to the centerline of the vehicle.

 

The vehicle will "dog track" if the thrust angle is not zero and the steering wheel will not be centered.

 

The best solution is to first adjust the rear toe to the centerline and then adjust the front toe. This is done during a all wheel alignment if the rear toe is adjustable. If the rear is not adjustable, then the front toe must be set to compensate for the thrust angle, allowing the steering to be centered.

If the thrust angle is not correct on a vehicle with a solid rear axle, it often requires a frame straightening shop to correctly reposition the rear axle.

A vehicles with independent rear suspension, the toe must be adjusted individually until it has reached the appropriate setting for its side of the vehicle, incorrect thrust angle is often caused by an out-of-position suspension or incorrect toe settings.

So in addition to the handling problems that are the result of incorrect toe settings, thrust angles can also cause the vehicle to handle differently when turning left vs. right.

 

Alignment Ranges

The vehicle manufacturers' alignment specifications usually identify a "preferred" angle for camber, caster and toe (with preferred thrust angle always being zero). The manufacturers also provide the acceptable "minimum" and "maximum" angles for each specification. The minimum and maximum camber and caster specifications typically result in a range that remains within plus or minus 1-degree of the preferred angle.

If for whatever reason your vehicle can't reach within the acceptable range, replacing bent parts or an aftermarket alignment kit will be required. Fortunately there is a kit for almost every popular vehicle due to the needs of body and frame shops doing crash repairs and driving enthusiasts tuning the suspensions on their cars.

 

Steering Center

Steering center is that the steering wheel is centered when the vehicle is traveling down a straight and level road. However most roads are crowned to allow for water drainage, this may cause the vehicle to drift to the right so the steering wheel will appear to be off-center to the left on a straight road. to compensate for this

    The left caster should be more negative than the right, but not more than 1/2 degree within the specified range.

    The left camber should be more positive than the right camber. Check the specs to see what the allowable differences.

A crooked steering wheel is one of the most common complaints after a wheel alignment. Steering center is controlled by the front and rear toe settings, when setting steering center, the rear toe should be set first bringing the Thrust Angle as close to the vehicle centerline as possible. the steering wheel is then locked in a straight-ahead position in order to set the front toe. Please note; before locking the steering wheel, the engine should be started and the wheel turned right and left a couple of times. This will take any stress off the power steering valve. Repeat the above starting and turning of the steering after setting front toe to ensure that the steering valve wasn't loaded again due to the tie rod adjustments.

 

Toe Out on Turns

When you steer a car through a turn, the outside front wheel has to navigate a wider arc than the inside wheel. For this reason, the inside front wheel must steer at a sharper angle than the outside wheel.

 

Toe-out on turns is measured by the turning angle gauges (turn plates) that are a part of every wheel alignment machine. The readings are either directly on the turn plate or they are measured electronically and displayed on the screen.  Wheel alignment specifications will usually provide the measurements for toe-out on turns. They will give an angle for the inside wheel and the outside wheel such as 20 for the inside wheel and 18 for the outside wheel. Make sure that the readings are at zero on each side when the wheels are straight ahead, then turn the steering wheel so that the inside wheel is at the inside spec. then check the outside wheel. The toe-out angles are accomplished by the angle of the steering arm. This arm allows the inside wheel to turn sharper than the outside wheel.  The steering arm is either part of the steering knuckle or part of the ball joint and is not adjustable. If there is a problem with the toe-out, it is due to a bent steering arm that must be replaced.

 

 

 

Wheel Offset

 

Offset diagram

The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types.

 

Zero Offset

The hub mounting surface is even with the centerline of the wheel.

 

Positive

The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

 

Negative

The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.

If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly.