Wheel Alignment Explained
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.
The primary static suspension angles that need to be
measured and adjusted are caster, camber, toe and thrust angle.
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.
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 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.
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.
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 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 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 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 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.
alignment equipment will measure set back and give you a reading in
inches or millimeters.
Setback is Caused By: Manufacture or Collision.
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.
The hub mounting surface is even with the centerline of the wheel.
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.
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.