12.6 Wheel Alignment

The term alignment means being positioned in a straight line. Relating to vehicles, alignment means to position the tires so they roll freely and evenly over the road surface. The main purpose of wheel alignment is to make the tires roll without scuffing, slipping, or dragging under all operating conditions. Correct wheel alignment is essential to vehicle safety, handling, extension of tire life, and maximum fuel economy.

The different types of wheel alignments are front end alignment, thrust angle alignment, and four-wheel alignment.

Not all vehicles are fully adjustable, so before any alignment always consult the manufacture's service manual. Regular wheel alignments will save you as much in tire wear as they cost and should be considered routine, preventive maintenance.

Steering Geometry

Steering geometry is the term manufacturers use to describe steering and wheel alignment. The six fundamental angles or specifications that are required for a proper wheel alignment are caster, camber, toe, steering axis inclination, toe-out on turns, tracking, and scrub radius.

Caster

Caster is the steering angle that uses the weight and momentum of the vehicle’s chassis to lead the front wheels in a straight path (Figure 12-45). Caster is the backward or forward tilt of the steering axis that tends to stabilize steering in a straight direction by placing the weight of the vehicle either ahead or behind the area of tire-to-road contact.


Figure 12-45 — Caster angle.

Caster controls where the tire touches the road in relation to an imaginary center line drawn through the spindle support. It is NOT a tire wear angle. The basic purposes for caster are as follows:

Caster is measured in DEGREES starting at the true vertical (plumb line). Manufacturers give specifications for caster as a specific number of degrees positive or negative. Typically, specifications list more positive caster for vehicles with power steering and more negative caster for vehicles with manual steering (to ease steering effort). Depending upon the vehicle manufacturer and type of suspension, caster may be adjusted by using wedges or shims, eccentric cams, or adjustable struts.

Negative caster tilts the top of the steering knuckle toward the front of the vehicle. With negative caster, the wheels will be easier to turn. However, the wheels tend to swivel and follow imperfections in the road surface.

Positive caster tilts the top of the steering knuckle towards the rear of the vehicle. Positive caster helps keep the wheels of the vehicle traveling in a straight line. When you turn the wheels, it lifts the vehicle. Since this takes extra turning effort, the wheels resist turning and try to return to the straight-ahead position.

Camber

Camber is the inward and outward tilt of the wheel and tire assembly when viewed from the front of the vehicle. It controls whether the tire tread touches the road surface evenly. Camber is a tire-wearing angle measured in degrees. The purposes for camber are as follows:

Positive and negative camber is measured from the true vertical (plumb line) (Figure 12-46). If the wheel is aligned with the plumb line, camber is zero.


Figure 12-46 — Camber angle.

With positive camber, the tops of the wheels tilt outward when viewed from the front. With negative camber, the tops of the wheels tilt inward when viewed from the front.

Most vehicle manufacturers suggest a slight positive camber setting from a 1/4 to a 1/2  degree. Suspension wear and above normal curb weight caused by several passengers or heavy loads tend to increase negative camber. Positive camber counteracts this.

Toe

Toe is determined by the difference in distance between the front and rear of the left and right side wheels (Figure 12-47). Toe controls whether the wheels roll in the direction of travel. Of all the alignment factors, toe is the most critical. If the wheels do NOT have the correct toe setting, the tires will scuff or skid sideways. Toe is measured in fractions of an inch or millimeters.


Figure 12-47 — Toe-in and out.

Toe-in is produced when the front wheels are closer together in the front than at the rear when measured at the hub height.

Toe-in causes the wheels to point inward at the front.

Toe-out results when the front of the wheels are farther apart than the rear.

Toe-out causes the front of the wheels to point away from each other.

The type of drive (rear or front wheel) determines the toe settings. Rear-wheel drive vehicles are usually set to have toe-in at the front wheels. This design is due to the front wheels moving outward while driving, resulting in toe-out. If the wheels are adjusted for a slight toe-in (1/16 to 1/4 in.), the wheels and tires will roll straight ahead when driving.

Front-wheel drive vehicles require different adjustment for toe, due to the front wheels driving the vehicle and being pushed forward by engine torque. This makes the wheel toe-in or point inward while driving. To compensate for this, front-wheel drive vehicles have the front wheels adjusted for a slight toe-out (1/16 inch). This adjustment will give the front end a zero toe setting as the vehicle travels down the road.

Steering Axis Inclination

Steering axis inclination (SAI) is the angle away from the vertical formed by the inward tilt of the kingpin, ball joints, or MacPherson strut tube (Figure 12-48).


Figure 12-48 — Steering axis inclination.

Steering axis inclination is always an inward tilt regardless of whether the wheel tilts inward or outward.

Steering axis inclination is NOT a tire- wearing angle. As with caster, it aids directional stability by helping the steering wheel to return to the straight-ahead position. Steering axis inclination is NOT adjustable. It is designed into the suspension of the vehicle. If the angle is not correct, then the suspension system should be checked for damaged or worn parts. Replace the parts to correct the problem.

Toe-Out On Turns

Toe-out on turns, also known as turning radius angle, is the amount the front wheels toe-out when turning corners. As the vehicle goes around a turn, the inside tire must travel in a smaller radius circle than the outside tire. To accomplish this, the steering arms are designed to angle several degrees inside of the parallel position. The exact amount depends on the tread and wheelbase of the vehicle and on the arrangement of the steering control linkage. Toe-out on turns is NOT an adjustable angle. If the angle is incorrect, it is an indication of damaged steering components.

Figure 12-49 shows toe-out on turns. Note how each front wheel turns a different number of degrees. This prevents tire scrubbing and squeal by keeping the tires rolling in the right direction on corners.


Figure 12-49 — Toe out on turns.

Tracking

Tracking is the ability of the vehicle to maintain a right angle between the center line of the vehicle and both front and rear axles or spindles (Figure 12-50). (The rear of the vehicle should follow the front wheels.) With improper tracking, the vehicle rear tires do NOT follow the tracks of the front tires. This causes the vehicle body or frame to actually shift partially sideways when moving down the road. Poor tracking will increase tire wear, lower fuel economy, and upset handling.


Figure 12-50 — Tracking.

Improper tracking has many causes, such as shifted or broken leaf springs, bent or broken rear axle mounts, bent frame, bent steering linkage, or a misadjusted front end alignment.

Scrub Radius

The scrub radius, sometimes called steering offset, is the distance between the lines of steering axis and the center line of the wheel at the contact point on the road. These lines are determined by drawing by a line through the center of the upper and lower ball joints, or the center line of the strut all the way to the road. The second line is drawn through the center of the tire, up and down as the tire sits on the road. The point at which these two lines intersect is known as the scrub radius.

The scrub radius is not adjustable and cannot be measured. The scrub radius can be zero, positive, or negative. A zero scrub radius results when the two lines intersect at the road surface. A positive scrub radius means that the two lines intersect below the surface of the road, and a negative means that the lines intersect above the surface of the road.

Negative scrub radius will cause the tire to toe-in during acceleration, braking, and traveling over bumps in the road. Zero scrub radius is acceptable. Positive scrub radius is less desirable because it causes the wheel to toe-out during acceleration, braking, or going over bumps and causes instability of the vehicle.

A bent spindle or changing the tire size can affect the center line location of the wheel. The height of the tire can also change the scrub radius, which can negatively affect the steering control. When larger diameter tires are installed on a vehicle, the scrub radius becomes positive, and the wheels tend to toe-out, which causes the vehicle to wander, handle poorly, and wear out the tires faster.

Wheel Alignment Tools and Equipment

The most basic types of equipment for wheel alignment are the turning radius gauge, the caster-camber gauge, and the tram gauge. These are the least complicated of all alignment equipment and easily illustrate the fundamentals for wheel alignment.

In larger shore facilities these basic types of equipment are normally replaced with a large alignment rack. The alignment rack consists of ramps, turning radius gauges, and specialized equipment for measuring alignment angles.

Turning Radius Gauges

Turning radius gauges measure how many degrees the front wheels are turned right or left (Figure 12-51). They are used when measuring caster, camber, and toe-out on turns.


Figure 12-51 — Portable turning radius gauge.

The portable type turning radius gauges are quite common. However, they are also mounted on alignment racks as integral units.

The front wheels of the vehicle are centered on the turning radius gauges. With the front wheels centered, the locking pins are pulled out, which allows the gauge and tire to turn together. The pointer on the gauge will indicate how many degrees the wheels have been turned.

The procedures for checking toe-out on turns using turning radius gauges are as follows:

  1. Center the front tires of the vehicle on the turning radius gauges and remove the locking pins.
  2. Turn one of the front wheels until the gauge reads 20 degrees.
  3. Read the number of degrees showing on the other gauge. Check toe-out on turns on both right and left sides. Note the readings.
  4. If not within the manufacturer’s specifications, check for bent or damaged components.

Caster-Camber Gauge

The caster-camber gauge is used with the turning radius gauge to measure caster and camber in degrees. The caster-camber gauge either fits on the hub magnetically or may be mounted on the wheel with an adapter (Figure 12-52). Caster and camber are adjusted together since one affects the other.


Figure 12-52 — Caster-camber gauge attached to a wheel.

The procedures for using a caster-camber gauge for measuring caster are as follows:

  1. With the vehicle centered on the turning radius gauges, turn one of the front wheels inward until the turning radius gauge reads 20 degrees.
  2. Turn the adjustment knob on the caster-camber gauge until the bubble is centered on zero. Then turn the wheel out 20 degrees.
  3. The degree marking next to the bubble will equal the caster of that front wheel. Compare the reading to the manufacturer’s specifications and adjust as needed.
  4. Repeat this operation on the opposite side of the vehicle.

The procedures for using a caster-camber gauge for measuring camber are as follows:

  1. With the vehicle on a perfectly level surface, turn the front wheels straight ahead until the turning radius gauges read zero.
  2. Read the number of degrees next to the bubble on the camber scale of the caster-camber gauge. This will show camber for that wheel. If not within the manufacturer’s specifications, adjust the camber.
  3. Double check the caster readings, especially when an excessive amount of camber adjustment is required.

 

Note

If shims are used to adjust camber, add or remove the same number of shims from the front and rear of the control arm. This will keep the caster set correctly.

Tram Gauge

The tram gauge is a metal rod or shaft with two pointers used to compare the distance between the front and rear of the tires of the vehicle for toe adjustment (Figure 12-53). The pointers slide on the gauge so they can be set to the distance between the tires. The tram gauge will indicate toe-out or toe- in in inches or millimeters.


Figure 12-53 — Tram gauge.

The procedures for using a tram gauge for measuring toe are as follows:

  1. Raise the front wheels of the vehicle and rub a chalk line all the way around the center rib on each tire.
  2. With a scribing tool, rotate each tire and scribe a fine line on the chalk line. This will give you a very thin reference line for measuring the distance between the tires.
  3. Lower the vehicle back on the turning radius gauges.
  4. Position the tram gauge at the back of the tires. Move the pointers until they line up with the scribe marks on the tires.
  5. Without bumping the tram gauge pointers, reposition the gauge to the front of the tires. The difference between the lines on the front and rear of the tires shows toe.

If the lines on the front of the tires are closer together than on the rear, the wheels are toed-in. If the lines are the same distance apart at the front and rear, toe is zero. Use the manufacturer’s service manual for specifications and adjustment procedures.

Test your Knowledge

6. Correct wheel alignment is essential to vehicle safety, handling extended tire life and _____?

A. Achieving maximum fuel economy
B. Achieving maximum speed
C. Achieving maximum ride height
D. Achieving maximum turning radius