Wheel positioning determines a number of performance aspects, including tire wear, braking control, vehicle stability in both straight lines and in turns, steering ease and overall vehicle 'handling.' For all of these reasons, it's imperative to gain an understanding of wheel angles, their purpose and their usefulness in obtaining proper chassis geometry.
While a shop may possess the most advanced computerized alignment equipment that requires no calculations on the part of the technician, it is nonetheless important to understand the basics. With this knowledge, the technician can more accurately diagnose problems and solve customer complaints.
Prior to performing any alignment job, the very first step involves measuring the vehicle's ride height. With the vehicle parked on a reasonably level surface, measure the body height at the specific points recommended by the vehicle maker, and refer to the specs supplied by your suspension parts supplier or your alignment equipment manufacturer. If the height is off-spec, spring replacement likely is needed, and must be done before the wheel alignment.
Spring replacement is probably one of the most overlooked and ignored aspects of chassis service. If the customer informs you that the car "seems to ride well enough and doesn't make any weird noises," don't automatically ignore the issue. Make an attempt to educate the customer: Tell the customer that although you can adjust the wheel angles to spec while the car is sitting in a parked condition, those angles change as the car travels down the road.
Inadequate ride height will compound the problem, allowing camber, caster and toe to travel beyond the optimum limits. The result is increased tire wear and possible unsafe handling. Although some customers may view this 'new' information as a scare tactic (they may be on the defensive at this point, suspecting that you're only trying to sell more parts), take the time to explain the geometry changes that take place. Ride height is the groundwork upon which wheel alignment takes place. If ride height is incorrect, there's no guarantee that the alignment will solve all of the vehicle's handling, ride and tire wear problems.
The toe reading is generally measured in fractions or tenths of an inch. The toe angle, as viewed from overhead, refers to the difference between the front tread centerline to the rear tread centerline of the two tires on the same axle.
As viewed from overhead, toe dictates whether or not the tires roll straight down the road. We can refer to toe as the 'angle of attack' for the tires as the vehicle is driven forward. If the front of the tires points towards the centerline of the vehicle, with both tires aiming inboard, this is called 'toe-in,' because the tires on that axle are aiming inboard. If the front of the tires is aiming in an outboard direction, away from the centerline of the vehicle, this is referred to as 'toe-out.'
Speaking in generalities, rear-drive vehicles usually require that front toe is set at a slight toe-in, usually 1/16 inch to 1/8 inch. This is done to anticipate the likely toe-out movement as the vehicle travels forward, due to the compliance in the front suspension bushings. Many front-drive vehicles often specify front toe at a toe-out setting, in anticipation of forward acceleration wherein the front tires may 'crawl' inboard. However, you should never assume that all front-drive vehicles use a static toe-out, because some require a toe-in, due to the design of the front suspension and the compliance movement of the suspension bushings.
In either case, the toe setting that will be achieved during the alignment is designed to compensate for the expected dynamic action during the vehicle's forward operation. In order to travel down the road in a theoretically perfect 'zero toe' condition, you must first compensate by adjusting the front wheels with a slight toe-in or toe-out, knowing that the front wheels will try to splay in or out to a zero toe when in motion. The object is always to achieve a zero toe angle as the car travels forward in a straight line.
In some cases, the rear wheels are adjustable for toe angle as well. Common examples involve independent rear suspensions like Corvette, Jaguar, Datsun Z and those with trailing axles as found in front-wheel drive (FWD) cars.
Toe adjustment at the front is performed easily by adjusting the threaded adjuster sleeves on multilink steering systems or the tie rod ends on rack systems. Rear toe on some adjustable designs require rotation of cam bolts, the use of special shims or in a few cases, attention to threaded adjuster rods.
Toe-out on turns -- also called the turning angle -- is measured during a toe check on an alignment rack. This is a look at how the inside wheel toes-out during a turn, compared to the outside wheel. This is a fixed angle, determined by the steering knuckle or 'arm.'
The camber angle refers to the inward or outward tilt of the tire when viewed from the front or rear of the vehicle. Negative camber exists when the top of the tire leans inward. If the top of the tire leans outward, positive camber exists. If the tire is positioned at a true vertical, it's set at zero-camber. Camber angles are measured in degrees. The camber angle is adjusted to maximize tire wear, handling and directional stability.
The majority of passenger vehicles are designed to use a slight positive camber angle at the front wheels. This is done to reduce steering effort, to increase highway-speed directional control (less road wander at speed), and to compensate for the added weight of driver and passengers. It's common for performance drivers and road-course competition drivers to request a negative camber at the front wheels, in order to maximize tire tread contact in severe turns. Though the front wheels may be set in a severe -- say, three degree -- negative camber and tire tread contact may be diminished when the vehicle travels straight (chances are, the inner tread only will actually contact the road), when the vehicle enters a hard right hand turn, the tire tries to flex and 'roll-over.'
The result is full-tread contact of the left front tire during the turn. With full-tread contact in a hard turn, the driver maximizes his tire 'bite,' and is able to better grip the road surface. Keep in mind that this is primarily true only in dry road conditions. In severe wet-surface conditions, there may not be sufficient tire grip to generate enough rollover to obtain a full-width tread contact patch. In that case, a camber setting closer to zero would likely perform better. However, because it's impractical to readjust the alignment every time the weather changes, performance drivers must choose between an optimum dry or optimum wet weather setting, or make a slight compromise between the two optimum settings.
Adjustment of the front axle camber angle varies with suspension design. On double-A-arm (upper and lower control arms) designs, the upper control arm's frame may require that flat shims be added or removed to adjust camber. If the upper control arm's pivot shaft is located outboard of the frame, adding shims will move the wheel in a positive direction, while removing shims will move the wheel in a negative direction. If the pivot shaft is located inboard of the frame, shims are added to create negative camber and removed to create positive camber.
On strut-type front suspensions, camber adjustment may take place at either the top strut tower (moving the top of the strut inboard for negative camber or outboard for positive) or at the bottom of the strut bracket where it bolts to the spindle. Far too often, vehicle manufacturers offer little or no camber adjustment provision. In this case, the use of aftermarket camber plates at the top or eccentric bolts or bushings at the bottom are required. Usually, if the adjust-ment is to be made at the lower strut bracket, the upper mounting hole will be the stationary point, while the lower hole will provide angle movement.
On vehicle with I-beam front suspensions, one practice is to adjust camber by bending the I-beams. Though some debate always has existed regarding this practice, special bending fixtures are available and the procedure can be successful in trained hands. However, this is not a job for the novice. If you plan to purchase a bending machine, make sure that you receive the proper training in its use.
On vehicles with rear camber adjustment (rear axles on some FWD cars, and independent rear suspensions), camber may be adjusted with the use of shims at the hubs or by adjusting eccentric pivot bushings.
It's important to realize that any change to camber will affect the toe setting. If the steering arm is located in front of the front axle centerline, the toe will move out as negative camber is induced; the toe will move inboard if positive camber is induced. When the steering arms are located behind the front axle centerline and you change to a more negative camber angle, toe will move inboard. If a more positive camber angle is achieved, the toe will move outboard. The point is to always check and readjust toe whenever a camber change is made. You cannot alter camber in any way without affecting the toe setting.
Caster refers to the tilt angle of the front spindles, as viewed from the side of the vehicle. It involves the relationship between the top of the spindle and the bottom of the spindle at its attachment and pivot points. Imagine a front suspension with upper and lower control arms. As viewed from the side of the vehicle, the caster angle refers to the location of the upper ball joint to that of the lower ball joint. If you draw an imaginary line from the lower ball joint to the upper ball joint, that angle represents caster. In the case of a strut-equipped front suspension, you can imagine a line drawn from the lower ball joint to the top of the strut tower.
When the top of the spindle is set back (placing the upper ball joint closer to the rear of the vehicle, with the lower ball joint closer to the front of the vehicle), this is called a positive caster angle. All vehicles use a positive caster angle.
The caster angle -- the reason we have an offset angle between upper and lower spindle points -- exists to allow optimum steering and handling for a given vehicle. A positive caster angle is used to allow predictable steering of the vehicle. If the caster was zero with the upper ball joint exactly above the lower ball joint, steering would be erratic, the vehicle would be very hard to control and the front wheels would wander all over the road. By creating a backward 'lean' (a positive caster angle), the vertical travel of the front wheels is 'dampened.' The more positive the caster angle, the more controllable the vehicle is at higher speeds. As caster is lessened (closer to zero), the vehicle's front wheels will react faster to steering input, but highway stability will be decreased.
Caster settings on many vehicles are fixed and can't be easily adjusted independently. Replacement of damaged parts is commonly required in some cases. Front double-A-arm suspensions allow easy caster adjustment at the upper control arm fulcrum shaft with the use of shims. Strut suspensions may be adjusted through use of camber/caster top tower plates.
Since camber and caster are both affected by changes at the same mounting points, a change in caster can affect camber, and vice-versa. In the case of an upper control arm fulcrum shaft adjustment via shims, adding or removing exactly the same thickness of shims at both front and rear of the shaft will only affect camber. By using a different thickness of shims at front and rear of the shaft, both camber and caster are affected.
Steering axis inclination (SAI)
The SAI is a fixed angle, determined by the spindle or the MacPherson strut. As viewed from the front of the vehicle, this refers to the relationship of two lines: From the spindle attachments (the line drawn from the upper to the lower ball joints, or from the lower ball joint to the strut tower) to a reference point. Depending on the type of alignment equipment being used, this reference point will either be a true vertical that passes through the center of the wheel hub, or the true centerline of the tire, from top center of the tread to the bottom center of the tread. It sounds confusing, but all we're doing is checking these two lines to make sure the spindle or control arms haven't been bent. A change to the specified SAI simply indicates that damage has occurred and parts need to be replaced.
This is simply an extension of reading the SAI. A change in scrub radius simply means that the camber is incorrect or damaged parts are in place. Viewed from the front of the vehicle, this is basically the measurement of distance between the camber angle of the wheel and the line drawn through the spindle's upper and lower ball joints, measured directly at the point where the tire actually contacts the road surface.
The need for four-wheel alignment
Dealing with rear axle thrust is vital in any alignment job. Addressing the issue of rear axle placement (thrust) has always been critical in vehicle alignment. While not always heeded in years past, its importance has been underscored with today's fleet of lighter, unitized body vehicles.
Essentially, we can consider three types of alignment approaches: two-wheel (also known as geometric centerline), thrust line and total four-wheel.
First, let's start with definitions of the terms we'll use in this article. Viewed from above the vehicle, the 'geometric centerline' is an imaginary line drawn from the center of the rear axle to the center of the front axle. This line simply follows the centerline of the vehicle chassis, from the halfway point between the rear wheels to the halfway point between the front wheels.
The 'thrust line' denotes the actual direction of the rear wheels (a right-pointing thrust line or a left-pointing thrust line). This is the real-world direction that the rear wheels are aimed, irrelevant of the geometric centerline. You also can view the thrust line as a line that divides the left and right rear wheel toe.
'Thrust angle' is the angle formed by comparing the chassis geometric centerline to the rear wheel thrust line. This angle is measured in degrees. A right- aimed thrust angle that deviates from the centerline is referred to as positive, while a left-aimed thrust angle is negative.
'Centerline steering' simply refers to a level steering wheel position when the vehicle travels straight ahead. A non-centered steering wheel is an indication of a probable thrust angle problem.
The desired result -- regardless of what type of vehicle is involved or what alignment approach is used -- is always the same: to create a parallel direction of travel for both front and rear wheels when the vehicle travels in a straight line.
In order to achieve this wheel-parallel state, there are three types of alignment approaches. Let's examine each.
Centerline two-wheel alignment
A 'geometric centerline' alignment involves aligning only the two front steer wheels, using the vehicle's geometric centerline as the only reference. This is the old-school approach to wheel alignment, and is now considered obsolete.
When you 'align' the wheels on a vehicle, you are adjusting the wheel direction based on a point of reference. That point of reference is critical, because it provides the entire basis of your alignment work. If you only refer to the geometric centerline, you're not considering the thrust direction of the rear wheels at all, and that's a critical mistake.
A thrust condition always causes the front wheels to steer into the direction of the thrust line in order to retain vehicle direction. If the front wheels are not adjusted parallel to the thrust line, constant steering input is needed by the driver, premature tire wear on both front and rear will result and poor directional control is a constant.
The center of the rear axle (the halfway point between the two rear wheels) can be located with total disregard to the rear axle setup. The rear axle may be offset from center, or it may be 'crooked,' causing the rear wheels to point right or left. When you only consider the geometric centerline of a vehicle as the point of reference for a front wheel alignment, you're assuming that the rear axle is located where it's supposed to be, and that's a big assumption. It's like trying to plot the shortest route from a specific address in Cleveland to Europe. In order to create an accurate point of reference, you need a specific address in a specific country. In other words, two-wheel geometric centerline alignment cannot be trusted.
'Thrust line' alignment considers the actual location and direction of the rear wheels. Never assume that the thrust line is parallel to the geometric centerline. When adjusting the direction of the front wheels, the front-wheel alignment should be set parallel to the direction of the rear wheels, in reference to the actual thrust line.
When aligning the front wheels on a vehicle that offers no rear-wheel adjustment, setting the front wheels according to the thrust line is the only accurate method of front-wheel alignment. The thrust line may not be parallel to the vehicle body, but at least you can make the front wheels parallel to the rear wheels, and that is the most important job. It's called doing the best you can, given the limitations set by either poor OE vehicle assembly or previous vehicle damage.
Check vehicle ride height and correct per customer agreement.
Always check and correct tire inflation.
Inspect tire condition and replace/rotate as needed. If tires or wheels are unidirectional, follow proper rotation procedure.
Check tire sizes. Brand and model must be same at all four wheels. Sizes must be the same on each axle.
Inquire about the operating condition. If the customer normally drives the vehicle heavily loaded, make sure similar load is in place during alignment.
Inspect steering components for looseness, wear, damage and improper installation. Remember, a customer may have had suspension work performed by another shop or may have attempted repairs on their own.
Inspect suspension parts for damage and wear.
Inspect the brake system for potential causes of vehicle pull in addition to an overall system check. This is a good time to perform a basic safety check, too. Check calipers, hoses, hard lines, master reservoir, etc.
Check driveline items such as driveshafts, U-joints and CV joints. A vibration complaint may be caused by a worn joint or by a driveshaft that has lost a balancing weight.
If the vehicle features front-wheel drive (FWD), check for evidence of recent engine or transmission work. The engine mounting cradle may have been reinstalled incorrectly, causing a pull. Remember, engine cradle placement also can affect front suspension geometry by creating incorrect/uneven steering axis inclination or included angle, which makes it impossible to perform a quality alignment.
Question the customer; ask when the problem started. If a FWD owner tells you that the car began to pull to the right or left after a clutch job, this is a tip-off that the engine cradle may be misaligned. If the vehicle is wearing out its tires prematurely, ask if the vehicle was recently used to carry heavy loads, etc. The more you can learn from the customer, the better armed you'll be for your diagnostic work.
If the vehicle is equipped with an electronic leveling system which lowers the vehicle at high speed and raises it during low speed, be sure to read the shop manual for the correct prealignment procedure. You'll likely have to neutralize the system's compressor, because the vehicle must remain at a specific ride height level in order to properly read and adjust the wheel angles. Naturally, follow the recommended procedure before attempting to measure ride height.