Performance Wheel Alignment
In order to play with wheel alignment angles in the pursuit of optimum handling, we first need to understand wheel alignment theory. Once you understand the terms and what various angles represent, then you´ll have a better understanding of what is changing as a result of individual adjustments. So we start with the basics.
FOLLOW OE SPECS... UNLESS
If the vehicle in question is to be driven only on the street (and at posted legal limits that we all should observe), in all likelihood the best approach is to follow OE alignment specifications, since these were determined as the best compromise settings for that particular vehicle.
If you wish to tune the suspension to achieve maximum performance under extreme conditions, we need to address both wheel angle settings and tire inflation pressures, since the two go hand-in-hand.
Bear in mind that if we alter the original ride height of a production vehicle, we change the wheel articulation (the geometry that the hub follows during compression and unloading). While accurate wheel alignment to factory specs is critical for tire wear and the safe handling and braking of any stock production vehicle, attention to "practical" wheel alignment angles is needed when altering the vehicle and/or placing the vehicle in extreme performance situations. In other words, align to fit the job at hand.
Viewing the tires from a head-on position (staring at the tread), the camber angle refers to the "lean" of the tire in comparison to a true vertical line drawn through the hub center. If the top of the tire leans inward, toward the center of the vehicle, this is negative camber. If the top of the tire leans outward, this is positive camber. If the tire is positioned straight up, and parallel to the true vertical, this is zero camber. Camber is measured in degrees and minutes of degrees.
The camber angle affects both tire tread wear and vehicle control. Our goal is to adjust camber in order to achieve a maximum tread contact patch when and where this maximum contact patch is needed.
This will vary according to the vehicle in question and the manner in which the vehicle is operated. For instance, if the vehicle is primarily driven on highways at speed for long periods of time, the camber angle needs to allow the tread to create its largest contact patch under those conditions. Depending on the specific vehicle, this might involve a zero camber angle or a slightly positive camber angle.
Remember, if the rear suspension provides camber adjustments, the rear wheel camber angles must be set to achieve the same goal -- maximum tread contact under the driver´s primary conditions.
If the vehicle is normally, or routinely, driven under changing lateral conditions (on curvy, twisty roads or on a road course), the alignment angles will require a bias toward negative camber. When the vehicle enters a turn, centrifugal force naturally moves the weight mass toward the outside of the vehicle (if turning right, weight is moved to the left; if turning left, mass momentum moves toward the right). This causes the tires to flex both downward (pushing the tire section toward the pavement) and laterally (to the outside, causing the sidewall to allow the tire to lean).
If the static camber was set at zero, for example, a hard right turn might cause the outside shoulder of the left front tire to bear the brunt of the weight and scrubbing action, while a large percentage of that tire´s center and inside tread area is lifted off the pavement. The result: You´re only using a very tiny portion of the tread (traction capability), while placing isolated wear on the outer shoulder. This causes lousy handling in the turns and tires with worn-out outer shoulders.
In order to compensate for this type of driving (lots of left/right turns), we adjust the camber angle accordingly in order to benefit from maximum tread contact in those turns. Once again, our goal is simply to create a maximum tread contact patch under the vehicle´s "typical" operating conditions.
Viewing the vehicle from the side, the caster angle (also called the steering axis) represents the angle created by drawing a straight line through the upper and lower suspension pivot points, relative to a true vertical line drawn through the hub center.
If the front suspension features a twin-arm suspension design, the steering axis is a line drawn to intersect the upper and lower ball joints. If the front suspension features a strut, the line is drawn through the upper strut bearing centerline and the lower ball joint. This angle, created by comparing it to the true vertical, is the caster angle.
If the upper ball joint (or strut bearing pivot) is located behind the lower ball joint, this creates a positive caster angle. If the upper ball joint or strut bearing is located forward of the lower ball joint, this is negative caster. If the two ball joints are vertically aligned, this is zero caster. Caster is measured in degrees, and has an enormous impact on both steering effort and high-speed stability.
The more positive the caster angle, the more stable the vehicle will be at speed. However, as we increase positive caster, this creates a greater demand for steering effort input. If the front suspension featured zero or negative caster, it would be easier to steer, but high-speed stability and directional control would suffer.
A compromise must be found between steering effort and control at speed. Generally, all vehicles will feature a positive caster angle. The amount of positive angle will vary among makes and models, and in some cases may not be readily adjustable (especially with many MacPherson strut-equipped production vehicles).
The caster angle will automatically change as a result of changes to ride height or camber. On a production vehicle that offers no caster adjustment, you may have no choice but to live with the increase or decrease in caster angle. If the suspension features twin (upper and lower) arms, caster adjustment likely will be offered.
The rule-of-thumb, whether ride height and alignment angles have been modified from the production values or not, is to dial the caster angle as close to OE specs as possible (in other words, try to keep caster constant during static alignment adjustment).
The toe angle refers to the difference in distance between the fronts of the tires and the rear of the tires on the same axle. In simple terms, if viewed from overhead, if the two front tires are closer together at the front of the tread than at the rear of the tread, this is a toe-in (also called positive toe). If the front of the tires are farther apart at the front and closer together at the rear, this is toe-out (also called negative toe).
If the two tires on the same axle are absolutely parallel to each other, this is zero toe.
Individual (right or left) toe angles always are adjustable on the steering axle, and usually adjustable on independent rear axles.
The purpose of the toe angle is to reduce rolling resistance, reduce tire wear and to aid in directional control. Too much of either toe-in or toe-out will create poor directional control in terms of wander, and the driver may feel a "shimmy."
In terms of tire wear, if tires are excessively toed-in, premature outer tread wear will be noticed, along with a feathering across the tread (the high points of the feathering will aim inboard). If we have too much toe-out, the outer tread areas will wear quickly and the feathering will be aimed outboard.
Our very basic goal in terms of toe adjustment of the front wheels is to provide zero toe during straight-line operation, which will be proper in most instances (certainly all instances on the street). Since we need to consider suspension pivot point movement on production vehicles, due to bushing compliance, we usually need to adjust in the opposite direction in order to compensate for anticipated movement.
For instance, when a rear-wheel-drive vehicle accelerates and moves forward, the leading face of the front wheels will tend to splay outward as the front axle is pushed forward. Because of this, we may need to adjust the front toe slightly inward (a static toe-in), in order to actually see a zero-toe result once the vehicle moves forward.
If the vehicle is rear-wheel driven, we would then expect the leading face of the front tires to "crawl" inboard upon acceleration, so we might set front static toe slightly outward (toe-out) to compensate. Again, our goal is to achieve zero toe as the vehicle is driven down the highway.
The subject of four-wheel alignment can and should require it´s own article, but we´ll touch on the basic points here. There are three approaches to wheel alignment: two-wheel, four-wheel thrust line and "total four-wheel" alignments.
1. Two-wheel alignment (also called geometric centerline) is an old, and far too basic, approach that should not be used, because it ignores the geometry picture created by the position of the two rear wheels (it only references the "center" of the rear axle, which has nothing to do with the angle of the rear wheels).
2. Thrust line alignment does consider the "thrust" direction of the rear wheels (viewed from overhead, we consider the total toe of the rear wheels). The direction of the rear wheels might aim forward (parallel to the geometric centerline) or it might angle off right or left. If it angles off, the total toe of the front wheels are adjusted to compensate.
If the thrust angle aims to the right, this is called positive thrust angle. If it aims to the left, it´s negative thrust angle.
If the thrust line aims to the right, this creates a thrust angle that would cause the vehicle to pull to the left. To compensate, the front total toe can be set to make the front wheels parallel to the thrust line of the rear wheels, even though the rear axle is "crooked" relative to the centerline of the vehicle.
3. If the vehicle offers toe adjustment at all four wheels, then we can perform the best type of alignment -- a "total" four-wheel alignment, in which the rear thrust angle is corrected to zero. Our goal is a simple one: to make all four wheels travel parallel to each other on a straight road.
TOE-OUT ON TURNS
Even though your front toe may have been statically set at zero with the wheels aimed straight ahead, individual wheel toe will change when the wheels are turned, simply due to the arc or travel around the steering axis.
When the steering wheel is turned, the front wheel on the inside of the turn will toe-out farther than the outboard wheel. This change in toe is designed into the suspension geometry. If the inside wheel did not toe-out further, the vehicle would tend to push badly (understeer).
STEERING AXIS INCLINATION
While not readily adjustable, steering axle inclination, or SAI, is a fixed angle that is created by the relationship between the steering axis and a true vertical when viewed from the front of the vehicle.
In simpler terms, if you look at the front of the vehicle, draw an imaginary line through the upper and lower ball joints (or from the top bearing center of the strut through the lower ball joint), and continue that line through the road surface. Then draw a true vertical line through the center of the tire tread where the tread touches the road. The created angle (the difference between the two lines) is the SAI.
Again, this angle is non-adjustable. It´s simply one of the references to check to see of the suspension is screwed up and a suspension part is bent. For further damage inspection, we can measure "included angle (IA)," which considers the combination of SAI and the actual wheel camber angle. This tosses camber into the SAI picture to further aid in diagnosing bent or improperly assembled components.
Scrub radius is the width of the area, at the ground, between the SAI line and the center of the tread. If these two lines intersect above the tread-to-ground area, this is negative scrub. If the lines intersect below the ground surface, this is positive scrub. If the two lines intersect at ground level, this is zero scrub.
Scrub radius involves the "pivot point" created by the force of the load and the steering axis. Most MacPherson strut front suspensions will feature a negative scrub radius. This helps to reduce the effect of torque-steer, which can be prevalent on many front-wheel-drive vehicles. If the front suspension features upper and lower control arms (a short arm/long arm, or SLA design), the scrub radius is likely to be positive.
In terms of modifications, the scrub radius easily can be affected. For instance, if you use eccentric bushings or eccentric "cam" bolts at the lower strut-to-steering knuckle connection in order to obtain negative camber, scrub radius will increase, but probably not enough to cause a steering effort problem.
If you install a grossly negative offset wheel (where the tire is moved severely outboard), this increases the scrub radius in a positive direction. Depending on how far the tread centerline is extended, the increased scrub radius can affect steering effort, handling and overall stability, not to mention improper loading of the wheel or hub bearings. You simply need to be aware of the many variables that can change as the result of a seemingly simple modification.