Understanding alignment angles and how they affect steering and handling is one of the most useful skills you can utilize when diagnosing issues with these systems. Many years ago, performing an alignment routinely involved adjusting the three primary angles of caster, camber and toe, and, as a result, we quickly became familiar with the effects these angles had on vehicle performance.
Today, improvements in vehicle design and suspension technology have eliminated the need, and, in many cases, even the possibility of making these adjustments, with the exception of toe. This has made alignments far easier, but ultimately had a negative effect on what we learn by working in the field. Since we rarely work with alignment angles other than toe, when we run across an unusual problem, it’s easy to find ourselves behind the knowledge curve.
In addition to the measurements of camber, caster and toe that are the most prevalent angles we associate with alignments, there are multiple non-adjustable measurements which affect steering characteristics and handling, and should be checked during an alignment, especially when a vehicle has been in a collision. These measurements can help identify bent or incorrectly installed components and help diagnose unusual problems, as well.
Familiarity with the following measurements and their related effects on steering is a vital aspect of mastering steering and suspension.
Camber
Camber is the angle formed by the inward or outward tilt of the wheel at the top, referenced to a vertical line through the center of the tire. Negative camber is when this line tilts inward at the top, positive when it tilts outward. Camber angles affect ride quality, directional stability and tire wear.
Positive camber improves ride quality and directional stability. Negative camber improves cornering performance, but reduces ride quality and increases wear on wheel bearings and ball joints. Cross camber is the difference in the camber angles from side-to-side, and too much cross camber may cause a pull to the side with the most positive measurement.
Caster
Caster is the angle formed by a line through the pivot axis of the ball joints, compared to a true vertical line as viewed from the side of the vehicle. Negative caster is when this line tilts forward at the top, positive when it tilts rearward.
Caster affects directional stability, steering effort and the rate of steering wheel return. More positive caster improves stability and steering wheel return, but requires greater steering effort and reduced comfort, and can increase tire wear on turns. Even though positive caster can increase tire wear on turns, caster is not considered a tire wearing angle because it is overall not a direct factor related to tire wear.
Toe
Toe is a comparison of the distance between the front and back of the wheels. The purpose of correct toe is so the wheels run parallel while driving. Toe-in is when the front of the wheels are closer together, toe-out is when the front of the wheels are further apart when compared to the rear. Toe affects tire wear, stability and handling performance. Toe can be adjusted without affecting other angles, but all other angle changes affect toe, so it is always the last adjustment.
Toe is reflected in two different ways during an alignment; as individual toe and total toe. Total toe is the difference in measurement between the front and rear of the tires, and it is the specification that affects tire wear, stability and performance. This can be considered the most critical toe setting; however individual toe is still important for the alignment process.
Individual toe is the difference between the front and rear of a tire as compared to a reference line. When performing a four-wheel alignment, front individual toe uses the thrust line (see thrust angle) as a reference. This method of measurement ensures a centered steering wheel. Front individual toe is responsible for steering wheel position.
In other words, total toe and alignment can be correct but the steering wheel can be off center. This was a common problem with a two-wheel alignment. Four-wheel alignment equipment utilizes thrust line measurement and individual toe to ensure the steering wheel is centered when the alignment is complete.
During the alignment process, as you adjust the toe on each side, you will be looking at the individual toe specification. Once both sides are adjusted, the total toe should be within specification.
Steering Axis Inclination
Steering Axis Inclination (SAI) is also known as king pin inclination (KPI) or ball joint angle. It is the angle formed between a line through the pivot axis of the ball joints and a true vertical line. On a strut suspension, it’s the line between the upper strut mount and the lower ball joint. SAI keeps the tire turning on its center point, reducing tire wear and steering effort.
SAI also affects the steering wheel return to center. SAI causes the wheel spindle to angle downward when the wheel is turned (caster affects this as well). The easiest way to visualize this is by turning the wheels all the way in either direction. The vehicle will lift slightly because as it responds to the spindle being angled downward, the wheel is forced downward. When you remove your hand from the steering wheel, it naturally wants to return to the point of least resistance, which demonstrates how SAI affects steering return to center.
Scrub Radius
Scrub radius is the distance between where a line through the pivot axis of the ball joints and a line through the center of the tire meet the road. It can be positive, negative or zero. Positive scrub radius reduces steering effort, but negative can increase stability. Scrub radius must be equal side-to-side, or the vehicle may pull.
Turning Radius
When a vehicle is turning, the inside wheel must follow a smaller circle than the outer wheel. In order for this to happen, the inside wheel must turn sharper. This is an angle that is built into the steering arms, and only bent or damaged suspension or steering components will affect this.
Thrust Angle
Thrust angle is determined by comparing a line drawn perpendicular through the rear axle or the direction the rear wheels are pointed (thrust line), to the geometric centerline of the vehicle.
Thrust angle is one of the most important angles in an alignment. An incorrect thrust angle can cause pulling or adversely affect handling, and since other alignment measurements are based off this angle, it must be correct. Bent or damaged components, or a shifted rear axle or rear cradle can cause incorrect thrust angle.
Included Angle
Included angle is the combination of SAI and camber. It is a calculated angle that is used for diagnostic purposes. By comparing SAI, camber and included angle, it can help you determine whether a component is bent or out of place, for example a misaligned cradle.
Setback
Setback is when one front wheel is set further back than the other. This can because by a damaged suspension component or a misaligned cradle.
Bump Steer
Bump steer is the tendency of a vehicle to steer itself during suspension travel due to changing toe angle. It’s controlled by the relationship between the control arm and tie rod locations, and affected by lowering or lifting a vehicle. On modified vehicles, a bump steer kit relocates the position of the tie rod to control arm, so the arc swing of the tie rod is relocated to the factory location.
Ultimately, it takes practice to become familiar with alignment angles and their effect on vehicle performance, but when diagnosing a particular symptom, utilizing the core knowledge of these alignment angles will help you toward the goal of mastering steering and suspension.
Coming up next in Part 4 of this series, we’ll discuss the workflow of diagnosing a problem from beginning to end, starting with the test drive.
