There’s more to wheel alignment than following the prompts that appear on a computerized alignment machine. While today’s very helpful wheel alignment systems make it easier and faster to achieve “factory” alignment, it’s important to gain a basic understanding of the terms and geometry involved in wheel angles and the results of angle adjustment.
Types of wheel alignment
The “original” approach of measuring and diagnosing vehicle wheel alignment is referred to as centerline two-wheel alignment, which basically allowed you to only measure (and adjust) front or steering axle wheel positions. This now-outdated method does not factor-in the rear wheel positions and isn’t effective, because it ignores the thrust direction of the rear axle.
The current accepted approach considers the actual location and direction of the rear wheels (even if the rear wheel angles are not designed to be adjustable). This method allows you to include and measure the rear axle thrust line/thrust angle. As a result, this allows you to adjust the front wheel angles relative to the rear wheel angles, regardless of geometric centerline. In the case of a vehicle that does not feature adjustment of the rear wheel angles, this allows us to perform a “four-wheel thrust
line” alignment.
If the specific vehicle at hand features an independent rear wheel toe adjustment, we can achieve optimum wheel alignment using the total four-wheel alignment approach, by referring to and adjusting the vehicle thrust angle to as close to zero as possible.
If the vehicle’s rear axle thrust angle is “off zero,” this can result in “dog-tracking” (where the vehicle body appears cocked/crooked relative to direction of vehicle travel), which in turn can contribute to increased tire wear and unequal left/right turning. If the rear wheel angles are adjustable, this allows us to perform a “total four-wheel” alignment.
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Understanding basic wheel angles
Toe angle
Wheel toe angle is represented by the relationship of the right and left wheels on the same axle, as viewed from overhead. Toe angle is measured by comparing the distance between the center of the front of the tires to a distance between the centers of the rear of the tires on the same axle.
“Toe-out” (negative toe) is present when the wheels are further apart at the front and closer together behind the axle centerline. “Toe-in” (positive toe) is present when the two wheels on the same axle are closer together at the front and wider apart at the rear. A toe-in condition is also called positive toe angle.
When the measured distance between the front of the wheels (ahead of the steering axle centerline) is identical to the distance between the wheels behind the axle centerline, this condition is called “zero toe.”
All front suspensions, regardless of design, feature toe angle adjustment, at a location on the steering tie rods/tie rod ends. Live rear axles will feature no toe angle adjustment, since this is a fixed angle. Independent rear suspensions usually feature rear wheel toe adjustment.
The toe angle effects directional control, turning response and tire tread life. Toe-related tread wear will cause a “feathering” wear pattern across the tread. If too much toe-in is present, the feathering will angle toward the center of the vehicle. If too much toe-out is present, the feathering will angle toward the outside of the vehicle.
Because of the compliance in control arm bushings and other dynamic variances in the suspension and steering system, the goal is to establish a static toe angle that will result in a zero-toe condition when the vehicle is driven down the road in a straight line. Depending on compliance issues and suspension design, the initial, or static toe setting may be slightly positive or slightly negative to compensate for toe movement when the vehicle moves forward.
Camber angle
As viewed from the front or rear of the vehicle, camber angle refers to the “lean” of the wheel from top to bottom. A wheel that leans outward at the top (compared to a true vertical) features positive camber.
A wheel that leans inward at the top features negative camber. If the wheel/tire is oriented following a true vertical, this is called zero camber.
Camber angle must always be adjusted to maximize the tread contact patch based on the driving requirements. In most cases, OEM specifications will recommend a slightly positive or zero camber angle in order to maximize tire wear and traction, and to provide easier steering and greater resistance to directional “darting” in a straight line.
If the wheel/tire features a static negative camber angle (vehicle sitting idle), this places more tread load at the road surface on the inner shoulder/tread area. Negative camber is regularly employed on performance vehicles (especially race cars on road courses) in order to increase the tire contact patch during hard turns. Since lateral loading (when the car goes into a hard turn) will try to push the top of the inside tire outward, adequate negative camber may be dialed in to compensate for this. So, while the front wheels may display negative camber as it rolls straight, when it goes into a hard turn, the wheel facing the direction of the turn will try to “straighten up,” achieving maximum tread contact with the road. If the camber angle isn’t sufficiently negative, this tire would lean too far, causing the inside of the tread to lift and placing excess stress and load only on the outside of the tread and outer shoulder.
Either by using OEM adjustment provisions or with the use of aftermarket “custom” adjustment components, all front suspension camber angles may be adjustable. If an upper/lower control arm is involved, the upper or lower arm will be adjustable by either adding or removing adjuster shims between the upper arm and frame, or by rotating an eccentric shaft or eccentric washers. In some cases, the lower arm may feature adjustability via eccentric shaft/washers. If more negative camber is required, the upper arm would move further inboard, or the lower arm would move further outboard. If more positive camber angle is needed, the upper arm would move outboard, or the lower arm would move inboard.
On strut-equipped vehicles, camber may be adjustable in one of two ways: by adjusting the top of the strut mount inboard/outboard at the upper towers, or by adjusting an eccentric at the lower mount, where the strut attaches to the steering knuckle upright. If the vehicle’s OEM design does not provide adjustment, aftermarket adjusters/kits are readily available for either top-strut or bottom-strut applications.
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Rear camber may or may not be adjustable, depending on the type of rear suspension. If a live axle is present (a rigid one-piece axle housing on a rear-drive vehicle), camber likely won’t be adjustable. However, if an independent rear axle is featured, camber should be adjustable either via eccentric bushings at the inboard control arm pivot points, or by means of an eccentric at the strut-to-rear upright. If adjustment is available (either through OE design or with the use of aftermarket adjusters), it’s best to always adhere to OE specifications for street driving.
The only need to vary this is if the vehicle is being set up for competition use, in which case the same rules apply that are required up front — to obtain the maximum tread contact patch based on track requirements.
Camber angle directly affects tire wear, since the camber angle may contribute to excessive inner or outer tread wear if not adjusted properly.
Caster angle
Caster angle is represented by a straight line drawn through the upper ball joint/pivot location through the lower ball joint (as viewed from the side of the vehicle).
Caster angle is measured in degrees. The caster angle is a major contributor to directional control. A too-small (not positive enough) caster angle may make the vehicle too twitchy (but would require less driver input to turn the wheel), especially as speed increases. In theory, the greater the caster angle, the more directional control you’ll have at higher speeds (which also requires slightly more driver input at the steering wheel). However, all suspension systems are designed to perform best at a specific caster angle, so always follow the OE specification in order to achieve the correct balance between turning effort and vehicle directional control.
Most vehicles feature a “positive” caster angle, where the upper suspension pivot point is located behind the lower pivot point (again, as compared to a true vertical). If the caster angle is zero (where the lower pivot is directly below the upper pivot), directional control would suffer, and there would be little if any steering wheel return, requiring the driver to manually drag the wheels back to a straight ahead direction following a turn.
Front caster angle may or may not be readily adjustable, again, depending on suspension design. If the front suspension features upper and lower control arms, the upper arm will likely be adjustable, either via the addition or removal of shims (between the upper arm and frame) or via eccentric bushings. If an upper/lower control arm system is featured, the two anchoring locations (where the upper arm attaches to the frame) can be adjusted (again, with shims or eccentrics). To alter camber, the adjustment must be performed equally at front and rear attachment points, in order to move the upper arm pivot inboard or outboard. If caster is to be adjusted, only one end would be adjusted.
If the vehicle features MacPherson type struts, since the top strut mount serves as the upper locating point (serving as the “upper ball joint”), the top of the strut can be moved forward or rearward to alter the caster angle. Commonly on most strut-equipped vehicle, no OE caster adjustment is readily offered. However, aftermarket adjustable strut mounts are available which allow you to move the top of the strut fore/aft and inboard/outboard for caster or camber.
While the caster angle itself is not a direct tire-wear angle, improper caster angle can contribute to excessive tire wear in conjunction with improper camber and toe angles.
Steering axis inclination (SAI) and included angle
Commonly referred to as SAI, steering axis inclination is the angle between a true vertical drawn through the center of the tire and a line drawn through the upper and lower ball joints when viewed from the front of the vehicle. SAI is a built-in angle designed into the suspension system.
Included angle (IA) represents the combination of SAI and wheel camber. Both SAI and IA are measured to verify that the fixed angles (those angles that exist by design) are correct. If SAI or IA are outside of the OE specification, it’s apparent that a chassis location has been damaged (for example, a strut is bent, the strut tower has deformed, a lower control arm is bent, etc.).
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Scrub radius
The scrub radius refers to the “fulcrum point” created by the force of the load and the steering axis. In simple terms, the scrub radius represents the distance from the king pin axis and the center of the tire contact patch. As viewed from the front of the vehicle, this is determined by considering the distance between the center of a front tire tread and the imaginary SAI line, when measured at the road surface. Since these two lines will eventually intersect, it’s this intersection point that we’re really interested in.
When the two lines crisscross exactly at the road surface, this is known as zero scrub. When the lines crisscross above the road surface, this condition is known as negative scrub. When the lines intersect below the road surface, the condition is called positive scrub.
An excessively negative scrub radius will require greater steering effort, while excessive positive scrub radius (where the tread center essentially moves further outboard) can not only affect handling and ease of steering, but can over-stress wheel bearings.
In most cases, twin-control-arm suspensions (that feature less caster angle and SAI) will benefit from a positive scrub radius.
Be aware that if a customer has added aftermarket wheels that feature a different offset, or has added wheel spacers, he has altered the scrub radius. This can result in complaints of a “squirrely” or “grabby” feeling during turns as the tire contact patches squirm.
Excessive positive or negative scrub radius will result in increased steering effort and tends to induce more road shock.
Centerline steering
This is a term that refers to a “straight and level” steering wheel position when the vehicle rolls in a straight line on a level road surface. If the steering wheel is not centered, this may indicate a possible incorrect off-zero thrust angle at the rear axle.
Understanding turning radius and toe-out in turns
When the front wheels are turned (left or right), the front wheel angles travel in a arc, primarily due to the caster angle and SAI). During turning, individual wheel toe angle will change as compared to the straight-ahead static setting.
For example, when the steering wheel is turned to the left, the left front wheel will exhibit greater toe-out as compared to the number of degrees that the right front wheel toes-in. This design characteristic decreases the turning radius of the vehicle (makes turning easier and allows the vehicle to make tighter turns) and helps to prevent tire scrubbing during turns.
Geometric centerline, thrust line and thrust angle
The geometric centerline is a line drawn from the center of the rear axle to the center of the front axle, as viewed from above the vehicle.
The thrust line refers to the “aimed” direction of the rear axle. This is different than the geometric centerline. The thrust line effectively divides left and right rear wheel toe. The thrust line may or may not follow the geometric centerline.
The thrust angle is the difference between the geometric centerline and the thrust line, measured in increments of degrees. If the thrust angle aims to the right, this is called a positive thrust angle. If the thrust angle aims to the left, this is negative thrust angle. A positive thrust angle will try to steer the vehicle to the left, while a negative thrust angle will try to steer the vehicle to the right. This will cause the driver to pull the steering wheel right or left to compensate.
Even the most technologically advanced wheel alignment system won’t allow you to properly adjust wheel alignment angles if suspension and steering components are worn or damaged. Always address the component condition first by inspecting ball joints, steering gear and tie rods, etc., before attempting to perform any alignment.
Bumpsteer
Bumpsteer refers to “unwanted” changes in toe angles during suspension travel that can cause steering to be darty and indecisive. In a nutshell, this occurs when toe angles change during suspension compression and rebound. This causes the vehicle to “steer itself” due to road conditions. Bear in mind that a certain amount of bumpsteer is often intentionally built into road race or circle track cars in order to tune the chassis for specific tracks. However, for a street application, you need to eliminate/neutralize the bumpsteer effect as much as possible.
Bumpsteer is affected by four basic variables, including tie rod length, steering arm vertical/rotating movement, steering wheel movement and tie-rod-to-lower-control-arm angle.
If the tie rod(s) length is too short (long rack with short rods), the travel arc will be too severe, resulting in quicker toe changes. If the tie rods are too long (short rack/long rods), this could result in more severe toe-in.
Especially in an excessive negative-camber setup, the spindle leans and causes the outer tie rods to move vertically in relation to the lower ball joints during suspension travel, causing unwanted toe changes whenever the suspension compresses and rebounds.
Also, with excessive negative camber, when the steering wheel is turned, the tie rods move up/down as the steering arm arc changes angles as mentioned previously. It’s the same effect, but induced by the driver as opposed to being caused by road surface irregularities.
NOTE: Front-end wheel alignment can play a role in changing/affecting bumpsteer. It’s vital to have adjustability at the outer tie rods. Be aware that if you play with caster angles, this will change the vertical location of the outer tie rod ends.
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Ackerman angle
Ackerman angle (or Ackerman effect) refers to the steering “differential” that occurs when the wheels are turned. The inboard wheel should turn at a greater degree than the outboard wheel during a turn, in order to facilitate turn-in and to minimize tire scrubbing.
In the case of a rack and pinion-equipped vehicle, theoretically the rack’s tie rods should be parallel to the rack body (when viewed from overhead) when the wheels are fully turned.
Road test diagnosis
Does the vehicle lead or pull?
If not, is the steering wheel off-center? If so, adjust toe to correct.
If the vehicle does lead or pull, check if the tires are uni-directional, making sure that the tires are installed in the correct orientation on the vehicle. If you suspect that tires are causing the pull, try switching the front tires left-right and road test to see if the pull remains. If the pull is still evident, check wheel alignment and correct as needed.
If you switched tires left-to-right, does the vehicle lead or pull in the same direction as before? If so, you have eliminated the tires as the variable. Check alignment.
If you switched tires and the lead or pull is stronger than before, consider re-mounting the tires in reverse rotation (unless the tires are unidirectional) and re-balance.
Does the vehicle pull to the right or to the left?
If it pulls to the left, consider increasing right front camber and decreasing the left front camber until the lead/pull is eliminated. If it pulls to the right, consider increasing left front camber and decreasing right front camber until the lead/pull is eliminated.
Does the vehicle wander?
Wander can be caused by a variety of problems including improper tire inflation, worn tires, worn hub bearings, worn suspension parts, loose suspension or steering connections/mounts or incorrect wheel alignment. If the vehicle seems to wander primarily in heavy crosswinds, also check condition of the struts and shock absorbers.
NOTE: If the front caster angle is incorrect, this will promote a lead/pull as well. For example, if the right front features a lesser caster angle than the left, the vehicle will pull to the right. If caster is less on the left side, the vehicle will pull to the left. The caster angle on most front-wheel-drive vehicles is not readily adjustable. If the caster angle is incorrect, this is likely the result of damage to the suspension attachment points, or damaged lower control arms, a bent strut, etc.
NOTE: If a vehicle is heavily loaded, this can result in out-of-specification wheel alignment angles. For example, if the vehicle’s rear cargo or trunk area is heavily loaded, this will cause a transfer of weight balance, effectively lightening the front axle load. This slight front rise can create improper toe, camber and caster angles.
Diagnosing tire wear patterns
Underinflation wear is revealed as excessive tread wear at both the inside and outside edges of the tire tread area. Note that some shoulder/tread edge wear is typical of front tires on FWD vehicles as a result of the forces involved during turning and weight distribution. Proper tire rotation will minimize this wear.
Causes of underinflation wear can include low inflation pressure, consistent overloading of the vehicle and lack of tire rotation.
Overinflation wear (excessive wear at the center tread area) is characteristic of excessive inflation pressure. The rear tires of trucks equipped with rigid rear axles may exhibit similar wear, which can be minimized by routine tire rotation.
Causes of overinflation wear include high inflation pressure and lack of regular tire rotation (on rigid axle vehicles).
Individual shoulder wear is evident by excessive tread loss on one shoulder of a tire. This wear can occur on one or both tires on the same axle. Wear at outside shoulders of both front tires is a characteristic of excessive toe-in with radial tires or excessive high-speed cornering. Wear at both inside shoulders of the front tires is characteristic of excessive toe-out or insufficient toe.
Causes of shoulder wear can include toe error or high-speed cornering.
Feathered wear is seen as a “sawtoothed” pattern across the tread area. Feathering is generally caused by toe errors as the tire is forced to slip sideways as the vehicle moves forward. Individual tires showing a feathered edge may indicate a turning angle problem.
Causes of feathering include toe error or turning angle errors.
Edge wear, or shoulder wear that retains a sharp edge where the tread meets the sidewall, is characteristic of camber error (if camber is too positive, excessive wear will occur on the outside; if camber is too negative, excessive wear will occur on the inside). Note that vehicles with high positive caster may exhibit this type of wear at both tread edges from primarily driving in urban conditions.
Causes of edge wear can include camber error, high caster/urban driving, and lack of tire rotation.
Cupping wear is described as “spotty” or localized cup-type wear patches. This is the result of the tire being allowed to slip or wobble at regular intervals.
Causes of cupping wear can include improper wheel balance and/or tire runout, worn or loose suspension components, worn or loose steering components, brake system problems or inherent tire quality problems.
Heel and toe wear refers to partial wear of individual tread blocks, appearing as sawtooth wear, but in the direction of tire rotation instead of across the tread. This wear pattern is most common on non-drive wheels (for example, rear wheels of a FWD vehicle) and can be corrected by more frequent tire rotation.
Causes of heel and toe wear can include rear toe error, soft tire tread compounds, tread block squirm (which can be common on some all-season tires) or lack of regular tire rotation.
Diagonal wear refers to a cupping-like condition which appears in a diagonal pattern across the tread surface. When operating at high slip angles, the entire tread surface cannot resist the twisting forces that are being applied, and slippage occurs, much like heel and toe wear. In fact, heel and toe wear can degenerate into diagonal wear.
It’s important to note that diagonal wear is not the same as cupping. Tire balance and shock/strut problems will not cause this wear pattern. As with heel and toe wear, diagonal wear is most likely to occur on non-drive wheels. Causes of diagonal wear can include rear toe/thrust error, soft tread compounds and tread block squirm.
