In this article, we’ll briefly cover the basics of wheel alignment angles as an aid for new technicians, followed by an overview of the need to consider how wheel angles affect ADAS (advanced driver assist systems) and the concern for ADAS recalibration following a wheel alignment job.
WHEEL ALIGNMENT BASICS
While today’s computerized and highly sophisticated wheel alignment systems provide substantial aid in walking the technician through the steps to achieve proper wheel angles, it’s nonetheless important for the technician to understand the basics of chassis alignment angles and the geometry involved, rather than blindly doing what the alignment system’s readings and calculations command. It’s obvious that incorrect wheel alignment conditions affect tire wear and can cause drifting/pulling during cruise, acceleration and braking, as well as poor directional control.
PRE-CHECK BEFORE ALIGNMENT
Prior to measuring or adjusting any wheel alignment angles, always perform a pre-check:
- Tire inflation. Make sure that all tires are inflated to specification for the specific vehicle;
- Tire size. Make sure that all tires are of the same size (at all wheels or front axle and rear axle, as some vehicles feature different front and rear sizes). Differences of outer tire diameter on the same axle are not acceptable;
- Tire condition. Obviously, inspect for tire damage such as cuts, severe tread wear, cupping, uneven tread wear, etc. Replace, if needed, prior to alignment;
- Vehicle ride height. Check ride height at the locations specified by the automaker. This will indicate if springs are worn/damaged. Correct, as needed, prior to alignment.
TOE ANGLE
Toe angle is achieved 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. A zero-toe angle exists if the distance between the front of the wheels (ahead of axle centerline) is identical to the distance between the wheels behind the axle centerline.
A toe-in condition (also referred to as a positive toe angle) is present when the two wheels on the same axle are closer together at the front and wider apart at the rear.
A toe-out condition (also called a negative toe angle) is present when the wheels are further apart at the front and closer together behind the axle centerline. 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 (with the exception of the potential use of shims to correct a rear toe issue), since this is a fixed angle. Independent rear suspensions usually offer rear wheel toe adjustment.
The toe angle is critical in terms of tire tread life. Ideally, the front steer wheels need to be parallel while cruising to avoid tread scrub. However, toe can also be used to alter a vehicle’s handling traits. An increased toe-in setting can help reduce an oversteer condition in turns and will improve the vehicle’s high-speed directional stability. An increase in toe-out can reduce an understeer tendency and will enhance initial turn-in during cornering. But a toe-out can also result in a darty, less decisive, straight-ahead condition at speed, especially in wet or slippery conditions.
With an excessive toe setting in or out, each front tire is pointed in a direction other than straight ahead.
When tires encounter a road surface with diminished traction (water, snow or ice), the tire that hits the less-tractive side of the road loses its grip, favoring the opposite tire on the same axle, which can tend to pull the vehicle in the direction of the toe angle.
For street-driven vehicles, always stay within the limit range specified by the vehicle maker.
TURNING RADIUS AND TOE-OUT ON TURNS
When the steer wheels are turned, individual wheel toe angle will change as compared to its 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 is a design feature that reduces the tendency of tire scrub during turns and reduces the turning radius of the outboard wheel, reducing the car’s tendency to turn-in too quickly, while providing reduced recovery effort when the vehicle direction changes. The inside wheel must turn in a tighter radius than that of the outside wheel in order to allow a smoother turn and reduce tire scrubbing. Inside and outside wheel positions refers to the direction of the turn. When turning left, the left front wheel is the inside wheel. When turning right, the right side wheel is the inside wheel.
While static toe (with wheels aimed straight ahead) allows both front wheels to rotate at the same speed and parallel to each other, the individual wheel toe angles differ when the steering wheel is turned more than 20 degrees.
This is referred to as the Ackerman principle, allowing the inside wheels to turn in a tighter radius while the outside wheel turns at a larger radius.
CAMBER ANGLE
Camber angle refers to the wheel’s angle from top to bottom when viewed from the front or rear of the vehicle, as compared to a true vertical. If the wheel leans out at the top, this is positive camber. If the wheel leans inward at the top, this is negative camber. If the wheel is set at a true vertical, this is zero camber.
CASTER ANGLE
The caster angle (steering axis angle) involves the relationship of the upper ball joint (or top of the strut mount) to the lower ball joint, as viewed from the side of the vehicle. Using a true vertical drawn through the hub center as a reference, caster angle is represented by a straight line drawn through the upper ball joint/pivot location through the lower ball joint.
A more-positive caster angle tends to contribute to directional stability at speed and aids in steering wheel return, helping the steering to return to a straight-ahead position after a turn.
A zero caster angle, where the lower pivot is directly below the upper pivot, would likely result in reduced directional control and poor steering wheel return, which would require the driver to manually drag the wheels back to a straight-ahead direction following a turn.
The steering axle’s caster angle has a major influence in directional control.
Since we are dealing primarily with production vehicles driven on the street, always follow the manufacturer’s specifications.
STEERING AXIS INCLINATION AND INCLUDED ANGLE
Viewed from the front of the vehicle, SAI (steering axis inclination) is a non-adjustable angle between a true vertical drawn through the center of the tire and a line drawn through the upper and lower ball joints. The SAI is determined at the point in which these two lines intersect. Simply stated, SAI is the factory-designed “camber” angle of a specific wheel’s suspension system.
Another fixed angle is IA (included angle), which is the combination of SAI and wheel camber. Both SAI and IA are measured to verify that the fixed-by-design angles are correct. If either the SAI or the IA are outside of the OE specification, this indicates that damage has occurred, such as a bent control arm, bent strut, dislocated strut tower, etc.
SCRUB RADIUS
If you’re not familiar with the term “scrub radius,” this represents the point of greatest load on the tire tread area, primarily during turns. 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 criss-cross 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 tends to increase steering effort, while excessive positive scrub radius, where the load of the tread is moved further outboard, can not only affect handling and ease of steering, but can over-stress wheel bearings.
Scrub radius is affected when aftermarket wheels featuring a different offset are installed, which moves the tire’s tread center from the original location. A wheel offset that moves the wheel further outboard places greater stress on wheel bearings and in the case of front-drive systems, this can also lead to overstress and wear on outer CV joints.
In most cases, a short arm/long arm suspension (upper and lower control arms, where the lower arm is longer) will exhibit a positive scrub radius.
Commonly, a front-wheel-drive MacPherson strut front suspension features a negative scrub radius, which aids in minimizing the torque steer effect that is a common trait of front-wheel-drive systems.
THRUST LINE AND THRUST ANGLE
A vehicle’s thrust line represents the “aim” of the rear axle, as viewed from above. 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 refers to the difference between the geometric centerline and the thrust line, measured in degrees. As viewed from above, if the thrust angle aims to the right (passenger side), this is a positive thrust angle. If the thrust angle aims left (driver side), this is a negative thrust angle.
CENTERLINE STEERING AND GEOMETRIC CENTERLINE
Centerline steering is simply a term that refers to a “straight and level” steering wheel clock position when the vehicle rolls in a straight line. If the steering wheel is not centered, this may indicate a possible thrust angle deviation. An off-center steering wheel will also “confuse” the steering angle sensor. The geometric centerline is a term that refers to a line drawn from the center of the rear axle to the center of the front axle, as viewed from above the vehicle.
TYPES OF WHEEL ALIGNMENT
The now-outdated method of centerline two-wheel alignment does not consider the rear wheel positions and should not be considered, because it ignores the thrust direction of the rear axle.
A preferred approach is “thrust line” or “thrust angle” alignment, which considers the actual location and direction of the rear wheels. This allows you to adjust the front wheel angles relative to the rear wheel angles, regardless of the geometric centerline.
If the vehicle in question features rear wheel toe adjustment, we can achieve optimum wheel alignment using the total four-wheel alignment, which considers and allows adjustment of all four wheels, allowing toe adjustment to bring the thrust angle to ideal zero or as close to zero as possible.
If the thrust angle is “off zero,” this can contribute to vehicle dog-tracking (crooked body relative to direction of travel), increased tire wear and unequal left/right turning. Total four-wheel alignment allows you to adjust and hopefully correct rear axle thrust angle, then allows you to adjust the front wheels parallel to the rear wheels.
Whether the rear toe is adjustable or not, always adhere to a four-wheel alignment. A four-wheel alignment approach allows you to refer to and consider the rear, while a total four-wheel alignment allows adjusting of both front wheel angles, as well as rear toe.
Loading the vehicle (cargo weight) in the manner in which it will be driven should be done with any wheel alignment job. In order to obtain the optimum wheel alignment angles for someone who expects maximized performance, the weight of the driver should be considered, as well as one or more passengers, depending on how many people will be riding in the vehicle for the majority of its operation.
While perhaps not practical in all cases, whenever possible, place the driver of the vehicle in the driver seat during the entire alignment process or another technician of the same weight. This will allow you to better tune the wheel angles in a loaded condition as the car will normally be driven.
This becomes more of a factor if the driver tends to be on the heavy side. While this can be a touchy subject to address if the person is self-conscious about their weight, simply explain that for optimum-tuned performance, it’s accepted practice to load the vehicle in its normal condition, regardless of the person’s size. Rather than potentially offending the customer, this tactic can be used to impress the customer regarding your attention to detail.
ADAS RECALIBRATION AFTER ALIGNMENT
If the customer’s vehicle is equipped with ADAS (advanced driver assistance system) and/or a steering angle sensor, simply performing a wheel alignment job doesn’t mean that the job is complete. Additional work may be required in the form of sensor/camera /radar calibration or reset. Here are some expert tips:
The following are tips provided by Autel.
ADAS calibration procedures should be performed on a level floor. If an alignment lift is used for leveling, Autel calibration software can provide adjustment information and the calibration frame can be easily raised or lowered to be at level with the floor.
As an example of available ADAS service systems, Autel offers its adjustable ADAS Calibration frame system with a millimeter-level accuracy, crossbar minor adjustment knob and laser positioning. The laser is positioned on a sliding plate to locate radar and front-mounted cameras. A modular frame assembly houses accessory tools needed for calibration.
The system’s software is intuitive and provides graphics, as well as step-by-step instructions. The system allows both pre-scan and post-scan reports, along with ADAS module identification and calibration.
Cameras, sensors, ultrasound and radar are some of the technologies used to capture driving environment data, including traveling or static vehicle position; pedestrian location; road signs; driving lanes; road curves; driving conditions, including poor visibility and evening driving; and more.
This information is used to instruct the vehicle to take its predetermined action.
Cameras, sensors and sensing systems are typically located in the front and rear bumpers, the windshield, the front grill and in the side and rearview mirrors. Depending on the vehicle type, either a stationary or a dynamic calibration procedure is required to calibrate ADAS sensors to vehicle modules. (In some vehicle types, both calibration procedures are required.)
The following are tips provided by Jordan Krebs, Snap-on product manager, alignment worldwide.
The Steering Angle Sensor (SAS) is a subcomponent of the steering system that typically only measured and helped maintain a straight steering wheel, but recently has been integrated with Lane Keep Assist and other ADAS components. Some vehicle manufacturers use different types of ADAS components that require different calibration types.
In static calibration, the vehicle and ADAS target holder are at set positions.
The scanner is connected to the OBD II port, which tells the sensors to calibrate.
In dynamic calibration, the scan tool is connected to the OBD II port with the technician driving the vehicle in a “target-rich environment,” until the sensors have collected enough data points.
Ford, General Motors, Honda, Subaru and other manufacturers may have static and dynamic sensors on the same vehicle, further increasing the amount of work required to properly calibrate the vehicle.
Vehicle manufacturers require wheel alignment to be checked before ADAS recalibration. We recommend it for all vehicles due to the potential positioning of the calibration stand based on alignment readings.
There are many ways to position the ADAS stand, including the following:
- Geometric centerline, based on the vehicle’s overall suspension components;
- Body centerline, based on the actual center of the vehicle’s body, and;
- Thrustline, based on the vehicle’s thrust angle.
Our current generation of aligners offer ADAS positioning information when the technician selects the vehicle.
This allows technicians to know before calibration what components may be needed and what will need to be calibrated, if and when adjustments are made to the vehicle’s alignment angles.
While we currently have our EZ-ADAS available in the market, we are continuously pushing ourselves to bring innovative products to the automotive industry, while striving to make work easier for our technician customers.
We have a new product coming soon. Using our experience in alignment, Tru-Point will enable technicians to place targets with a higher degree of accuracy, all without the need for lasers, tape measures or plumb bobs.
Tru-Point will provide an audit of vehicle alignment and will prove the target’s placement to guarantee that a proper calibration is completed.
The following comments were provided by Kaleb Silver, Hunter’s director of products management.
Recalibrating a steering angle sensor is a relatively easy process. Using Hunter’s CodeLink Tool, you’re guided through a few quick and easy steps, with the steering wheel centered. That’s the most common reset needed. In some cases, other sensors are also calibrated as part of the process, like the torque angle and yaw sensors.
ADAS sensors/cameras/radar are tied to the wheel alignment, as the wheels determine the direction of vehicle travel, which can affect the aiming of various sensors.
The variety of ADAS encompasses many different technologies, including cameras and radar, all of which are involved in attaining a semi-autonomous driving experience. However, keep in mind that not all components necessarily require recalibration attention after a wheel alignment. As long as the vehicle has not been involved in a collision or service work that has disturbed any sensors, cameras or radar systems, the requirement for recalibration may or may not be needed. In many cases, a reset may or may not be required by the OEM. As an example, for vehicles that feature a front camera, Honda says you don’t need to recalibrate. Audi, as another example, says that you do need to reset. In short, always adhere to the OEM requirements, as they may be OEM-specific.
However, a great deal of floor space is not needed in all cases. There are two basic categories: some that require expensive fixtures, but not a lot of floor space (perhaps as little as five feet), and others that are much more basic and less expensive, but require more extensive floor space. A full-function scanner tool will be required in order to perform pre- or post-ADAS scans.
The two common methods of calibration involve dynamic or static procedures. A dynamic calibration is done by placing the vehicle in drive. Static calibration is performed in a bay with stationary target fixtures.
Each vehicle manufacturer has its own requirements, often based on vehicle model and year. The Big Three Detroit automakers tend to require dynamic calibrations, while most Asian and European OEMs tend to require static, fixture-based calibrations.
ADAS systems employ various “terrain-reading” component, including the use of radar, lasers and cameras. These systems are intended to detect road markings, other vehicles, pedestrians and additional objects. In addition, these driver-assist systems rely on information provided by steering angle sensors, steering torque and sensors that detect yaw and lateral acceleration.
Whether nor not the vehicle is equipped with ADAS or is simply equipped with a steering angle sensor, the steering angle sensor must be recalibrated anytime toe angle adjustments are made.
Thrust angle is critical, not only for proper wheel alignment and vehicle directional stability, but also to allow the ADAS sensors/systems to properly “read” driving conditions.
Wheel alignment must be performed before attempting to recalibrate ADAS, as the system relies on existing thrust angles and wheel toe settings.
RECALIBRATION ISN’T FOR WHEEL ALIGNMENT ONLY
It’s important to note that whether or not the vehicle will receive a wheel alignment, recalibration will be required whenever an ADAS sensor or camera is unmounted during a non-related repair — for example, if a camera or sensor needs to be removed in order to perform a body or engine repair or as a result of a windshield replacement, where a camera is removed/reinstalled. If a camera or sensor is not mounted in the correct location or at the correct angle, the system may not detect an approaching obstacle or it might activate when not needed. Alignment and calibration of the system’s cameras and sensors are critical.
