Section 3.5: Four-Wheel Alignment Procedures

Four-Wheel Alignment Procedures

Modern passenger vehicles are designed with independent front and rear suspensions, requiring a precise four-wheel alignment to ensure correct handling and tire wear. A four-wheel alignment measures and adjusts all four wheels relative to the geometric centerline and the thrust line of the vehicle. To achieve accurate results, a technician must follow a strict, sequential adjustment workflow and perform critical electronic calibrations after completing the mechanical adjustments.

The Sequential Adjustment Workflow

Fasteners and adjustment points on a vehicle suspension interact with one another. Changing one angle almost always alters another. Therefore, alignments must follow a specific sequence:

• Step 1: Rear Camber - Rear camber is adjusted to ensure that the rear tires maintain flat contact with the road. While rear camber does not cause a steering pull (unless the cross-camber is extremely high), it prevents inside or outside shoulder tire wear.
• Step 2: Rear Toe (Establishes Thrust Line) - Adjusting the rear toe establishes the vehicle's thrust line. If rear toe is adjusted to point the rear wheels straight down the geometric centerline, the thrust angle is zero. If the rear wheels are adjusted to point slightly to one side, the rear axle will steer the vehicle, creating a non-zero thrust angle. If front toe is adjusted before the rear toe is set, any subsequent adjustment to the rear toe will change the vehicle's thrust line. This will instantly make the steering wheel crooked when the vehicle is driven in a straight line, as the front wheels must be steered to match the new rear thrust line.
• Step 3: Front Caster - Moving to the front axle, caster is adjusted first. Caster is a stability angle and is typically adjusted by moving the upper or lower control arm forward or backward. Because adjusting caster rotates the control arm, it usually changes the camber angle.
• Step 4: Front Camber - Front camber is adjusted next. Adjusting camber involves moving the top of the tire inward or outward. Because this shift moves the steering knuckle and tie rod attachment point, adjusting camber will directly alter the toe angle.
• Step 5: Front Toe - Front toe is always the final adjustment. Since any changes to caster or camber will pull or push the tie rod ends and change the toe, adjusting toe first would require it to be readjusted multiple times.

Steering Wheel Centering and Front Toe Adjustment

To ensure the steering wheel is straight when the vehicle is driving straight:

1. Center the Steering Wheel: The technician must center the steering wheel exactly and lock it in place using a steering wheel holder.
2. Adjust Tie Rods Equally: Front toe is adjusted by loosening the tie rod lock nuts and rotating the inner tie rod sleeves. To change the toe without throwing the steering wheel off-center, the technician must adjust both sides. For example, to adjust toe-in to toe-out, the technician must lengthen the tie rod on one side and shorten the tie rod on the other side by the same number of turns (or matching alignment machine readings).
3. Remove the Steering Wheel Holder: Only after front toe is locked and torqued to specification is the steering wheel holder removed.

Steering Angle Sensor (SAS) Calibration

On modern vehicles equipped with Electronic Stability Control (ESC), Electric Power Steering (EPS), and Advanced Driver Assistance Systems (ADAS), a mechanical wheel alignment is not complete without an electronic calibration.

• The Steering Angle Sensor (SAS): The SAS measures the rotational position of the steering wheel and the speed at which it is turned. It sends this data to the ESC and EPS control modules via the vehicle's CAN bus network.
• The Problem: During a mechanical alignment, the position of the front wheels relative to the steering column may be altered slightly. If the steering wheel is centered mechanically but the SAS is not calibrated, the computer may believe the steering wheel is turned (e.g., indicating 3 degrees of steer to the left). If the vehicle is driving straight, the ESC module detects that the yaw rate and wheel speed sensors indicate straight travel, but the SAS indicates a turn. The ESC module interprets this discrepancy as a skid or loss of control and may apply individual brakes or active steering assist to correct the "skid."
• Symptoms of Missing Calibration: This can cause the vehicle to actively pull or drift, illuminate the traction control/ESC warning light, or disable safety features like Lane Keep Assist (LKA) and Automatic Emergency Braking (AEB).
• SAS Calibration Procedure: After completing the alignment and tightening all locknuts:
  1. Connect a scan tool to the vehicle's OBD-II port.
  2. Access the ABS/ESC or EPS control module.
  3. Select the "Steering Angle Sensor Calibration" or "SAS Reset" utility.
  4. Ensure the front wheels are pointed straight ahead and the steering wheel is centered.
  5. Initiate the reset process, which writes the current sensor position as the new "zero-point" (straight-ahead) reference in the module's non-volatile memory.

Road Testing Protocols

The final step of any wheel alignment is a road test. The technician must drive the vehicle on a road with minimal crown to verify:

• The steering wheel is perfectly level and centered when driving straight.
• The vehicle drives straight without pulling or drifting.
• The steering feels stable, with good returnability.
• No warning lights (ABS, ESC, or EPS) are illuminated.