WinAlign® HD Wheel Alignment Education Guide for Heavy-Duty Trucks

Toe is the most critical alignment setting for steer axle tire wear. It is measured in inches, millimeters or degrees.

Individual Toe is the angle drawn by a line drawn through a plane of one wheel referenced to the thrust line of the vehicle. Toe-in is when the horizontal lines intersect in front of the wheel. Toe-out is when the lines intersect behind the wheel.

Excessive toe-in wears the outside of the tire.

Excessive toe-out wears the inside of the tire.

Tire Wear Due to Improper Camber Settings
Camber is the angle formed by the inward or outward tilt of the wheel referenced to a vertical line. This angle is measured in degrees. Camber is positive when the wheel is tilted outward at the top and is negative when the wheel is tilted inward at the top.

Tire wear from excessive camber: Wear from positive camber is on the outside shoulder of the tire; with negative camber, wear is on the inside shoulder.

Caster: A Factor in Vehicle Handling
Caster is the forward or rearward tilt of the steering axis in reference to a vertical line. The angle is measured in degrees. Caster is positive when the top of the steering axis is tilted rearward and is negative when the tilt is forward. Caster is usually a factor in vehicle handling, but can affect tire wear. Proper caster is important for directional stability and returnability. Improper caster can cause shimmy, excessive steering effort, pulling and shoulder wear on the steer tires.

Turning Angle

Tandem Axle Angles
Geometric centerline of a vehicle is a line drawn through the midpoints of the front axle and the rear reference axle.

Thrust line is the bisector of the total toe angle of an axle. It represents the direction the axle “points” compared to the centerline of the vehicle.

Thrust angle is the angle formed by the geometric centerline and the thrust line of an axle.

Tandem scrub angle is the angle formed by the two thrust lines of a tandem axle vehicle.

In the diagram below, misalignment causes the tandem axles to work against each other.

The steer axle must be turned to offset the “push” of the axles and keep the vehicle moving straight ahead. This causes every tire on the vehicle to scrub.

Tire wear from tandem scrub occurs at the leading edge of the steer tires, in a pattern called “inside/outside” wear. For example, on the front axle of this vehicle, wear would occur on the outside of the left steer tire and on the inside of the right steer tire. Tire wear would occur on all drive axle tires.

Trailer Alignment and Tire Wear

The same conditions that cause tandem scrub on tractors also apply to tractor-trailer combinations.

Misaligned trailer axles cause tandem scrub, resulting in rapid wear on all tires.

If the trailer doesn’t track correctly, it exposes more area to wind resistance. This can affect handling and fuel economy.

Alignment Angles Affect Rolling Resistance and Fuel Consumption

While the effects of misalignment show clearly in tire wear, the effects on fuel consumption are less easy to quantify. Fuel consumption is affected by many factors.

However, it is obvious that misalignment must increase rolling resistance – and rolling resistance is a major cause of fuel consumption.

Geometric Centerline Alignment

Geometric Centerline Alignment can be used as a reference from which to compute individual toe angles. The Geometric Centerline of a vehicle is established by placing a line from the midpoint of the front axle and the midpoint of the rear-most axle.

The Geometric Centerline is not based on frame rails or cross member reference points.

The alignment system will establish the Geometric Centerline.

Frame Centerline Alignment

Separation

Separation is the distance between the reference axle adjustment points. This distance may be measured and entered into the aligner before adjusting thrust angle to allow the aligner to calculate how much the axle must be moved at the adjustment point.