How does the Included Angle affect your Vehicle

Included Angle Explained: Core Maintenance for Your Car and Truck

Understanding wheel alignment is crucial for vehicle longevity, safety, and performance. While terms like camber and caster are common, the **Included Angle** is a fundamental, often overlooked measurement that connects the components of your suspension and steering systems. A proper included angle is essential for correct steering feel and minimizing tire wear.

What is the Included Angle in a Car or Truck?

The Included Angle (IA), also known as the Steering Axis Inclination (SAI) or Kingpin Inclination (KPI) Angle, is a geometric measure defined by the sum of two other angles:
Included Angle = Steering Axis Inclination (SAI) + Camber Angle.

It is the angle between the true vertical line and the steering axis (the imaginary line through the upper and lower pivot points of the steering knuckle). Because SAI is fixed by the manufacturer's design, the IA acts as a critical diagnostic tool. If the camber changes due to an accident or worn part, the resulting change in the Included Angle (while SAI remains constant) immediately signals a problem that is NOT adjustable by standard alignment procedures.

Why is the Included Angle Important in a Car or Truck?

• Diagnostic Tool: Since the SAI component is non-adjustable, the Included Angle provides a baseline. Any significant deviation from the manufacturer’s specification (after correcting camber/caster) indicates that a component is bent or damaged, such as a steering knuckle, strut, spindle, or control arm.
• Load Management: It helps the spindle or wheel support the vehicle's weight. The angle ensures the center of the tire contact patch is vertically beneath the steering axis, reducing stress on wheel bearings and ball joints.
• Steering Return: Along with caster, the IA creates a necessary geometric force that helps the steering wheel return to the center (straight-ahead) position after a turn.

How Does the Included Angle Affect Camber and Caster?

Effect on Camber:

Camber is directly related to the Included Angle by definition (IA = SAI + Camber). If a vehicle suffers impact damage that bends the spindle or strut assembly:

• If the camber reading is incorrect, but the Included Angle is correct, the problem is related to an adjustable component (e.g., control arm position).
• If both the camber reading and the Included Angle are incorrect (by the same amount), it strongly suggests a non-adjustable part like the steering knuckle or strut is bent. This is the primary diagnostic use of the IA.

Effect on Caster:

The Included Angle has **no direct mathematical relationship** with the Caster Angle. Caster is the tilt of the steering axis when viewed from the side, while IA is the tilt viewed from the front. However, any collision severe enough to damage the steering knuckle (and thus change the IA) is likely also severe enough to affect the components that control caster, necessitating a full four-wheel alignment inspection.

How Does the Included Angle Affect Steering?

The Included Angle is a major contributor to the "steering pivot radius" or "scrub radius."

• Steering Effort: A properly set IA reduces the effort required to turn the wheels at low speeds.
• Steering Feel (Road Feedback): IA, along with SAI and caster, creates the "self-centering" action. This means the wheels naturally try to return to a straight-ahead position after a turn, giving the driver a stable and predictable steering feel.
• Tire Scrub: If the IA is off, it can push the center of the steering axis far away from the center of the tire contact patch. This offset causes the tire to "scrub" (drag sideways) as it turns, leading to poor handling and rapid, uneven tire wear.

What Specific Areas Are Affected by Changes in the Included Angle?

Since the Included Angle is a geometric measure based on the fixed Steering Axis Inclination (SAI), changes to the IA usually indicate a deformation in one of the following non-adjustable or primary structural components:

• Steering Knuckle/Spindle: These are the hub and pivot point of the wheel. They are designed to be rigid, and bending here immediately changes the IA.
• Strut Assembly (MacPherson Struts): A bent strut shaft or damaged lower mounting bracket will alter the SAI, which in turn changes the IA.
• Control Arms: While control arms primarily affect camber (which changes the IA), a severe impact can bend the arm itself, leading to misalignment.
• Frame or Subframe: In the most severe cases (major collisions), damage to the mounting points of the suspension components on the chassis itself will change the fixed geometry, affecting the IA across the board.

Additional Relevant Information

The difference in the Included Angle between the left and right sides of the vehicle is known as the Cross Included Angle. This is perhaps the most important value. A large cross IA (over half a degree) indicates a problem in the suspension geometry on one side, almost always due to collision damage. Ignoring a discrepancy in the cross included angle can result in severe directional instability, poor handling, and premature failure of other steering and suspension components. Always check the cross included angle when performing an alignment inspection.