Ackermann Steering Geometry: Vehicle Dynamics

Ackermann Geometry

• Ackermann geometry is achieved by angling the steering arms inward slightly.
• When the steering wheel is straight, the steering arms are not parallel to each other.
• This inward angling means that when the vehicle turns, the inner wheel steers at a greater angle than the outer wheel.
• This difference in steering angle accommodates the different turning radii of the inner and outer wheels.
• The center point of the turning circle for each wheel should ideally lie on a common axis.
• Perfect Ackermann geometry is difficult to achieve across all steering angles.

Over-Ackermann and Under-Ackermann

• Over-Ackermann describes a condition where the inner wheel steers at an angle greater than what is geometrically ideal.
• This can cause the inner tire to scrub more than necessary, especially at high steering angles.
• Under-Ackermann is the opposite, where the inner wheel steers at an angle less than ideal.
• Under-Ackermann can lead to the outer tire scrubbing more.
• The ideal amount of Ackermann depends on several factors.

Impact on Vehicle Dynamics

• Correct toe out on turns improves handling and reduces tire wear.
• It helps maintain consistent grip levels across the front tires during cornering.
• Incorrect Ackermann can lead to unpredictable handling.
• It contributes to increased tire wear due to scrubbing.
• It can reduce the overall efficiency of the steering system.

Factors Affecting Ackermann

• Vehicle wheelbase.
• Track width.
• Steering arm length.
• Steering arm angle.
• Suspension geometry changes (roll, dive, and squat) also affect Ackermann.

Design Considerations

• Designers choose the Ackermann angle based on the vehicle's intended use.
• A road car prioritizes predictable handling and reduced tire wear.
• Race cars are often set up with less Ackermann or even reverse Ackermann.
• Adjustments can be made via shims or adjustable components in the steering linkage.

Reverse Ackermann

• Reverse Ackermann is where the outer wheel steers at a greater angle than the inner wheel.
• This is sometimes used in racing.
• It can improve turn-in and reduce understeer at high speeds.
• It often comes at the cost of increased tire wear, and potential instability at lower speeds.

Measurement and Adjustment

• Ackermann is measured in degrees of difference in steering angle between the inner and outer wheels at a given steering input.
• Alignment machines calculate Ackermann angle based on steering geometry measurements.
• Adjustments involve shimming or replacing steering components to move the pivot points.
• Skilled technicians are necessary to accurately measure and adjust this setting.

Toe Angle vs. Ackermann

• Toe angle is the static angle of the wheels relative to the vehicle's centerline when the steering is straight.
• Ackermann describes how the toe angles change dynamically as the wheels are steered.
• Toe affects straight-line stability and tire wear.
• Ackermann affects handling during turns.
• Toe and Ackermann adjustments must work in coordination to optimize overall performance.

Explore Ackermann steering geometry in vehicle design. This configuration ensures each wheel follows its intended path during turns, preventing excessive scrubbing. It achieve this by angling the steering arms inward, causing the inner wheel to steer at a greater angle than the outer wheel.