SUSPENSIONS : Régler, Améliorer, Comprendre

Understanding Vehicle Suspension Geometry

Components and Functionality of Suspension

• The suspension system of a vehicle consists of multiple moving parts, with their kinematics and angles defined by the manufacturer based on specific characteristics such as occupant comfort, grip level, and desired vehicle behavior.

• Various factors like driving on rough roads, replacing components (e.g., ball joints or steering racks), or even minor impacts can lead to dimensional misalignments in suspension parts.

Importance of Suspension Geometry Adjustment

• When misalignments occur, it is essential to implement an adjustment procedure known as suspension geometry. This requires consulting a specialist.

• Key reference axes for suspension geometry include:

• Axis of Caster: Concentric axis between two spindles on the same axle.

• Symmetry Axis: Theoretical line connecting the centers of front and rear axles.

• Thrust Axis: Determines the direction of the rear axles.

• Vertical Axis: Perpendicular to the ground.

Key Dimensions Affecting Suspension Geometry

• Important dimensions include:

• Kingpin Inclination: Angle between spindles and perpendicular to geometry axis; tolerances vary widely among vehicles.

• Thrust Angle: Critical angle between symmetry axis and thrust axis; improper adjustment can cause steering issues.

Wheel Alignment Angles

• Camber Angle: Inclination of wheels relative to vertical; positive camber helps distribute weight effectively but may reduce grip in modern vehicles with wider tires.

• Negative camber is preferred for better cornering stability.

• Excessive negative camber can lead to uneven tire wear but improves handling during turns.

Parallelism Adjustments

• Individual parallelism refers to the angle formed between each wheel's rolling direction and the vehicle's symmetry axis. Total parallelism aims for balance despite other angle effects during movement.

• Misalignment leads to rapid tire wear due to deformation caused by camber changes while driving straight.

Effects of Incorrect Alignment

• Poor alignment results in steering wheel off-center, pulling towards one side, and irregular tire wear patterns.

Additional Angles Influencing Stability

• Caster Angle: Positive caster enhances directional stability; negative caster causes self-aligning tendencies similar to shopping cart wheels.

• The pivot angle affects how wheels align with vehicle movement direction, impacting overall handling dynamics.

Conclusion on Ground Offset

Geometry of Vehicle Suspension

Impact of Wheel Offset and Tire Width

• A positive offset leads to toe-in during acceleration, which is beneficial for rear-wheel drive vehicles. Conversely, a negative offset causes toe-in during braking, ensuring the vehicle maintains a straight line in case of wheel slip.

Geometry Adjustment Procedure

• The geometry adjustment process varies by vehicle type; typically adjustable angles include thrust angle, alignment (parallelism), and sometimes camber on both front and rear axles.

Equipment Used in Geometry Measurement

• Most setups consist of two main components: a light-emitting gantry and individual reflectors attached to each wheel that return beams to the gantry for measuring discrepancies.

Steps in the Geometry Adjustment Process

• The procedure consists of nine phases starting with placing the vehicle on a dedicated lift, ensuring it is level with the ground.

• Subsequent steps involve installing reflectors on wheels, adjusting the gantry height, calibrating the system longitudinally by moving the vehicle forward or backward.

Final Adjustments and Reporting

• After locking brakes using specialized tools and finding extreme steering points, technicians adjust suspension angles manually while monitoring real-time data displayed on screens.

• Angles outside tolerance are shown in red; those close to limits appear yellow, while compliant angles are green. Technicians aim for all values to be within manufacturer specifications before generating an intervention report.

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