Vehicle Dynamics & Control - 20 Anti-dive and anti-squat suspension geometry

Understanding Vehicle Pitch Dynamics

Introduction to Pitch Dynamics

• The video discusses the pitch dynamics of a vehicle, focusing on anti-dive and anti-squat suspension geometry influenced by inertial forces and vertical elasticity.

• During acceleration, vehicles experience positive pitching motion (squat) due to rear suspension compression; during braking, negative pitching motion (dive) occurs from front suspension compression.

Anti-Dive and Anti-Squat Geometry

• Anti-dive and anti-squat geometries do not significantly affect weight distribution; pitch angles are small enough that the center of gravity remains largely unchanged.

• These geometries alter how vertical tire forces are transmitted through springs versus control arms, impacting suspension compression.

Virtual Pivot Points in Suspension Kinematics

• Understanding virtual pivot points is essential for grasping longitudinal dynamics and suspension kinematics.

• The instantaneous center of rotation for wheel movement relative to the chassis can be considered a virtual pivot point, which is finite rather than at infinity.

Examples of Suspension Types

Double Wishbone Suspension

• In double wishbone suspensions, the upper wishbone's rigid body rotates around an axis fixed to the chassis, influencing hub carrier movement.

• The fixing point's velocity vector is perpendicular to its rotation axis, determining where the virtual pivot point lies along dashed axes.

McPherson Strut Suspension

• Similar principles apply to McPherson strut suspensions; lower control arms rotate about a fixed axis while maintaining specific relative velocities.

• The construction of the virtual pivot point involves analyzing both control arm movements and strut orientations for accurate placement.

Conclusion on Virtual Pivot Points

• While examples illustrate well-defined virtual pivot points in certain suspensions like double wishbone and McPherson strut designs, all independent and solid axle suspensions have such points.

Understanding Virtual Pivot Points in Suspension Systems

The Importance of the Virtual Pivot Point

• A well-defined virtual pivot point allows designers to maintain control over suspension positioning, which is crucial for vehicle dynamics.

• The relationship between front and rear suspension virtual pivot points significantly influences anti-dive and anti-squat characteristics during acceleration.

Analyzing Forces During Acceleration

• Focus shifts to dynamic forces rather than static ones; only deviations from static equilibrium are considered, particularly the longitudinal tire force (FX).

• FX creates an inertial force at the center of gravity that counteracts its direction, leading to a moment balance involving vertical tire forces.

Moment Balance and Squat Dynamics

• A moment balance about the front tire contact point reveals how additional vertical tire forces (Delta FC R) relate to FX through distances L and H.

• There exists a fixed ratio between longitudinal tire force (FX) and additional vertical tire force due to this relationship.

Conditions for Rear Suspension Squat

• Rear suspension squat occurs when the resulting force has a moment about the virtual pivot point; if it lies on a specific line, no squat occurs.

• The further below this line the virtual pivot point is located, the greater the squat effect due to increased moments.

Achieving Anti-Squat Characteristics

• A formal expression for total additional moments around the virtual pivot shows that 100% anti-squat is achieved when certain ratios align.

• X percent anti-squat can be calculated based on these ratios, indicating how effectively squat is controlled during acceleration.

Exploring Braking Dynamics with Virtual Pivot Points

Front Suspension Dynamics During Braking

• In braking scenarios, different fractions of brake force are distributed between front and rear axles affecting overall vehicle dynamics.

• The total inertial force acting on the center of gravity equals negative mass times longitudinal acceleration during braking events.

Moment Balances in Braking Scenarios

• A moment balance about rear tire contact points helps determine changes in vertical tire forces due to braking effects.

• Total moments responsible for compressing front suspension can be expressed using relationships among various distances related to brake forces.

Conditions for Zero Compression in Front Suspension

• The locus of virtual pivot points achieving zero compression during braking forms a dashed line defined by specific distance ratios.

Practical Considerations for Anti-Dive and Anti-Squat