Formation sur la géométrie des trains roulants (parallélisme chasse carrossage épure direction...)

Key Concepts

Trains roulants (front and rear axles), wheel alignment, carrossage (camber), inclinaison de pivot (kingpin inclination), angle inclus (included angle), déport au sol (scrub radius), chasse (caster), parallélisme (toe), pures de direction (Ackermann steering geometry), hauteur de crémaillère (rack height), position des essieux (axle position), angle de set back, angle de VII, assiette de référence (reference attitude).

Carrossage (Camber)

Definition: Inclination of the wheel relative to the vertical, viewed from the front.
Positive Camber: Wheel leans outward at the top.
Negative Camber: Wheel leans inward at the top.
Modern Cars: Front wheels typically have near-zero or slightly negative camber.
Rear Wheels: Often have significant negative camber for improved stability.
Derive (Drift): Positive camber causes the wheel to drift when pushed. Negative camber causes the wheel to brake.
Effet de Côte (Camber Thrust): Camber induces a side force that tends to push the wheel away from its intended path, causing wear.
Compensation: Parallélisme (toe) adjustments can compensate for the drawbacks of carrossage.

Inclinaison de Pivot (Kingpin Inclination)

Definition: Angle formed by the vertical and the kingpin axis, viewed from the front.
Function: Helps return the steering to center, especially at low speeds.
Mechanism: During steering, the vehicle lifts slightly, and the weight creates a torque on the kingpin axis, returning the wheel to the straight position.

Angle Inclus (Included Angle)

Definition: Sum of the inclinaison de pivot (kingpin inclination) and carrossage (camber).
Alternative Definition: Angle of inclination of the pivots plus the carrossage, angle that forms the axis of the spindle with the horizontal plus 90°.
Significance: Determined during manufacturing by the shape of the pivot. A value outside tolerance indicates a deformed pivot.

Déport au Sol (Scrub Radius)

Definition: Distance at the ground between the kingpin axis and the center of the tire contact patch.
Positive Déport: Contact point is outside the kingpin axis.
Zero Déport: Contact point and kingpin axis coincide.
Negative Déport: Contact point is inside the kingpin axis.
Determination: Achieved through the design of hubs and wheels. Wheel offset is marked on the wheel.
Caution: Mixing wheels with different offsets can cause instability during braking.

Chasse (Caster)

Definition: Angle formed by the kingpin axis and the vertical, viewed from the side.
Positive Chasse: Kingpin axis is inclined rearward.
Negative Chasse: Kingpin axis is inclined forward.
Function: Places the kingpin axis ahead of the tire contact patch.
Chasse Distance: Distance between the kingpin axis and the contact point. This distance creates the force that returns the wheels to the straight position.
Demonstration: A model with zero chasse will not return to a straight line after steering. A model with positive chasse will.
Negative Caster with Offset: Some manufacturers use negative caster but compensate with wheel offset to achieve a positive chasse distance.

Parallélisme (Toe)

Definition: Angle formed by the wheel plane and the longitudinal axis of the vehicle, viewed from above.
Measurement: Can be expressed in millimeters as the difference in distance between the front and rear edges of the wheels on the same axle.
Toe-Out (Ouverture): Wheels converge towards the rear of the vehicle (negative toe). Compensates for negative carrossage.
Toe-In (Pincement): Wheels converge towards the front of the vehicle (positive toe). Compensates for positive carrossage.
Misalignment: Causes tire slippage in a straight line, leading to rapid tire wear (rough contact surface).

Pures de Direction (Ackermann Steering Geometry)

Function: Ensures that the wheels travel in circles with a common center during turns, preventing tire slippage.
Mechanism: The outer wheel describes a larger circle than the inner wheel, so its steering angle is smaller. This is achieved by the orientation of the steering levers, which form a constant angle with each wheel.
Jeantaud's Principle: The extensions of the steering levers should intersect at the center of the rear axle.
Modern Adjustments: The intersection point may be located before or after the rear axle to account for tire deformation, front-wheel drive/rear-wheel drive configurations, and desired handling characteristics (oversteer/understeer).

Hauteur de Crémaillère (Rack Height)

Effect: Parallélisme (toe) varies with changes in vehicle attitude during suspension movement.
Influence: The position of the rack directly affects the magnitude of this variation. Too high or too low, the variation is maximized.
Aggravation: The problem is worsened if the rack has been displaced due to an impact. Each wheel opens and closes alternately, with a movement opposite to that of the opposite wheel.
Mounting: The rack position is generally determined during manufacturing and fixed to a subframe or the front panel. When replacing the front panel, a template must be used.

Train Arrière (Rear Axle) - Peugeot 605 Example

Advanced Design: The rear axle of the Peugeot 605 has a system for managing parallélisme (toe) that produces an effect similar to that of a rack, used to improve handling.
Mechanism: Each half-axle is mounted on a pivot and stopped from rotating by an adjustable link. This link allows adjustment of the parallélisme of each wheel using an eccentric.
Additional Adjustment: A second eccentric allows adjustment of the height of the anchor point of each hub carrier.
Benefit: This design maintains a practically constant parallélisme regardless of changes in vehicle attitude and trajectory.
Misadjustment: A bad adjustment causes a variation of the parallélisme which degrades the road holding.

Position des Essieux (Axle Position)

Characteristics: Defined by their position on the car, their width (track), and the distance between them (wheelbase).
Alignment: Even if the values of the front and rear axles are correct, the axles must be aligned with the vehicle to ensure good handling and avoid premature tire wear.
Angle de Set Back: Measures the misalignment of the wheels on the same axle.
Angle de VII: Measures the misalignment of the centers of the front and rear axles.

Assiette de Référence (Reference Attitude)

Problem with Old Method: The old method called "en ordre de marche" (in running order) was affected by weight or suspension sagging.
New Method: Automobile Peugeot developed a method of controlling the trains roulants called "en assiette de référence" (in reference attitude).
Mechanism: By clamping the vehicle's suspensions, this method eliminates all problems of weight or suspension sagging.
Availability: This method is described in workshop documentation.

Conclusion

The video provides a detailed overview of the geometric parameters of trains roulants (front and rear axles) and their impact on vehicle handling and tire wear. It emphasizes the importance of proper alignment and adjustment of these parameters, including carrossage (camber), inclinaison de pivot (kingpin inclination), déport au sol (scrub radius), chasse (caster), parallélisme (toe), and pures de direction (Ackermann steering geometry). The Peugeot 605 rear axle design and the "assiette de référence" (reference attitude) method are presented as examples of advanced techniques for optimizing wheel alignment and handling.