EASY TO KEEP THE PATH

The stability of trajectory retention is characterized by the ability of the car to steadily maintain a given direction of movement and turnability, an indicator that indicates the behavior of the car in a turn.

Turnability can be:

• understeering (insufficient) – the lateral deflection of the front wheels is greater than the lateral deflection of the rear wheels.
• oversteering (excessive) – the lateral deflection of the rear axle wheels is greater than the deflection of the front axle wheels. In this case, the car's rear axle is blown off, right up to the turn.
• neutral is the behavior of a car when the degree of deflection of the wheels of the front and rear axles are equal and the trajectory of the center of mass of the car corresponds to a given radius.

Cornering is most affected by the type of vehicle drive:

• a rear-wheel drive vehicle is prone to oversteering
• A front-wheel drive vehicle is prone to understeering
• a four-wheel drive vehicle is closest in characteristics to neutral cornering

The ability of a car to sustainably maintain a given direction of movement is maximally influenced by the type of suspension:

• dependent suspension does not provide acceptable performance due to changing wheel angles under different conditions
• independent suspension provides a better level of stability.

Dependent suspension is the suspension of a car in which the movement of one axle wheel depends on the movement of the other wheel. The transfer of forces and moments from the wheels to the body (frame) here can be carried out by such an elastic element as a spring, and in the case of other types of elastic elements, by lever mechanisms of various kinematic schemes.

Independent suspension is a car suspension in which the movement of one axle wheel does not depend on the movement of the other wheel. Regardless of the type of elastic element, the transfer of forces and moments from the wheels to the body (frame) is always carried out by lever mechanisms of various kinematic schemes.

Dependent suspensions are used on vehicles that carry heavy loads or require extreme traction in off-road conditions.

Independent suspensions are more advanced, but complex and provide better handling and riding comfort.

The main suspension alignment parameters affecting directional stability include:

• scrub radius
• caster angle
• steering axis inclination (SAI).

The scrub radius is the straight-line distance between the point where the steering axis intersects the road surface and the center of the tire’s contact patch with the road (in the vehicle’s unladen condition). The scrub radius can be zero, positive, or negative.

For many years, most vehicles used relatively large positive scrub radius values. This design made steering effort lower during parking maneuvers, since the wheel rolled slightly while turning rather than pivoting in place, and it also allowed more space in the engine bay by positioning the wheels further outward. However, when the wheels on one side of the vehicle traveled over a surface with a different friction coefficient, struck a bump, or when one tire was punctured, the steering wheel tended to pull sharply out of the driver’s hands.

Starting from the 1970s–1980s, as vehicle speeds increased and power steering systems became widespread, cars with zero or even negative scrub radius began to appear in large numbers. This design helped minimize the previously mentioned undesirable effects and improved steering feel and vehicle handling.

Caster is the angle of the steering axis in the longitudinal plane of the vehicle. It is defined as the angle between the vertical line and the line passing through the centers of the wheel’s steering pivot points.

This line passes through the upper and lower ball joints in a “double wishbone” suspension, or through the upper and lower mounting points of the strut in a MacPherson suspension. The caster angle can be either positive or negative.

Positive caster (when the steering axis tilts rearward) generates a stabilizing moment at speed, providing dynamic self‑centering. The greater the angle, the stronger the stabilizing effect when the wheels are turned. This occurs because the tire’s contact patch lies slightly behind the steering axis intersection point, producing lateral forces that tend to return the wheel to its straight‑ahead position when it is deflected.

Negative caster (when the steering axis tilts forward) was used in earlier vehicles before the widespread adoption of power steering systems. It reduced steering effort during low‑speed maneuvers but did not provide self‑centering, requiring the driver to constantly correct for road irregularities.
In front‑wheel‑drive vehicles, dynamic stabilization is also achieved because the driven wheels “pull” the car out of a turn, so smaller caster angles are typically used. In rear‑wheel‑drive and all‑wheel‑drive vehicles, the caster angle is usually greater.

The angle of lateral inclination of the axis of rotation of the wheel (SAI – Steering Axis Inclination) is the angle between the vertical and the projection of the axis of rotation of the wheel on the transverse plane of the car. This angle provides self–leveling of the steering wheels due to the weight of the car - weight stabilization. Dynamic stabilization works only at sufficiently high speeds. When moving and maneuvering at walking speed, the weight stabilization system works.

When the wheel is rotated, the pivot axis lowers relative to the starting point 0 due to the angle of the transverse inclination, thereby lifting the car. The weight of the lifted part of the car tries to return everything to its original, stable position, i.e. turn the steering wheel to a neutral position.

Choosing the SAI value is a search for the optimum. As the transverse angle decreases, the effectiveness of weight stabilization decreases. Excessive tilt leads to excessive steering force when maneuvering, accompanied by turning the wheels at a large angle, for example, when parking.