The directional stability of a car is related to the handling stability of the car and directly related to the driving safety of the car. It has become one of the important performances of modern cars.
Any car has a turning radius when turning. Let R be the distance from the longitudinal symmetry plane of the car to the instantaneous turning center O. Assuming that the wheelbase of the car is L, the distance between the two front wheel kingpin axes is K, and β is the turning angle of the outer steering wheel, the turning radius R is approximately L/sinβ. If the actual steering trajectory circle deviates from R, understeer or oversteer will occur.
When it comes to understeer and oversteer, the term slip angle is involved. When the car starts to turn at high speed, due to the inertia of the car moving forward, the car will instantly resist the steering, and the tire slip angle is generated, that is, the angle formed by the direction of the car and the direction of the wheels. The side slip angle of the wheel is not only caused by the cornering characteristics of the tire, but also caused by the structural factors of the suspension, such as the stiffness and geometric characteristics of the suspension.
If the sideslip angles of the front and rear wheels are equal, then the actual turning radius of the car is equal to the turning radius corresponding to the steering wheel angle, which is called "neutral steering"; if the front wheel yaw is greater than the rear wheel, the actual turning radius of the car is greater than the turning radius corresponding to the steering wheel angle , It is called "understeer"; if the rear wheel slip is larger than the front wheel, the actual turning radius of the car is smaller than the turning radius corresponding to the steering wheel angle, which is called "oversteer". Although the neutral steering can make better use of the lateral force (component perpendicular to the forward direction of the wheels) to achieve the maximum steering speed, it weakens the driver's subjective perception of the car's stability and cannot predict the car's braking flick. When the vehicle speed reaches a certain limit, the steering radius will decrease sharply, and the car will turn sharply, which will make the steering difficult or lose the steering, and even cause an accident. Understeer produces a relatively large steering radius, weakens the lateral force, and the car has good stability that automatically resumes straight driving and is easy to maneuver. Therefore, most automobile manufacturers make the cars have slight understeer. In this case, the occurrence of braking deviation will cause the car to return to the original straight route. However, in a specific analysis of the specific circumstances, the car must adopt an over-steer design in order to obtain the shortest turning time.