Influencing factors of vehicle handling stability

There are many factors that affect the handling stability of automobiles, among which the main factors are drive system, steering system and transmission system. The main factors that affect the handling stability in the drive system are: front wheel alignment parameters, rear suspension structure parameters and lateral stabilizer bar, tires, front axle or frame deformation, suspension and so on.

(1) Influence of front wheel alignment parameters

The front wheel alignment parameters include camber, kingpin camber, kingpin caster and toe-in (toe angle).

The camber of the front wheel refers to the angle formed by the intersection line between the transverse plane of the front wheel center and the front wheel plane and the vertical line on the ground. The camber of the front wheel is generally around 1, as shown in figure 1. Its main function is to press the hub into the inner bearing when the car is running, so as to reduce the bearing load at the outer end. At the same time, it can prevent the front wheel from inclining inward due to the deformation of the front axle and the excessive clearance between the kingpin hole and the kingpin, and reduce the distance between the tire landing and the intersection of the kingpin axis and the ground, thus making the steering light.

The inclination angle of the kingpin refers to the angle between the kingpin axis and the ground vertical line in the transverse plane of the automobile, as shown in Figure 2. The influence of kingpin inclination angle on handling stability is mainly the righting moment, which is the forward turning moment generated by the improvement of system potential energy when the front wheel rotates, and has nothing to do with lateral force. Therefore, it can be said that the inclination angle of the kingpin mainly plays a righting role at low speed, and the "backward inclination distance" mainly plays a righting role at high speed.

The caster angle of the kingpin refers to the included angle between the kingpin axis and the ground vertical line in the longitudinal plane of the automobile, as shown in Figure 3. The influence of caster angle on vehicle handling stability is mainly caused by "caster angle resistance", which makes the lateral force on the ground produce a righting moment on the tire, and this moment produces an additional steering angle similar to the lateral angle of the tire, which is proportional to the lateral force, making the vehicle tend to increase understeer, which is conducive to improving the steady-state steering characteristics of the vehicle. If the caster angle of the kingpin decreases, the righting torque decreases, and when the disturbance torque to the steering wheel is greater than the righting torque of the steering wheel, shimmy will occur.

The camber angle of the front wheel changes with the load. When the vehicle turns, under the action of centrifugal force, the vehicle body leans outward, the outer wheel suspension is in a compressed state, and the camber angle of the wheel gradually decreases (becomes negative camber angle); The inner wheel suspension is in an extended state, which weakens the camber angle that originally became the negative camber angle of the road. So as to improve the ability of the wheel to bear lateral force and greatly improve the stability of the automobile when turning. The toe-in of the front wheel (as shown in Figure 4) should not be too large. If the toe-in is too large, the camber angle and caster angle of the wheel will decrease, and the front wheel will swing, resulting in unstable hunting and steering operation.

The geometric parameters of the front suspension guide mechanism determine the change trend and rate of the front wheel positioning parameters. When the wheel jumps, the change of camber angle includes two parts: the camber angle change caused by the body roll and the camber angle change caused by the wheel jumping relative to the body. In the double wishbone independent suspension, the former change makes the wheels tilt to the side of the car body, that is, the camber angle increases, which leads to the decrease of tire cornering stiffness, thus increasing the understeer effect of the whole car; The latter change depends on the geometric relationship between the upper arm and the lower arm of the suspension. In the double wishbone structure, the camber angle often decreases with the increase of the compression stroke of the spring, which is contrary to the camber angle change caused by the body roll, and will produce the trend of oversteering.

(2) Influence of structural parameters of rear suspension.

The influence of structural parameters of rear suspension on vehicle handling stability is similar to the "interference steering" of front suspension. When the car turns, the distance between the left and right wheels of the independent suspension changes due to the roll of the car body, the outer wheel jumps up, the distance from the car body is shortened, the inner wheel pulls down, and the distance from the car body is increased. With the different suspension structural parameters, the toe angle of the wheel will inevitably change when the wheel jumps up and down. Toe-in refers to the inward contraction of the steering front end of the automobile, so that the distance between the front ends of the two front wheels is smaller than the distance between the rear ends. The difference between the front and rear distances of two wheels, called toe-in, is generally not more than 8 ~ 12 mm, and its function is to eliminate the phenomenon that the front wheels are separated outward when rolling, which leads to the wheel dragging and rolling, and guide the front wheels to drive in a straight line.

(3) Influence of stabilizer bar

Stabilizer is often used to improve the roll stiffness of suspension or adjust the roll stiffness ratio of front and rear suspension. When the car turns, it can prevent the car body from generating large roll and lateral angular vibration, thus ensuring the good driving stability of the car. After the stabilizer bar stiffness is improved, the roll stiffness of the front suspension increases, the load change of the left and right wheels increases, the average roll stiffness of each wheel of the front axle decreases, and the vehicle understeer increases. The use of hard stabilizer in front suspension is helpful to improve the understeer and the hunting performance of the car.

(4) the influence of tires

Tire is an important factor affecting the handling stability of automobile. Improving the bearing capacity of tires, especially the bearing capacity of rear wheels, such as increasing the size or level of tires, or changing the rear wheels from single tires to double tires, will improve the steady-state steering characteristics of automobiles. Changing the camber angle of the rear tire can also improve the handling stability of the automobile, because the negative camber angle of the rear tire can increase the cornering stiffness of the rear tire, thus reducing the tendency of over-steering.

(5) Influence of deformation of front axle or frame

Because the frame is the foundation of the car, its deformation will directly affect the connection and cooperation of all parts, thus directly affecting the handling stability. The deformation of the front axle of an automobile will change the axial position of the kingpin hole, increase the inclination angle of the kingpin and decrease the camber angle. On the contrary, camber angle will decrease and camber angle will increase, resulting in heavy steering, tire abrasion and no automatic righting ability.

(6) Effect of suspension

When the vehicle is subjected to lateral force, the vertical load change of the front and rear axles of the vehicle is the main reason that affects the handling stability. If the vertical load of the left and right wheels of the front axle of the automobile changes greatly, the automobile tends to increase understeer; If the vertical load of the left and right wheels of the rear axle changes greatly, the vehicle tends to reduce understeer. The main factors affecting the vertical load change of the left and right wheels of the front axle and rear axle are: lateral stiffness of the front and rear suspensions, suspension mass, centroid position, roll center position of the front and rear suspensions, etc. These parameters also affect the handling stability of suspension system. The longitudinal driving force will increase the understeer trend of front-wheel drive vehicles. Of course, when the engine is used for braking, it will make the car tend to increase too much steering. Therefore, if the accelerator pedal is suddenly released after stepping on the ground during the acceleration of a high-power front-wheel vehicle, the steering characteristics of the vehicle will change obviously, and even become oversteer. Therefore, the car will suddenly and unexpectedly drive to the inside of the curve. Using automatic transmission, limited differential (LSD) and suspension which can deform and understeer the driving wheels during braking can reduce and eliminate the phenomenon of involvement.

When the rear-wheel drive vehicle brakes the engine, the braking force increases the side slip angle of the rear axle, which leads to the tendency of oversteer. Coupled with the comprehensive influence of other factors, rear-wheel drive vehicles tend to "roll in".