Axle（2）

Author: QQ Rush

Link: https://zhuanlan.zhihu.com/p/653013512

Source: Zhihu

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2、 The steering wheel positioning parameters of the steering axle should ensure that the steering function of the car is ensured, while also ensuring that the steering wheels have an automatic centering effect to ensure stable straight driving of the car. When the steering wheel deviates due to occasional external force, once the applied force disappears, it should be able to automatically return to the original straight driving position. This automatic return function is ensured by the positioning parameters of the steering wheel, which means that the installation between the steering wheel, main pin, and front axle should have a certain relative position. The positioning parameters of these steering wheels include kingpin inclination, kingpin inclination, front wheel camber, and front wheel toe in. 1. When designing the steering axle with a kingpin tilt angle, the kingpin is located in the longitudinal plane of the car, and there is a backward tilt angle γ on its upper part, which is the angle between the kingpin axis and the ground vertical line in the longitudinal plane of the car, as shown in Figure 20-6. The inclination angle of the main pin can form a stable torque for correction. When the kingpin has a caster angle γ, the intersection point a between the kingpin axis and the road surface will be located in front of the contact point b between the wheel and the road surface, as shown in Figure 20-6a. When a car is driving in a straight line, if the steering wheel is accidentally deflected by external forces (such as turning to the right as shown by the arrow in the figure), it will cause the car to deviate to the right. At this point, due to the centrifugal force of the car itself, at the contact point b between the wheels and the road surface, the road surface exerts a lateral reaction force Fy on the wheels. The reaction force Fy creates a torque FyL that acts on the wheel's pivot axis, and its direction is exactly opposite to the direction of wheel deflection. Under this torque, the wheels will return to their original center position, ensuring stable straight driving of the car. Therefore, this torque is called the stabilizing torque. But this torque should not be too large, otherwise in order to overcome this stable torque during steering, the driver needs to apply a large force on the steering wheel (known as heavy steering). The magnitude of the stabilizing torque depends on the value of the force arm L, which in turn depends on the magnitude of the inclination angle γ. The commonly used gamma angle nowadays does not exceed 2 ° -3 °. Modern high-speed cars experience an increase in stability torque due to a decrease in tire pressure and an increase in elasticity. Therefore, the gamma angle can be reduced to near zero, or even negative. Figure 20-6b is a schematic diagram of the caster angle of the Jiefang CA1091 car. 2. When designing a steering axle, the inclination angle of the main pin is defined as the inclination angle between the main pin axis and the vertical line of the ground, which is inclined inward by a β angle (i.e. the angle between the main pin axis and the ground in the horizontal plane of the car), as shown in Figure 20-7a. The kingpin inclination angle β also has the effect of automatically aligning the wheels, as shown in Figure 20-7b. When the steering wheel is deflected by an angle from the middle position under external force (for convenience of explanation, it is drawn as 180 ° in the figure to turn to the position shown by the double dotted line), the lowest point of the wheel will sink below the road surface. But in reality, it is impossible for the lower edge of the car wheels to sink below the road surface. Instead, lift the steering wheel and the entire front of the car up to a corresponding height. In this way, the gravity of the car itself has the effect of causing the steering wheel to return to its original center position. In addition, the inward tilt of the main pin also reduces the distance from the intersection point of the main pin axis and the road surface to the intersection line between the wheel center plane and the ground (Figure 20-7a), thereby reducing the force exerted by the driver on the steering wheel during steering, making steering control light and reducing the impact force transmitted from the steering wheel to the steering wheel. But the value should not be too small, that is, the inclination angle should not be too large. Otherwise, during the process of the wheel turning around the kingpin, there will be significant sliding between the tire and the road surface, which increases the frictional resistance between the tire and the road surface. This not only makes the steering heavier, but also accelerates tire wear. Therefore, the inclination angle β is generally not greater than 8 °, and the distance is generally 40-60mm. Figure 20-7c shows the kingpin inclination angle and front wheel camber angle of the Jiefang CA1091 car. The inclination angle of the main pin is ensured in the design of the front beam and is achieved through mechanical processing. When processing, tilt the upper end of the main pin hole axis at both ends of the front beam inward to form an inward inclination angle. 3. The camber angle of the wheels, in addition to the two angles of the caster angle and the camber angle mentioned above, ensures stable straight driving of the car, and the front wheel camber angle α also plays a positioning role. α is the angle between the intersection of the transverse plane of the car passing through the center of the wheel and the plane of the wheel, and the vertical line of the ground, as shown in Figure 20-7c. If the wheels are installed perpendicular to the road surface when the car is empty, the axle may tilt inward due to load deformation when fully loaded, which will accelerate the tire wear of the car. In addition, it increases the load on the outer small bearings and wheel hub fastening nuts, reducing their service life. Therefore, in order to ensure uniform tire wear and reduce the load on the outer bearings of the wheel hub, the wheel should be installed with a certain camber angle in advance to prevent the wheel from tilting inward. At the same time, the camber angle of the wheels can also adapt to the arched road surface. However, the camber angle should not be too large, otherwise it will cause biased wear on the wheels. The camber angle of the front wheels is determined in the design of the steering knuckle. When designing, make the axis of the steering knuckle journal at an angle with the horizontal plane, which is the front wheel camber angle α (usually around 1 °). 4. The constraint of the front wheel tie and axle makes it impossible for the wheels to roll outward, resulting in rolling and sliding on the ground, thereby increasing tire wear. In order to eliminate the adverse consequences caused by wheel camber, when installing the wheels, the center planes of the two front wheels of the car are not parallel, and the distance B between the front edges of the two wheels is less than the distance A between the rear edges. The difference between A-B is called front wheel toe in, as shown in Figure 20-8. This can make the wheels approach the straight ahead direction at each instant of rolling, thus greatly reducing and eliminating the adverse consequences caused by the wheel tilting outward. The front wheel toe in can be adjusted by changing the length of the tie rod. When adjusting, the distance difference A-B between the front and rear of the two wheels can be adjusted according to the measurement positions specified by each manufacturer to meet the specified toe in value. The general toe in value is 0-12mm. In addition to the position shown in the diagram, the measurement position can also be determined by taking the difference between the front and rear at the center planes of the two tires, or by selecting the difference between the front and rear at the inner surfaces of the two wheel rims. In addition, the toe in can also be represented by the angle - toe in angle, as shown in Figure 20-8 at the angle of phi. 5. The camber angle and toe in wheel alignment parameters of the rear wheels usually refer to the front steering wheel of the car's front wheels. However, modern cars not only have front steering wheels with camber and toe in, but some cars also have rear wheels with camber and toe in. Like the Red Flag CA7220 sedan, the rear wheels are equipped with a front toe angle of 8 '+5' and an camber angle of -58 '± 10'. The car is equipped with front engine and front drive, while the rear wheels are driven by passive wheels. The driving force of the car acts on the rear axle through the longitudinal arm (Figure 20-9). If the wheels do not have a toe angle, when the car is in motion, the rear axle will bend to a certain extent under the driving force, causing the wheels to experience forward tension. The pre-set toe angle is used to counteract this forward tension. The rear wheel camber angle has two functions: ① because the camber angle is negative, it can increase the span of the wheel contact point and increase the lateral stability of the car; ② Negative camber angle is used to counteract the negative toe in (front tension) of the wheels when the car is driving at high speeds and with high driving force, in order to reduce tire wear. The toe angle and camber angle of the wheel cannot be adjusted. On some heavy-duty vehicles with rear wheel drive, due to the use of independent suspension and spine frame, in order to maintain the tires in the correct ground position and reduce wear when the vehicle is loaded, the rear wheels are also designed with a certain positive camber angle, such as the Tatra 138 model.