Four wheel positioning

The steering axle shall not only ensure the steering function of the vehicle, but also meet the requirements for handling stability. For example, when the steering wheel deflects under the force, once the applied external force disappears, it shall automatically return to the original straight driving position. This automatic return action is guaranteed by the positioning parameters of the steering wheel. The steering wheel positioning parameters include kingpin caster, kingpin inclination, front wheel camber and front wheel toe in. Rear wheel alignment parameters include rear wheel camber and rear wheel toe in.

1. Caster angle of kingpin

In the longitudinal plane of the car, the kingpin upper part inclines backward, which is called kingpin backward. The included angle y between the kingpin axis and the vertical line of the ground in the longitudinal plane of the vehicle is called kingpin caster.

When the kingpin has a caster angle y, the intersection point a between the extension line of kingpin axis and the road surface is located in front of the contact point b between the wheel and the road surface, and the car runs in a straight line. If the steering wheel is slightly deflected due to accidental external forces (for example, it deflects to the right, as shown by the arrow in the figure), the driving direction of the car will deflect to the right. Due to the centrifugal force of the car itself, at the contact point b between the wheel and the road surface, the road surface exerts a lateral reaction force F10F on the wheel, which generates moments F, L relative to the kingpin axis, and its direction is just opposite to the wheel deflection direction. This torque plays the role of making the wheels return to their original positions, which is called the stabilizing torque, and it maintains the stability of the car in a straight line. However, the stabilizing torque should not be too large, otherwise, in order to overcome the stabilizing torque during steering, the driver will exert greater force on the steering wheel, causing heavy steering. Since the caster angle y of kingpin determines the size of force arm L and the stability torque, the stability torque can be adjusted by adjusting the size of y. Generally, the y angle does not exceed 20~3. The stability torque of modern high-speed vehicles increases due to the reduction of tire pressure and the increase of elasticity, and the y angle can be reduced to close to zero or even negative. The caster angle of kingpin is generally obtained by tilting the front axle section backward when the front axle, leaf spring and frame are assembled together.

2. Kingpin inclination

The phenomenon that the upper part of the kingpin inclines inward in the transverse plane of the car is called kingpin inclination. The included angle B between the kingpin axis and the vertical line of the ground in the transverse plane of the vehicle is called the kingpin inclination.

The kingpin inclination B also has the function of automatically aligning the wheels. When the steering wheel deflects an angle from the middle position (as shown by the double dot line drawing in the figure) under the external force, the lowest point of the wheel will fall below the road surface theoretically. But in fact, this is impossible, but the steering wheel and the whole front of the car are lifted up to a corresponding height. At this time, the gravity of the car itself will make the steering wheel return to the original middle position. The kingpin inclination also reduces the distance c between the intersection point of kingpin axis and road surface and the intersection line c between the wheel center plane and the ground, which can reduce the force on the steering wheel during steering, make steering easy to operate, and reduce the impact force transmitted from the steering wheel to the steering wheel. However, the value of c should not be too small, that is, the kingpin inclination should not be too large, otherwise, during the steering process when the wheel deflects around the kingpin, large sliding will occur between the tire and the road, and the friction resistance between the tire and the road will increase. It causes heavy steering and increased tire wear. General inclination angle β Not more than 8 °, the distance c is generally 40~60mm.

The kingpin inclination is guaranteed in the front beam design. During machining, tilt the upper end of the kingpin hole axis at both ends of the front beam inward to form the inclination B.

As mentioned above, kingpin roll back and kingpin roll in both have the function of automatically returning the steering of the car and maintaining straight driving. However, the caster angle is related to the vehicle speed, and the caster angle is almost independent of the vehicle speed. Therefore, at high speed, the kingpin recline is dominant, while at low speed, the kingpin recline is dominant. In addition, when the front wheel occasionally encounters impact and deflects when the car is driving in a straight line, the kingpin also leans inward to play a positive role.

3. Front wheel camber

When the front wheel is installed on the axle, the phenomenon that the rotation plane of the front wheel slightly inclines outward is called front wheel camber. The included angle a between the intersection line of the transverse plane of the vehicle passing through the wheel and the wheel plane and the vertical line of the ground is called the front wheel camber. If the wheel is installed exactly perpendicular to the road surface when the vehicle is empty, the axle may tilt inward due to load deformation when the vehicle is fully loaded, which will accelerate the eccentric wear of the vehicle tires. In addition, the axial component of the vertical reaction force of the road to the wheel along the wheel hub will make the wheel hub press against the small bearing at the outer end of the wheel hub, increasing the load on the small bearing at the outer end and the fastening nut of the wheel hub, which is easy to loosen the nut and cause the wheel to fly out, causing danger. Therefore, in order to make the tire wear even, reduce the load of the outer bearing of the wheel hub, and improve the driving safety, the wheels should be installed with a certain camber angle to prevent the wheels from tipping inward. At the same time, the camber of the wheels can also be adapted to the arched road surface. However, the camber angle should not be too large, otherwise it will cause partial wear of tires.

The camber of the front wheels is determined when designing the steering knuckle. It is obtained by making the axis of steering knuckle journal form an angle with the horizontal plane during design (generally a is about 1).

4. Front wheel toe in

After the two front wheels of an automobile are installed, in the plane parallel to the ground through the wheel axis, the toe in of the front ends of the two wheels is called the toe in of the front wheels. The front edge distance B between the left and right wheels is less than the rear edge distance A. The difference between A and B is called the front toe in value.

Toe in of the front wheel is to eliminate the adverse effects of wheel camber. Due to the camber of the front wheels, the wheels on both sides will roll outward when rolling. As the constraints of the steering tie rod and axle make it impossible for the wheels to roll outward, the toe in (top view) of the wheels will roll and slip on the ground, thus increasing the wear of the tires. When two wheels have toe in, they will slide inwards when rolling forward. In this way, the slippage caused by toe in can offset the slippage caused by camber, so as to basically ensure that there is no sliding forward movement of the two front wheels. The front and rear distance of the front wheels can be adjusted by changing the length of the tie rod. The adjustment can be made according to m of each plant. The measuring position except for the position shown in the figure - B) conforms to the specified toe in value. The toe in value is generally the front and rear difference at the inner side of the steel ring of the 0 ring. Outside, the front and rear difference at the center plane of two tires is usually taken. In addition, toe in can be expressed by angle toe in angle.

5. Camber angle and toe in of rear wheel

For front wheel drive vehicles and independent rear suspension, with the improvement of road conditions, the driving speed of modern cars is getting higher and higher, which is due to the maneuverability and early tire wear. For the sake of the car, if the rear wheel is not properly positioned, even if the front wheel is well positioned, there will still be some defects. The design should ensure that the car has the steering characteristics that do not prevent "jerk" and automatic steering phenomena when driving at high speeds. The rear wheels of cars have a certain degree of camber and toe in, which can make the rear wheels get a proper side slip angle and improve the handling stability of high-speed driving.

Ideally, the camber of all four wheels is zero, so that the tires have good contact with the road surface, thus obtaining the best traction performance and handling performance.

The camber is not static, it changes with the suspension moving up and down. After the vehicle is loaded, the suspension sinking will cause the camber angle to change.

In order to compensate the load, most vehicles with independent rear suspension often have a small front and rear wheel camber. Failure or dislocation of the strut cylinder, bending of the strut, failure of the upper control arm bushing, bending of the upper control arm, spring compression or overload of the suspension will cause the rear wheel camber to become negative. Bent steering knuckle and lower control arm will cause excessive camber of rear wheels.

The toe in of the rear wheels will change when the suspension moves and rebounds. Rolling resistance and engine torque also affect the toe in of the rear wheels. For front drive vehicles, the front drive wheel should have positive toe in and the rear driven wheel should have negative toe in. On the contrary, front and rear drive vehicles should have negative toe in, and independent rear suspension drive wheels should have positive toe in as far as possible.

If the toe in of the rear wheel does not meet the technical requirements, the tire wear and steering stability will be affected to the same extent as the toe in of the front wheel. Even if the toe in measurement value is within the specified range, it does not mean that the wheel alignment is correct, especially for the rear wheel toe in measurement. If the inward deflection of the front end of one side of the rear wheel is equal to the outward deflection of the front end of the other side of the rear wheel, the toe in value will be within the specified range. However, because the rear wheel is not parallel to the longitudinal axis of the vehicle, the vehicle will still pull away.

When the car is running on the road, the ideal state is that the toe in of all wheels is zero. This is especially true for anti lock vehicles. Because when braking on slippery and wet roads, incorrect toe in will affect the braking balance. In order to prevent slip, the anti lock brake will keep on and off. When there is no anti lock braking system, the ground driving force may be disturbed, which may cause uncontrollable slip.

If the two rear wheels are parallel to each other and the whole vehicle, the driving force line will be perpendicular to the rear axle and coincide with the longitudinal axis of the vehicle. However, if the front end of one or two rear wheels deviates inward or outward, or one wheel retracts slightly with respect to the other, the driving line will deviate from the centerline, thus generating a driving force deviation angle and causing the vehicle to deviate in the direction opposite to the deviation angle. For example, when the line of driving force action is to the right, the car pulls to the left.

The appearance of driving force deviation angle makes the directional stability of vehicles on ice, snow or wet roads worse. When the vehicle is braked or accelerated sharply, it will sometimes make the vehicle run away and aggravate the tire wear. Therefore, the front wheels used for steering control should overcome this effect of the rear wheels.

The above problems can only be solved by eliminating the driving force deviation angle. By resetting the toe in of the rear wheel, the driving force line can be returned to the center.

For some heavy vehicles with independent suspension and center frame rear wheel drive, the rear wheels are also designed to have a certain positive camber angle in order to keep the tires at the correct grounding position when the loaded vehicle is running and reduce wear.