The vibration characteristics are related, and the natural vibration characteristics of the car body are related to the characteristics of the suspension. so,carSuspension is an important part to ensure riding comfort. At the same time, automobile suspension, as a force transmission part connecting the frame (or body) and the axle (or wheel), is also an important part to ensure the safety of the automobile. Therefore, car suspension is often listed as an important component in the technical specification table of the car, as one of the indicators to measure the quality of the car.
If the car frame (or body) is directly installed on the axle (or wheel), the goods and people will feel very uncomfortable due to the uneven road and ground impact. This is because there is no suspension device. Automobile suspension is a general term for the elastic coupling device between the frame (or body) and the axle (or wheel). Its function is to elastically connect the axle and the frame (or body) to alleviate the impact of the vehicle during driving. Ensure the cargo is intact and the personnel are comfortable; attenuate the vibration introduced by the elastic system, so that the car maintains a stable posture while driving, and improves the handling stability; at the same time, the suspension system is responsible for transmitting the vertical reaction force, the longitudinal reaction force (traction and braking force) and The lateral reaction forces and the moments caused by these forces act on the frame (or body) to ensure the smooth running of the car; and when the wheels bounce relative to the frame, especially when turning, the wheel trajectory must meet certain requirements. Therefore, the suspension also plays a guiding role to make the wheels jump relative to the vehicle body according to a certain trajectory.
Whether the selection of the suspension structure and performance parameters is reasonable or not has a great impact on the ride comfort, handling stability and comfort of the car. This shows that the suspension system is one of the important assemblies in modern cars.
The general suspension is composed of elastic elements, guiding mechanisms, shock absorbers and transverse stabilizers. The elastic element is used to bear and transmit the vertical load, and alleviate the impact on the car body caused by uneven road surface.Types of elastic components includeLeaf spring, Coil springs, torsion bar springs, oil-air springs, air springs and rubber springs. The shock absorber is used to attenuate the vibration caused by the elastic system. The types of shock absorbers include cylindrical shock absorbers, new-type shock absorbers with adjustable resistance, and inflatable shock absorbers. The guiding mechanism is used to transmit the force and moment between the wheel and the vehicle body, while keeping the wheel jumping relative to the vehicle body according to a certain movement trajectory. Usually the guiding mechanism is composed of a control swing arm type rod. There are single rod type or multi-link type. When the leaf spring is used as an elastic element, there is no need for additional guiding mechanism, and it also plays a guiding role. In some cars and buses, in order to prevent excessive lateral tilt of the body during steering, etc., a horizontal stabilizer bar is added to the suspension system to increase the lateral rigidity, make the car have insufficient steering characteristics, and improve the steering stability of the car Performance and driving comfort.
The natural frequency of a car is an important parameter to measure the ride comfort of the car, which is determined by the stiffness of the suspension and the mass of the suspension spring (sprung mass). The vertical vibration frequency that the human body is accustomed to is about 1 to 1.6 Hz. The natural frequency of vehicle body vibration should be close to or within the frequency range adapted to the human body to meet the comfort requirements. The natural frequency is calculated as follows:
Where: g-gravity acceleration; f-suspension vertical deformation (deflection) M-suspension sprung mass
C(=Mg/f)- Suspension stiffness refers to the vertical load applied to the suspension required to produce unit vertical compression deformation of the suspension. From the natural frequency formula, it can be seen that when the suspension vertical load is constant, the suspension stiffness is Smaller, the lower the natural frequency
However, the smaller the stiffness of the suspension, the greater the vertical deformation of the suspension when the load is constant. In this way, if there is no sufficient limit stroke, the probability of hitting the limit block will increase. If the natural frequency is selected too low, it is likely that there will be braking nodding angle, turning side cargo angle, unloaded and fully loaded vehicle height changes too much. The natural frequency of general trucks is 1.5~2Hz, station wagons are 1.2~1.8Hz, and luxury cars are 1~1.3Hz. In addition, when the suspension stiffness is constant, the greater the sprung mass, the greater the vertical deformation of the suspension, and the lower the natural frequency. The natural frequency of an empty vehicle is higher than that of a fully loaded vehicle. The sprung mass changes in a large range, and the natural frequency changes in a large range. In order to keep the natural frequency of no-load and full-load at a certain or small change, the stiffness of the suspension needs to be made variable or adjustable.
Another suspension index that affects car ride comfort is sprung quality. The sprung mass is divided into two parts: the sprung mass and the unsprung mass. The part of the mass carried by the elastic element, such as the body, the frame, and all other components and loads above the spring, belong to the sprung mass. Wheels, axles of non-independent suspensions, etc. belong to unsprung mass, also called unsprung mass M. If the unsprung mass is reduced, the vibration frequency of the vehicle body can be reduced, and the vibration frequency of the wheel can be increased, which is beneficial to reduce resonance and improve the ride comfort of the car. The influence of unsprung mass on ride comfort is usually evaluated by the ratio of unsprung mass to sprung mass m/M. The smaller the ratio, the better.
Another important indicator that affects car ride comfort is the damping ratio Ψ, which is expressed as:
k- represents the drag coefficient of the suspension damping element.
A large value of Ψ can quickly attenuate the vibration, but will transmit the larger impact of the road to the car body. A small value of Ψ will slow the vibration attenuation, and the vibration lasts for a long time after being impacted, which makes passengers feel uncomfortable. In order to give full play to the role of the spring in the compression stroke, the damping ratio Ψ of the compression stroke is often designed to be smaller than the extension.
The roll angle stiffness and front-to-back matching of the suspension are important parameters that affect the handling and stability of the car. When the car body rolls under the action of lateral force, if the roll angle is too large, the passengers will feel unsafe and uncomfortable. good. Therefore, the roll angle stiffness of the whole vehicle should satisfy: when the body is subjected to 0.4g lateral acceleration, its roll angle is in the range of 2.5～4°, the vehicle has certain under-steer characteristics, and the roll angle stiffness of the front suspension should be greater than that of the rear suspension. Inclination stiffness. Generally, the ratio of the front suspension roll angle stiffness to the rear suspension roll angle stiffness should be in the range of 1.4 to 2.6. If the front and rear suspensions themselves cannot meet the above requirements, transverse stabilizers can be installed in the front and rear suspensions to improve the steering stability of the car .
Active suspension
There are two types of suspensions in modern cars, one is driven suspension and the other is active suspension.
The driven suspension is the traditional suspension, which is composed of springs, shock absorbers (damping cylinders), guiding mechanisms, etc. Its function is to weaken the impact force transmitted from the road to the vehicle body, and attenuate the load caused by the impact force. Vibration of the system. Among them, the spring mainly plays the role of damping the impact force, and the main role of the shock absorber is to attenuate the vibration. Since this type of suspension is driven by external forces, it is called a driven suspension.
In the active suspension control link, a device capable of generating twitching is installed, and a force suppression method is adopted to suppress the impact of the road on the vehicle body and the tilt force of the vehicle body. Because this kind of suspension can generate force by itself, it is called active suspension.
Active suspension has been developed in the past ten years. A new type of suspension controlled by a computer has three conditions: (1) It has a power source capable of generating force; (2) The actuator can transmit this force. And can work continuously; (3) has a variety ofsensorAnd collect the relevant data to the microcomputer to calculate and determine the control method. Therefore, the active suspension brings together the technical knowledge of mechanics and electronics, and is a relatively complex high-tech device.
For example, the French Citroen Santilla equipped with active suspension. The center of the suspension system of the car is a microcomputer. There are 5 types of suspensions.sensor, Respectively send data to the microcomputer such as vehicle speed, front wheel brake pressure, accelerator pedal speed, amplitude and frequency in the vertical direction of the body, steering wheel angle and steering speed. The computer continuously receives these data and compares it with the preset threshold value to select the corresponding suspension state. At the same time, the microcomputer independently controls the actuators on each wheel, and generates twitching by controlling the change of oil pressure in the shock absorber, so that the suspension motion can be produced on any wheel at any time and meet the requirements. Therefore, Santilla Bridge is equipped with a variety of driving mode options. The driver only needs to pull the "normal" or "sports" button on the sub-instrument panel, and the car will automatically be set in the best suspension state. The best comfort performance.
In addition, the active suspension has the function of controlling the movement of the body. When the inertia of the car braking or cornering causes the spring to deform, the active suspension will generate a force that opposes the inertia, reducing the change of the body position.E.g. GermanyBenzFor 2000 CL sports cars, when the vehicle is turning, the suspension sensor will immediately detect the inclination and lateral acceleration of the vehicle body. The computer will compare and calculate with the preset threshold value based on the sensor information, and immediately determine what position it will be. The load is added to the suspension to minimize the inclination of the body.
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