Suspension

Classification of suspension system

According to different control forms, it is divided into passive suspension and active suspension. According to the different guiding mechanism of automobile, it can be divided into independent suspension and non independent suspension.

Dependent suspension

The structural feature of non independent suspension is that the wheels on both sides are connected by an integral axle, and the wheels together with the axle are suspended under the frame or body through elastic suspension. Non independent suspension has the advantages of simple structure, low cost, high strength, easy maintenance and small change in front wheel positioning during driving. However, due to its poor comfort and handling stability, it is basically no longer used in modern cars and is mostly used in trucks and buses.

independent suspension

Independent suspension means that the wheels on each side are hung separately under the frame or body through elastic suspension. The utility model has the advantages of light weight, reducing the impact on the vehicle body and improving the ground adhesion of the wheels; The soft spring with small stiffness can be used to improve the comfort of the car; It can reduce the position of the engine and the center of gravity of the vehicle, so as to improve the driving stability of the vehicle; The left and right wheels beat separately and are irrelevant to each other, which can reduce the inclination and vibration of the vehicle body. However, the independent suspension has the disadvantages of complex structure, high cost and inconvenient maintenance. Most modern cars use independent suspension. According to their different structural forms, independent suspension can be divided into cross arm type, longitudinal arm type, multi link type, candle type and McPherson type suspension.

Wishbone suspension

Wishbone suspension refers to the independent suspension in which the wheel swings in the transverse plane of the vehicle. According to the number of wishbones, it is divided into double wishbone suspension and single wishbone suspension.

Single wishbone has the advantages of simple structure, high roll center and strong anti roll ability. However, with the improvement of the speed of modern vehicles, the excessive roll center will cause the large change of track width when the wheels jump and the aggravation of tire wear. In addition, the excessive vertical force transfer between the left and right wheels during sharp turns will increase the camber of the rear wheels and reduce the lateral stiffness of the rear wheels, resulting in the serious working condition of high-speed tailspin. Single wishbone independent suspension is mostly used in the rear suspension, but it is not widely used because it can not meet the requirements of high-speed driving.

Double wishbone independent suspension is divided into equal length double wishbone suspension and unequal length double wishbone suspension according to whether the upper and lower wishbones are of equal length. The equal length double wishbone suspension can keep the kingpin inclination unchanged when the wheels jump up and down, but the wheel track changes greatly (similar to the single wishbone type), resulting in serious tire wear, which is rarely used now. For unequal length double wishbone suspension, as long as the length of upper and lower wishbones is properly selected and optimized, and through reasonable layout, the changes of track width and front wheel positioning parameters can be within an acceptable limit, so as to ensure good driving stability of the vehicle. Unequal length double wishbone suspension has been widely used in the front and rear suspension of cars. This suspension structure is also used in the rear wheels of some sports cars and racing cars.

multi-link suspension

Multi link suspension is a suspension in which (3-5) rods are combined to control the position change of wheels. The multi link type can make the wheel swing around the axis at a fixed angle with the longitudinal axis of the vehicle. It is a compromise scheme between the cross arm type and the longitudinal arm type. Properly selecting the included angle between the swing arm axis and the longitudinal axis of the vehicle can obtain the advantages of the cross arm type and longitudinal arm type suspension to varying degrees and meet different performance requirements. The main advantage of multi link suspension is that the change of track width and toe in is very small when the wheels jump. No matter the vehicle is in the driving and braking state, it can turn smoothly according to the driver’s intention. Its disadvantage is that the vehicle has axle swing at high speed.

Trailing arm suspension

Longitudinal arm independent suspension refers to the suspension structure in which the wheel swings in the longitudinal plane of the vehicle, which is divided into single longitudinal arm type and double longitudinal arm type. When the wheels of the single trailing arm suspension jump up and down, the caster angle will change greatly, so the single trailing arm suspension is not used on the steering wheel. The two swing arms of double trailing arm suspension are generally made of equal length to form a parallel four-bar structure. In this way, when the wheels jump up and down, the caster angle of the kingpin remains unchanged. Double trailing arm suspension is mostly used on steering wheels.

Candle suspension

The structural feature of candle suspension is that the wheels move up and down along the kingpin axis rigidly fixed on the frame. The advantage of candle suspension is that when the suspension is deformed, the positioning angle of kingpin will not change, only the wheel base and wheelbase will change slightly, so it is especially conducive to the steering, handling and driving stability of the vehicle. However, the candle suspension has a big disadvantage: the lateral force when the car is driving will be borne by the kingpin sleeved on the kingpin sleeve, resulting in increased friction resistance and serious wear between the sleeve and the kingpin. Candle suspension is not widely used.

McPherson Suspension

The wheels of McPherson suspension also slide along the kingpin, but it is not exactly the same as candle suspension. Its kingpin can swing. McPherson suspension is a combination of swing arm and candle suspension. Compared with the double wishbone suspension, the advantages of McPherson suspension are: compact structure, small change of front wheel positioning parameters during wheel runout, good handling stability, and convenient arrangement of engine and steering system due to the cancellation of the upper wishbone; Compared with candle suspension, the lateral force on its sliding column has been greatly improved. McPherson suspension is mostly used in the front suspension of small and medium-sized cars. The front suspension of Porsche 911, domestic Audi, Santana, Xiali, Fukang and other cars are McPherson independent suspension. Although McPherson suspension is not the suspension structure with the highest technical content, it is still a durable independent suspension with strong road adaptability.

Active suspension

Active suspension is a new type of suspension developed in recent ten years and controlled by computer. It brings together the technical knowledge of mechanics and electronics. It is a relatively complex high-tech device. For example, for the French Citroen santia with active suspension, the center of the suspension system of the car is a microcomputer. The five sensors on the suspension respectively transmit data such as vehicle speed, front wheel braking pressure, the speed of stepping on the accelerator pedal, the amplitude and frequency in the vertical direction of the body, steering wheel angle and steering speed to the microcomputer. The computer continuously receives these data and compares them with the preset threshold value to select the corresponding suspension state. At the same time, the microcomputer independently controls the actuator on each wheel and generates twitch by controlling the change of oil pressure in the shock absorber, so as to produce satisfactory suspension movement at any time and on any wheel. Therefore, Sandia car has a variety of driving mode options. As long as the driver pulls the “normal” or “Sports” button on the auxiliary instrument panel, the car will be automatically set in the best suspension state for the best comfort performance.

Active suspension has the function of controlling body movement. When the inertia of the car when braking or turning causes the spring deformation, the active suspension will produce a force relative to the inertia force to reduce the change of body position. For example, when the vehicle turns a corner, the suspension sensor will immediately detect the inclination and lateral acceleration of the body. According to the information of the sensor, the computer compares and calculates with the preset critical value, and immediately determines where to add the load to the suspension to minimize the inclination of the body.

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