Front suspension of medium-capacity vehicle

Purpose and requirements for suspension

Car suspension is a set of devices that provide elastic communication between the carrier system and the bridges or wheels of the car, reduce dynamic loads on the carrier system and wheels and attenuate their vibrations, as well as adjust the position of the car body during movement.

According to the purpose, the suspension parts are divided into an elastic element, which includes a transverse stability stabilizer, a guiding device and a damping device. The elastic element transmits vertical loads and reduces the level of dynamic loads that occur when the car moves along the irregularities over the road, while ensuring the necessary smoothness of the car.

The suspension guide device transmits forces and moments between the wheel and the body to the vehicle carrier system and determines the nature of wheel movement relative to the vehicle carrier system. Depending on the design, the guide device fully or partially releases the elastic element from additional loads transmitted by the wheels to the frame (body) of the car.

The damping device, as well as friction in the suspension, provide attenuation of vibrations of the body and wheels of the car, in which the mechanical energy of the vibrations changes to thermal. Pendants as an elastic element are subdivided into spring, spring, torsion, rubber, pneumatic, hydraulic and combined.

Depending on the type of guide, all suspensions are divided into dependent and independent ones. A feature of a dependent wheel suspension is the presence of a rigid beam connecting the left and right wheels, so the movement of one wheel in the transverse plane is transmitted to another. With independent suspension, there is no direct connection between the wheels. Each wheel of this bridge moves independently of each other.

Independent suspensions by the nature of the movements accompanying the vertical lifting of the wheel are divided into suspensions with the wheel moving in the transverse, longitudinal plane or in two planes (transverse and longitudinal) and candle.

Damping action in suspension is provided mainly by shock absorber. Currently, hydraulic shock absorbers are most widely used. By the nature of the work, one-sided and two-sided shock absorbers are distinguished. Shock absorbers of one-sided action create resistance and dampen oscillations only during deflection, and two-sided action - both during deflection and during compression. According to the design feature, telescopic and lever shock absorbers are distributed.

The main requirements for suspensions are as follows.

1. The elastic characteristic of the suspension must ensure high smoothness of travel, absence of impacts on the travel limiters, counteract rolls during turning, "clues" during braking and "squats" during acceleration of the car.

2. Kinematic scheme shall create conditions for possible minimum change of track and angle of pivots and controlled wheels installation; correspondence of kinematics of wheels movement to kinematics of steering drive, which excludes oscillations of controlled wheels around pivots.

3. Optimum attenuation of body and wheel vibrations.

4. Reliable transmission from wheels to the body or frame of longitudinal and transverse forces and their moments.

5. Small weight of suspension elements and especially unsprung parts.

6. Sufficient strength and durability of suspension parts and especially elastic elements, which are among the most heated parts of the car.

The human body is affected by the amplitude, frequencies, accelerations and intensity of accelerations of oscillatory motion. In the practice of estimating the total intensity of oscillations of the compressed mass of the car, the acceleration dispersion when driving along a given road is widely used. This criterion is integral and takes into account the whole spectrum of acting oscillations.

Dispersion is a statistically well-defined factor, which allows comparative tests or corresponding calculations to assess the effect on its value of the most insignificant changes in the design of the car and its suspension.

The International Organization for Standardization (ISO) has developed a standard that establishes permissible mean quadratic accelerations in the range of 1-80 Hz with a duration of action of 8 hours. The body fluctuates when the car moves along an uneven road, not with slightly changing frequencies, but with frequencies of natural oscillations. Reducing these frequencies helps to reduce accelerations in body vibrations when the car moves along any road and leads to an improvement in the smoothness of the car.

Physiologically, the most familiar for a person are vibrations with frequencies characteristic of normal walking. With good suspension quality, the intrinsic frequencies for cars are 0.8-1.2 Hz, and for trucks and buses - 1.2-1.9 Hz. If the natural frequencies lie within the limits specified above, the human body tolerates oscillations well.

Description of the design of the designed suspension

The main drawback of the prototype suspension structure is the constant stiffness of the elastic element - a metal semi-eleptic 13 sheet spring. Such a design provides the necessary natural oscillation frequency of the body - n = 70-90 kol/min, i.e. low natural oscillation frequencies of the body should ensure a satisfactory state of the human body during oscillations - only with a fully loaded car. While with a partially loaded or empty car, this elastic element is excessively rigid, which leads to an increase in vibrations, tremors from the road surface and, as a result, a decrease in the comfort of the car, damage to the load, a decrease in the durability of the body and frame of the car.

In order to eliminate the above mentioned drawbacks of the prototype suspension design, we will conduct a set of measures:

design an elastic element that meets the conditions of rigidity and strength for an empty car;

in addition to the main elastic element, the additional elastic element is calculated.

Thus, a combined pneumatic-spring suspension is obtained, in which the role of the guiding and partially elastic element is performed by a small-leaf spring, and as an additional - a pneumatic element, the air supply of which comes from the braking system of the car.

The operation of such a suspension is as follows: when the car is empty or partially loaded, only the spring works, when the load on the car increases, the pneumatic element control unit receives a signal from the body position sensor and the load on the car, opens the air supply valve. As a result, the pressure in the pneumatic element increases to the necessary value - its rigidity and the entire suspension increases.