1. Purpose and significance (including analysis of current research situation at home and abroad)

Automobile suspension is the general name of all the connecting devices between the frame (or the bearing body) and the axle (or wheel). It is an important part to ensure the normal running of the vehicle. It consists of three parts: spring, shock absorber and guide mechanism ^[1]. The maximum total mass of medium-sized truck is 6.0t lt; GA ≤ 14 t according to the maximum total mass of (GA) set at the time of operation of highway. The main domestic manufacturers of light and medium-sized trucks are: Dongfeng, Chongqing Lifan, Zhejiang FeiDie, Chengdu Dayun, Heavy Automobile, Jianghuai, Qingling, Beiqi Futian and so on. At present, the suspension of domestic medium-sized trucks is mainly non-independent suspension. Suspension elastic elements are generally leaf springs. Because it acts as a guiding mechanism, the suspension system is greatly simplified ^[2]. This suspension is widely used in front and rear suspension of trucks. In the middle of it, a U-bolt is used to fasten the leaf spring to the bridge.

The car model designed in this design is Jianghuai Gerfa K6L medium card, K6 medium card as Jianghuai Gerfa's 2015 masterpiece, in the appearance, power, safety and other aspects of the brand-new upgrade. The main task of suspension is to deliver the forces and moments between the wheel and the frame (or body); to ease the impact load transmitted to the frame (or body) of the road surface; to attenuate the vibration of the carrying system; to ensure the ride comfort of the car; and to ensure that the wheels have ideal motion characteristics when the road surface is uneven and the load changes; to ensure the handling and stability of the car, so that the car can achieve high-speed driving capacity. The choice of numbers will affect the dynamic performance of the vehicle, which is mainly reflected in the three aspects of vehicle performance: ride comfort, handling stability and comfort. Therefore, in the design of a vehicle model, we should pay special attention to the structure selection and parameter setting of automobile suspension. Choosing a suitable suspension for the car can not only guarantee the performance of the car, but also reduces the difficulty of research and development and saves the development cost. The non-independent suspension has the advantages of simple structure, low cost, high strength, easy maintenance and small change of front wheel positioning. However, due to its poor comfort and handling stability, and the cab of the truck which is arranged on the front suspension, it is particularly important to design and optimize parameters such as suspension size, which is of great significance to the experience of the truck driver. The design and optimization of parameters such as "inch" is particularly important to the experience of truck drivers. The design chooses the front suspension form composed of leaf spring and cylinder shock absorber, selects the vehicle type to design the front suspension reasonably, makes the suspension match the truck reasonably, and provides better ride, maneuverability, comfort and enough safety, makes the van run normally and drive comfortably. Therefore, this design will provide some advice to freight cars and manufacturers, and it will also be helpful to their own research and study.

1.1 Domestic Research Status

Compared with the wide application of independent suspension and active suspension system in car, most of the suspension systems in China are non-independent suspension with leaf spring as elastic element and guidance device at present. There are more than 160 domestic automobile suspension spring production enterprises, which are all over the country. There are about 80 specialized production enterprises with large scale. The product quality level will soon reach the advanced foreign countries. Therefore, at the present stage of our country, there is still a higher research value. The research on the performance of passive spring suspension mainly focuses on three aspects: through the analysis of the force on the automobile, the mathematics is established. The model is then used to find the optimal parameters of suspension by means of computer simulation or finite element method (FEM). The variable stiffness spring and variable damping vibration absorber are studied to keep the suspension running well in most road conditions. The research of guiding mechanism makes the stability of automobile suspension improve greatly on the premise of satisfying ride comfort. The research of leaf spring suspension mainly starts from the above three parts, and a lot of researches have been carried out on a certain part.

In 2014, Chi Lijun^[3] presented his point of view on the research and design of truck front and rear suspension. Specifically, the truck suspension mechanism, in which non-independent leaf spring suspension is used as front suspension, and non-independent suspension is divided into main and auxiliary spring parts, which are used as rear suspension.

First of all, confirm the structural form of the suspension object, determine the main parameters of each part of the suspension structure, plan the structure design scheme of the front suspension, and apply the allowable stress., types of metal materials and leaf springs used and materials and types of original parts that reduce vibration. Next, check the stiffness and stiffness proportion distribution of the suspension leaf spring, and analyze the main and pair steel plate projectiles, the structural layout of the spring in the rear suspension.

A number of degrees of freedom are used to calculate the comfort level and smoothness of the rear suspension by computer programming, so that the comfort level and smoothness of the rear suspension structure can be judged by the original design intention and ergonomics. Therefore, it is beneficial to improve the handling of automobile . This paper provides clear ideas and steps for the design of the front suspension, and provides a case for the design of the front suspension of the truck, which is helpful to the research and production of the design and production of the front suspension.

The important design component of leaf spring suspension is the leaf spring, for which the research and analysis are of special importance in the design. Yu Anhe ^[5]et al. mentioned the finite element analysis method which is more commonly used in the engineering. The stiffness characteristics of a type of leaf spring are analyzed by the linear contact element provided by Nastran finite element software (or other finite element software). The influence of friction characteristics on the stiffness calculation of leaf spring is discussed. The load-deformation characteristic diagram of leaf spring under dynamic and static loads is given ^[4]. According to the distribution diagram of force and deformation, the position where the leaf spring is easy to fail can be obtained, and the leaf spring suspension is designed for the freight car. There is evidence to be found on the shelf, and it is clear and simple.

Presently, there are many researches on the failure of leaf spring in our country, which makes the technology of leaf spring suspension mature and reliable. Southwest Institute of Technology and Engineering studied the failure behavior of automobile leaf spring by means of chemical composition analysis and hardness test, and got their conclusion: by controlling the heat treatment conditions, the machining allowance was increased and the decarburization layer was removed. The corrosion resistance and fatigue resistance of leaf spring surface can be greatly improved by surface shot peening treatment, which can effectively avoid corrosion fatigue fracture caused by decarbonization. This also provides more attention and optimization method for the design of leaf spring.