According to the stress situation, the body shell can be divided into three types: non-load-bearing, semi-load-bearing and load-bearing (or full load-bearing). Non-load-bearing car body is characterized by flexible connection between car body and frame through spring or rubber pad. In this case, the car body installed on the frame has little reinforcement effect on the frame, and the car body only follows its own gravity, the gravity of the people and goods it carries, and the inertia force and air resistance generated when the car runs. The frame bears the gravity of the engine and chassis components, the force transmitted by these components through its bracket when working, and the force transmitted by the road surface through the wheels and suspension when the car is running (the last item has the greatest influence on the frame or body). The semi-loaded car body is characterized in that the car body and the frame are rigidly connected by means of screws, riveting or welding. In this case, the car body not only bears the above load, but also helps to strengthen the frame and share part of the load of the frame to some extent. The characteristic of the load-bearing car body is that the car has no frame, and the car body is the installation basis of the engine and chassis assembly. In this case, all the above loads are borne by the car body. In order to reduce the weight of vehicles and save materials, most intermediate cars, ordinary cars, mini-cars and some passenger cars often adopt load-bearing structures. The cab of a freight car only accounts for a small part of the length of the car, so it is impossible to adopt a load-bearing structure. Without a completely closed frame, it is difficult to adopt an open body (convertible) load-bearing structure. If the body of a luxury car is to improve the comfort of the car, reduce the vibration from the engine and chassis assembly at work and the impact transmitted to the body by the road through the wheels and suspension when the car is running, the non-load-bearing structure can be adopted. The car body and truck cab have no obvious skeleton, but are spatial structures welded by external covering and internal sheet metal parts. The floor of a load-bearing car body has relatively complete longitudinal and transverse load-bearing elements, and there are two longitudinal beams 1 1 at the front of the floor, which are often welded together with the front fender 8 and the front radiator fixing frame 9 at both sides to form a rigid space frame, and the engine, front suspension and other components can be directly installed and bear their working loads. On the contrary, the front of non-load-bearing cars (long-headed trucks, too) is weak, and its front sheet metal parts are usually not welded on the body shell, but connected with each other by screws and installed on the frame.