The frame is the skeleton of the vehicle, also known as the girder, is an important bearing part of the vehicle, connected with each main assembly, bearing the role of complex space forces. In general, the frame should have sufficient strength, appropriate stiffness, minimum mass under the premise of ensuring stiffness and strength, and the structure should be as simple as possible. With the development of the highway, the speed continues to increase, so it is required that the frame should have enough resistance to bending deformation and torsion deformation.

The characteristics of the semi-trailer frame are mainly that the front of the frame should be installed with the traction pin and its bottom plate, and the traction seat of the tractor is connected through the traction pin. In this way, the top of the frame is much higher than the frame of the tractor. In order to reduce the loading center of mass of goods and ensure the necessary exercise stability of the initial train, it is necessary to make the bottom plane of the cargo box at the rear of the frame as low as possible.

At the same time, the wheelbase of the semi-trailer is generally long, and most of the current market products are steel frames. In order to reduce the total mass of the trailer frame, some round or square holes are sometimes made on the web of the longitudinal beam, which also provides some convenience for the passing of the electric and gas routes of the trailer and the assembly, maintenance and repair of the frame (Figure 1). Therefore, the frame design parameters should meet the following requirements:

(1) Meet the general layout requirements of the tractor and trailer: that is, the tractor can pull normally after the connection with the trailer, and there is no collision or scratching outside the connection device, that is, the tractor does not interfere with the leg or other parts of the trailer after turning, and the trailer's front turning radius does not interfere with the cab or exposed parts of the tractor.

(2) There must be sufficient strength: to ensure that the frame is used under the required working conditions and that no serious damage occurs during the design life cycle.

(3) To have the appropriate stiffness: on the one hand, under various conditions of use, the frame can not have great deformation, so that the relative position of the assembly and components fixed on the frame on the frame is not changed much, to ensure that they can work normally without motion interference or running noise; On the other hand, the stiffness of the frame, especially the torsional stiffness, should not be too large, so as to avoid unnecessary breakage of the frame when passing through the uneven road, and at the same time, it should match the suspension Angle stiffness of the vehicle.

(4) Save materials and reduce quality: on the premise of ensuring that the frame has sufficient stiffness and necessary strength, the structure of the frame should be simplified as far as possible, and the consumption of materials should be reduced to reduce production costs.