A commercial vehicle frame structure lightweight design method under a whole vehicle environment

Abstract

The present application relates to a kind of commercial vehicle frame structure light-weight design method under whole vehicle environment, comprising the following steps: S1, for the characteristics of commercial vehicle, establish frame light-weight design element, including optimization target, optimization variable and constraint condition;S2, according to the topological relationship of commercial vehicle, establish whole vehicle multi-rigid body dynamics model;S3, establish frame finite element model, and carry out parameterization operation, input optimization variable, output optimization target;S4, establish whole vehicle rigid-flexible coupling dynamics model, carry out commercial vehicle structure performance analysis under whole vehicle environment, and the result of commercial vehicle structure performance analysis is used as constraint condition;S5, judge whether the weight of frame is reduced to meet design requirements, while judge whether the structure performance of commercial vehicle meets design requirements;If not satisfied, then a new set of parameters is obtained to carry out iterative calculation;If satisfied, then output frame light-weight design scheme.

Description

Technical Field

[0001] This invention relates to the field of automotive technology, and in particular to a lightweight design method for the frame structure of a commercial vehicle in a whole-vehicle environment. Background Technology

[0002] Lightweight design in the automotive industry, as a crucial means to improve fuel economy, achieve energy conservation and emission reduction, enhance handling experience, and improve product market competitiveness, has become a key focus for automotive manufacturers. The chassis, as a vital component of commercial vehicle chassis, withstands bending torque generated by movement and random excitation from uneven road surfaces. The rationality of its lightweight design directly affects the structure's strength, modal dynamics, and fatigue durability, thus impacting the vehicle's ride comfort, handling stability, NVH (noise, vibration, and harshness), passive safety, and other performance characteristics. Therefore, developing lightweight chassis design for commercial vehicles is of great significance.

[0003] Current lightweight design methods for commercial vehicle chassis have certain limitations. When establishing a finite element model for analysis, they only treat single or a subset of components as the lightweight design domain for optimization calculations, simplifying the effects of other components to boundary conditions and load parameters. While this approach is convenient for modeling, the simplified boundary conditions contain errors compared to actual engineering connections, and the static and dynamic characteristics of individual structures differ significantly from those in a vehicle-wide environment. Individual structural optimization schemes may not be optimal in a vehicle-wide environment. It is necessary to combine multi-objective, multi-condition optimization methods within the vehicle environment, employing multi-objective collaborative optimization and simulation verification to obtain a lightweight solution that better meets the performance requirements of the entire vehicle. Therefore, conducting lightweight chassis design within a vehicle-wide environment has significant engineering implications. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a lightweight design method for commercial vehicle frame structures in a whole-vehicle environment. Based on a rigid-flexible coupling dynamic model of the whole vehicle that can take into account the influence of frame flexibility, the method ensures that the performance indicators of commercial vehicle structures are more accurate and reliable during the solution process by automatically satisfying the frame boundary and load conditions. This results in a lightweight design scheme that is closer to practical applications, providing a more scientific and efficient solution for commercial vehicle frame design.

[0005] This invention is achieved using the following technical solution: A lightweight design method for commercial vehicle frame structure in a vehicle environment includes the following steps: S1, for the characteristics of commercial vehicles, establishes the lightweight design elements of the chassis, including optimization objectives, optimization variables and constraints; S2. Based on the topological relationships of commercial vehicles, establish a multi-rigid-body dynamics model for the entire vehicle; S3. Establish the finite element model of the chassis and perform parametric operations, input optimization variables and output optimization objectives; S4. Establish a rigid-flexible coupling dynamic model of the whole vehicle, conduct structural performance analysis of the commercial vehicle in the whole vehicle environment, and use the results of the structural performance analysis of the commercial vehicle as constraints. S5 determines whether the weight reduction of the chassis meets the design requirements, and at the same time determines whether the structural performance of the commercial vehicle meets the design requirements; if not, a new set of parameters is obtained for iterative calculation; if it meets the requirements, the lightweight chassis design scheme is output.

[0006] As a further explanation of the invention, step S1 includes: The optimization variables include the frame shape and dimensions, the spatial position of the frame crossbeams, the thickness of the frame crossbeams and longitudinal beams, and the frame material. Optimization targets include frame quality; The constraints include the structural strength of the chassis, the constraint modes of the chassis, the fatigue life of the chassis, and the equivalent stiffness of the whole vehicle.

[0007] As a further explanation of the invention, step S2 includes: Using computer software, a multi-rigid-body dynamics model of the whole vehicle is established, including the cab system, powertrain system, frame system, front and rear suspension systems, steering system, and tires; The cab system, powertrain system, frame system, front and rear suspension systems, steering system, and tires are identified as key subsystems, and connection relationships are established within and between these subsystems through constraint pairs.

[0008] As a further explanation of the invention, step S3 includes: Using computer software, a finite element model of the chassis is established and parametric operations are performed. The chassis topography and dimensions, and / or the spatial position of the chassis crossbeams, and / or the thickness of the chassis crossbeams and longitudinal beams, and / or the chassis material are input to change the chassis components. At the same time, the topological connection relationship between the changed components and related components is maintained, and an updated chassis finite element model is generated, and the updated chassis quality is output. Superelements are defined as degree-of-freedom reduction boundaries at the connection points between the frame system and the associated subsystems, as well as at the points of force application.

[0009] As a further explanation of the invention, step S4 includes: S41: Based on the finite element model of the chassis, the modal synthesis method is applied to solve the chassis modal neutral file and the chassis modal stress file; based on the multi-rigid-body dynamics model of the whole vehicle, the chassis modal neutral file is read, the chassis system and the connection relationship between the chassis system and related subsystems are replaced, and then the rigid-flexible coupling dynamics model of the whole vehicle is established.

[0010] As a further explanation of the invention, step S4 also includes: S42: Based on the rigid-flexible coupling dynamic model of the whole vehicle, apply force load or road excitation to obtain the stiffness analysis results of the frame structure; S43: Conduct modal analysis to obtain the results of frame-constrained modal analysis and mode shape animation; S44: Apply road surface excitation and output the chassis modal displacement files under different road surface conditions; based on the modal stress recovery theory, superimpose the chassis modal stress files and chassis modal displacement files to obtain the chassis stress time history; use the rainflow counting method to count the number of stress cycles; calculate the fatigue life of each node of the chassis according to the SN method, compare the fatigue life of each node of the chassis, and take the minimum value as the chassis fatigue life; S45: Apply a force load to put the whole vehicle in a torsional state and obtain the equivalent torsional stiffness of the whole vehicle; and apply a force load to put the whole vehicle in a bending condition and obtain the equivalent bending stiffness of the whole vehicle; and then obtain the stiffness analysis results of the whole vehicle system.

[0011] As a further explanation of the invention, step S45 includes: The six degrees of freedom at the wheel center end are constrained. Equal nodes are taken on the lower flanges of the left and right longitudinal beams within the front suspension support area. Force is applied to each node. The force on the left longitudinal beam is downward and the force on the right longitudinal beam is upward, so that the whole vehicle is in a torsional state. The torsional angle of the frame on the plane of the applied action section can be obtained to obtain the equivalent torsional stiffness of the whole vehicle. Equal nodes are selected on the upper flanges of the left and right longitudinal beams at the midpoint between the front axle and the rear axle. Force is applied to each node with the direction of the applied force downward, so that the whole vehicle is in a bending condition. The displacement of the force application point on the frame in the vertical direction can be obtained to obtain the equivalent bending stiffness of the whole vehicle.

[0012] As a further explanation of the invention, step S5 includes: An optimization algorithm is applied to obtain a new set of parameters. After updating the finite element model of the chassis, iterative calculations are continued.

[0013] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention establishes a rigid-flexible coupling dynamic model of the whole vehicle. By automatically satisfying the frame boundary and load conditions, the accuracy and reliability of performance indicators such as frame structural strength, constraint mode and fatigue life are improved, thereby improving the overall design quality. (2) The script of this invention realizes the automation of simulation such as finite element model parameter modification, modal neutral file generation and output, rigid-flexible coupling dynamic model establishment, structural strength analysis, constraint mode solution, modal displacement output, structural fatigue durability performance analysis and vehicle equivalent stiffness solution, laying the foundation for the lightweighting of the chassis; (3) The present invention applies multi-objective optimization technology to improve the robustness of commercial vehicle frame structure design, shorten the design and debugging cycle, and improve product development efficiency; (4) This invention is applicable to commercial vehicle frame structures of various structural forms, and is also applicable to lightweight design of other structural components. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the method flow of the present invention; Figure 2 This is a schematic diagram of the rigid-flexible coupling dynamics model of the whole vehicle according to the present invention; Figure 3 This is a comparison diagram of the frame quality before and after optimization according to an embodiment of the present invention; Figure 4 This is a comparison diagram of the frame structure strength before and after optimization according to an embodiment of the present invention; Figure 5 This is a comparison diagram of the vehicle frame constraint modal optimization before and after an embodiment of the present invention; Figure 6 This is a comparison diagram of the frame fatigue life before and after optimization according to an embodiment of the present invention; Figure 7 This is a comparison diagram of the equivalent torsional stiffness of the whole vehicle before and after optimization according to an embodiment of the present invention; Figure 8 This is a comparison diagram of the equivalent bending stiffness of the whole vehicle before and after optimization according to an embodiment of the present invention. Detailed Implementation

[0015] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0016] Preliminary notes: ADAMS software refers to automatic analysis software for mechanical system dynamics; the ADAMS / Car module is a functional module in ADAMS software used for modeling, simulation, and optimization analysis of the whole vehicle; the ADAMS / Durability module is a specialized module in ADAMS software, mainly used for durability analysis. Bushing connection, also known as bushing connection, is a common connection method in engineering simulation and finite element analysis, used to simulate the behavior of elastic connections; Joint connection, a common connection method, is a kinematic pair connection method, generally used to simulate kinematic pairs. HyperMesh software refers to professional finite element preprocessing software; the HyperMesh / Morphing module is a functional module in HyperMesh software used for mesh deformation and shape optimization. SFE CONCEPT software refers to an implicit parametric modeling software; ANSYS software refers to a large-scale general-purpose finite element analysis software; Modal synthesis is an analytical method used to reduce the degrees of freedom, and it is particularly suitable for the dynamic analysis of complex and large structures. Modal stress recovery theory is a method used for fatigue analysis that assesses the fatigue life of materials by recovering the stress information of the structure. Rainflow counting is an algorithm used to analyze and process time history data, and it is widely used, especially in the fields of fatigue testing and fatigue life prediction. nCode software is an integrated fatigue design software for engineering. The nCode / SNCAEFatigue module is a functional module in the nCode software used for high-cycle fatigue life analysis. The Goodman correction method is a correction method in materials fatigue analysis used to predict the fatigue life of materials under cyclic loading. The SN curve method is a classic method for analyzing the fatigue life of materials; The NSGA-II algorithm is a dominance-based multi-objective optimization algorithm used to find a set of optimal solutions, rather than a single optimal solution.

[0017] like Figure 1-8 As shown: A lightweight design method for commercial vehicle frame structure in a whole vehicle environment, including the following steps: S1, for the characteristics of commercial vehicles, establishes the lightweight design elements of the chassis, including optimization objectives, optimization variables and constraints; S2. Based on the topological relationships of commercial vehicles, establish a multi-rigid-body dynamics model for the entire vehicle; S3. Establish the finite element model of the chassis and perform parametric operations, input optimization variables and output optimization objectives; S4. Establish a rigid-flexible coupling dynamic model of the whole vehicle, conduct structural performance analysis of the commercial vehicle in the whole vehicle environment, and use the results of the structural performance analysis of the commercial vehicle as constraints. S5 determines whether the weight reduction of the chassis meets the design requirements, and at the same time determines whether the structural performance of the commercial vehicle meets the design requirements; if not, a new set of parameters is obtained for iterative calculation; if it meets the requirements, the lightweight chassis design scheme is output.

[0018] Step S1 includes: The optimization variables include the frame shape and dimensions, the spatial position of the frame crossbeams, the thickness of the frame crossbeams and longitudinal beams, and the frame material. Optimization targets include frame quality; The constraints include the structural strength of the chassis, the constraint modes of the chassis, the fatigue life of the chassis, and the equivalent stiffness of the whole vehicle.

[0019] Step S2 includes: Using computer software, a multi-rigid-body dynamics model of the whole vehicle is established, including the cab system, powertrain system, frame system, front and rear suspension systems, steering system, and tires; The cab system, powertrain system, frame system, front and rear suspension systems, steering system, and tires are considered as key subsystems. Connections within and between these subsystems are established using constraint pairs. Preferably, software including but not limited to ADAMS is used to create a multi-rigid-body dynamics model of the entire vehicle within the ADAMS / Car module. This model includes key subsystems such as the cab system, powertrain system, frame system, front and rear suspension systems, steering system, and tires. Bushing and joint simulations are applied to simulate the connections between the components in detail.

[0020] Step S3 includes: Using computer software, a finite element model of the chassis is established and parametric operations are performed. The chassis topography and dimensions, and / or the spatial position of the chassis crossbeams, and / or the thickness of the chassis crossbeams and longitudinal beams, and / or the chassis material are input to change the chassis components. At the same time, the topological connection relationship between the changed components and related components is maintained, and an updated chassis finite element model is generated, and the updated chassis quality is output. Hyperelements are defined as degree-of-freedom reduction boundaries at the connection points between the frame system and related subsystems, as well as at the points of force application. Preferably, a finite element model of the frame is built using software including but not limited to HyperMesh, and a parametric geometric model covering the frame and its accessories is constructed using the HyperMesh / Morphing module and SFE CONCEPT software. This model can not only define design variables, but also allow arbitrary changes in the position, size, and shape of the geometry, ensuring that the model maintains accuracy while avoiding mesh distortion or quality degradation caused by structural changes, thus providing great flexibility for design optimization. Through adaptive updates, every change in the geometry is ensured to be quickly and accurately reflected in the finite element model. Using finite element software such as HyperMesh and ANSYS, hyperelements are defined as degree-of-freedom reduction boundaries at the connection points between the frame and other components, as well as at the points of force application, to apply corresponding loads and constraints in subsequent simulations.

[0021] Step S4 includes: S41: Based on the finite element model of the chassis, the modal synthesis method is applied to solve the chassis modal neutral file (*.mnf) and chassis modal stress file (*.op2). Based on the multi-rigid-body dynamics model of the whole vehicle, the chassis modal neutral file is read, and the connection relationships between the chassis system and related subsystems are replaced to establish the rigid-flexible coupling dynamics model of the whole vehicle. On this basis, the above functions are automated by writing scripts.

[0022] Step S4 also includes: S42: Based on the rigid-flexible coupling dynamic model of the whole vehicle, apply force load or road excitation to obtain the stiffness analysis results of the frame structure; preferably, use the ADAMS / Durability module to obtain the nodal stress, strain and cloud map in the flexible body to complete the structural strength analysis, and write scripts to automate the above functions. S43: Conduct modal analysis to obtain the results of the frame constraint modal analysis and the mode shape animation; preferably, conduct modal analysis, view the results of the frame constraint modal analysis and the mode shape animation in the whole vehicle environment in the ADAMS software post-processing interface, and write scripts to automate the above functions; S44: Apply road surface excitation and output frame modal displacement files (*.dac) under different road surface conditions; based on modal stress recovery theory, superimpose the frame modal stress files (*.op2) and frame modal displacement files (*.dac) to obtain the frame stress time history; use the rainflow counting method to count the number of stress cycles; calculate the fatigue life of each node of the frame according to the SN method, compare the fatigue life of each node of the frame, and take the minimum value as the frame fatigue life; preferably, apply road surface excitation and output frame modal displacement files (*.dac) under different road surface conditions; based on modal stress recovery theory, superimpose the frame modal stress files (*.op2) and frame modal displacement files (*.dac) to obtain the frame stress time history; use the rainflow counting method to count the number of stress cycles; call the nCode / SN CAE Fatigue module, calculate the fatigue life of each node of the frame according to the SN method modified by the Goodman correction method, and output it in the form of a cloud map, where the minimum value of the life of all nodes is the fatigue life of the frame; write a script file to automate the above functions; S45: Apply a force load to put the whole vehicle in a torsional state and obtain the equivalent torsional stiffness of the whole vehicle; and apply a force load to put the whole vehicle in a bending condition and obtain the equivalent bending stiffness of the whole vehicle; and then obtain the stiffness analysis results of the whole vehicle system.

[0023] Furthermore, step S45 includes: Constrain the six degrees of freedom at the wheel center end (referring to translation along the X-axis, translation along the Y-axis, translation along the Z-axis, rotation about the X-axis, rotation about the Y-axis, and rotation about the Z-axis), take equal number of nodes on the lower flanges of the left and right longitudinal beams within the front suspension support area, apply a force (e.g., a force of 40N) to each node, with the left longitudinal beam having a downward force and the right longitudinal beam having an upward force, so that the whole vehicle is in a torsional state. Obtain the torsional angle of the frame on the plane of the applied action section, and the equivalent torsional stiffness of the whole vehicle can be obtained. Take equal nodes on the upper flanges of the left and right longitudinal beams at the midpoint between the front axle and the rear axle. Apply a force (e.g., 100N) to each node, with the force direction downward, so that the whole vehicle is in a bending condition. Obtain the displacement of the force application point on the frame in the vertical direction, and the equivalent bending stiffness of the whole vehicle can be obtained.

[0024] Step S5 includes: An optimization algorithm is applied to obtain a new set of parameters. After updating the finite element model of the chassis, iterative calculations are continued.

[0025] The following is a specific example illustrating the application of the lightweight design method for commercial vehicle chassis in a vehicle environment described in the above embodiments: For a certain light truck, the lightweight design method of commercial vehicle frame structure provided by this invention is applied to carry out optimization design, so as to minimize the frame mass while ensuring that the structural performance of the commercial vehicle meets the design requirements.

[0026] The optimization objective is to minimize the chassis mass. The optimization variables include the thickness of the chassis longitudinal beams, each crossbeam and connecting plate (3~6mm), and the distance of the third and fourth crossbeams from the front axle center in the direction from the front to the rear of the vehicle (1200~1600mm). The spatial position of the third and fourth crossbeams and the chassis beams are used as optimization variables. The parameters are continuously adjusted and changed to seek optimization. The constraints are that the chassis structural strength, chassis constraint modes, chassis fatigue life under a certain test track (6km) road spectrum, and the equivalent stiffness performance index of the whole vehicle meet the design requirements. Comparison of frame quality before and after optimization is as follows: Figure 3 As shown, the optimized frame has a significantly reduced weight; The comparison of the frame structure strength before and after optimization is as follows: Figure 4 As shown, the optimized vertical impact strength, braking strength, and torsional strength are slightly lower, while the turning strength is increased. The comparison of the frame constraint modes before and after optimization is as follows: Figure 5 As shown, the optimized first-order vertical bending mode and first-order horizontal bending mode are improved, while the first-order torsional mode is basically the same.

[0027] Comparison of frame fatigue life before and after optimization is as follows: Figure 6As shown, the fatigue life of the optimized frame is significantly increased; The comparison of the equivalent stiffness of the whole vehicle before and after optimization is as follows: Figure 7 and Figure 8 As shown, the optimized vehicle's equivalent torsional stiffness and equivalent bending stiffness are significantly increased. The above comparative analysis shows that the method provided by this invention has successfully achieved lightweight chassis design in the whole vehicle environment, while meeting performance indicators such as strength, modal characteristics, and fatigue durability, thus improving R&D efficiency and product performance.

[0028] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the invention should also be covered within the protection scope of the invention. Therefore, the protection scope of the invention should be determined by the scope of the claims.

Claims

1. A lightweight design method for the frame structure of a commercial vehicle under whole-vehicle conditions, characterized in that, Includes the following steps: S1, for the characteristics of commercial vehicles, establishes the lightweight design elements of the chassis, including optimization objectives, optimization variables and constraints; S2. Based on the topological relationships of commercial vehicles, establish a multi-rigid-body dynamics model for the entire vehicle; S3. Establish the finite element model of the chassis and perform parametric operations, input optimization variables and output optimization objectives; S4. Establish a rigid-flexible coupling dynamic model of the whole vehicle, conduct structural performance analysis of the commercial vehicle in the whole vehicle environment, and use the results of the structural performance analysis of the commercial vehicle as constraints. S5 determines whether the weight reduction of the chassis meets the design requirements, and at the same time determines whether the structural performance of the commercial vehicle meets the design requirements; If the conditions are not met, a new set of parameters is obtained and iterative calculation is performed. If the requirements are met, a lightweight chassis design solution will be output.

2. The lightweight design method for commercial vehicle frame structure under whole vehicle environment as described in claim 1, characterized in that, Step S1 includes: The optimization variables include the frame shape and dimensions, the spatial position of the frame crossbeams, the thickness of the frame crossbeams and longitudinal beams, and the frame material. Optimization targets include frame quality; The constraints include the structural strength of the chassis, the constraint modes of the chassis, the fatigue life of the chassis, and the equivalent stiffness of the whole vehicle.

3. The lightweight design method for commercial vehicle frame structure under whole vehicle environment as described in claim 2, characterized in that, Step S2 includes: Using computer software, a multi-rigid-body dynamics model of the whole vehicle is established, including the cab system, powertrain system, frame system, front and rear suspension systems, steering system, and tires; The cab system, powertrain system, frame system, front and rear suspension systems, steering system, and tires are identified as key subsystems, and connection relationships are established within and between these subsystems through constraint pairs.

4. The lightweight design method for commercial vehicle frame structure under whole vehicle environment as described in claim 3, characterized in that, Step S3 includes: Using computer software, a finite element model of the chassis is established and parametric operations are performed. The chassis topography and dimensions, and / or the spatial position of the chassis crossbeams, and / or the thickness of the chassis crossbeams and longitudinal beams, and / or the chassis material are input to change the chassis components. At the same time, the topological connection relationship between the changed components and related components is maintained, and an updated chassis finite element model is generated, and the updated chassis quality is output. Superelements are defined as degree-of-freedom reduction boundaries at the connection points between the frame system and the associated subsystems, as well as at the points of force application.

5. The lightweight design method for commercial vehicle frame structure under whole vehicle environment as described in claim 4, characterized in that, Step S4 includes: S41: Based on the finite element model of the chassis, the modal synthesis method is applied to solve the chassis modal neutral file and the chassis modal stress file; based on the multi-rigid-body dynamics model of the whole vehicle, the chassis modal neutral file is read, the chassis system and the connection relationship between the chassis system and related subsystems are replaced, and then the rigid-flexible coupling dynamics model of the whole vehicle is established.

6. The lightweight design method for commercial vehicle frame structure under whole vehicle environment as described in claim 5, characterized in that, Step S4 also includes: S42: Based on the rigid-flexible coupling dynamic model of the whole vehicle, apply force load or road excitation to obtain the stiffness analysis results of the frame structure; S43: Conduct modal analysis to obtain the results of frame-constrained modal analysis and mode shape animation; S44: Apply road surface excitation and output the chassis modal displacement files under different road surface conditions; based on the modal stress recovery theory, superimpose the chassis modal stress files and chassis modal displacement files to obtain the chassis stress time history; use the rainflow counting method to count the number of stress cycles; calculate the fatigue life of each node of the chassis according to the SN method, compare the fatigue life of each node of the chassis, and take the minimum value as the chassis fatigue life; S45: Apply a force load to put the whole vehicle in a torsional state and obtain the equivalent torsional stiffness of the whole vehicle; and apply a force load to put the whole vehicle in a bending condition and obtain the equivalent bending stiffness of the whole vehicle; and then obtain the stiffness analysis results of the whole vehicle system.

7. The lightweight design method for commercial vehicle frame structure under whole vehicle environment as described in claim 6, characterized in that, Step S45 includes: The six degrees of freedom at the wheel center end are constrained. Equal nodes are taken on the lower flanges of the left and right longitudinal beams within the front suspension support area. Force is applied to each node. The force on the left longitudinal beam is downward and the force on the right longitudinal beam is upward, so that the whole vehicle is in a torsional state. The torsional angle of the frame on the plane of the applied action section can be obtained to obtain the equivalent torsional stiffness of the whole vehicle. Equal nodes are selected on the upper flanges of the left and right longitudinal beams at the midpoint between the front axle and the rear axle. Force is applied to each node with the direction of the applied force downward, so that the whole vehicle is in a bending condition. The displacement of the force application point on the frame in the vertical direction can be obtained to obtain the equivalent bending stiffness of the whole vehicle.

8. The lightweight design method for commercial vehicle frame structure under whole vehicle environment as described in claim 7, characterized in that, Step S5 includes: An optimization algorithm is applied to obtain a new set of parameters. After updating the finite element model of the chassis, iterative calculations are continued.

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