【正文】 er to determine the tire/road interaction forces, and a graphics format to view the road during postprocessing animation. These files are stored in a mon directory for easy retrieval. Custom control algorithms were developed to control vehicle speed, steering, and drive torque. These functions can be quickly modified to execute different vehicle maneuvers such as roll stability, a high speed lane change, or durability bumps similar to a proving ground. After the simulations are run, the forces and torques acting on the frame are written to data files. A custom software program is then used to extract the loads at specific time steps and write them to an ANSYS load file. The load file is then read into ANSYS and applied to a finite element model of the frame. The frame stresses are then calculated using an inertial relief solution. In summary, the model uses custom software routines and the existing links between the CAD and CAE codes to create a custom environment for evaluating the performance and durability of a heavyduty truck. However, the model assumes that the truck frame is a rigid, under formable body. In reality, the truck frame contains a great deal of flexibility which can impact vehicle performance and stability. As a result, these effects must be captured in the multibody system simulation. CAE SOLUTION FOR FRAME FLEXIBILITY PREVIOUS TECHNIQUES – In the past, several techniques have been employed to capture frame flexibility in a MSS model. Three popular methods are: bushings, mass beam elements, and FEA super 13 element reduction. In the first method the frame is divided into two or more rigid bodies connected together with force elements having bushinglike properties: stiffness and damping in three translational directions and three rotational directions. The bushing properties are adjusted to give the overall frame bending and torsional stiffness. As can be expected, this method is cumbersome to use, and if properly tuned, it will be capable of capturing only the fundamental bending and torsional modes of the frame. In the second method the frame is divided into a large number of rigid bodies interconnected by massless beam elements. This is similar to the bushing method but many more rigid bodies are usually used, and they are connected wi th massless beam elements whose equations (Timoshenko beam theory) are better suited to modelling truck frame rails and crossmembers. Noheless, it istime consuming to build a frame with this method and careful tuning of the beam elements is still required to capture the frame’s flexural response. The third method is the most accurate of the three methods and is based on a finite element representation of the frame. In this method the finite element model is reduced to a super element representation with the overall stiffness and mass properties condensed to a set of master nodes. The reduced model is checked against the original finite element model to ensure that the important frame dynamics are still captured. It is then imported into the MSS environment where the super elements and master nodes are converted to an equivalent representation of rigid bodies and force elements. Although this method is based on a finite element solution, it can still be difficult to achieve accurate results. For example, care must be taken in selecting the master nodes to ensure that the mass and stiffness condensation process is accurate. All the methods described above are difficult to use for creating an accurate flexible model of a truck frame. In general, they are only capable of capturing the basic frame response: the first few bending and torsional modes and the gross frame stiffness. If each method is to work, a significant effort is required to tune its properties to match some reference, such as static deflection testing, modal testing, or finite element simulation results. Consequently, neither method is suitable for use in a concurrent design and analysis environment it would simply take too long to make changes to the model, and it would not have adequate spatial resolution to capture subtle design changes to the frame. COMPONENT MODE SYNTHESIS TECHNIQUE – Recent advances in the integration of FEA and MSS have overe the difficulties in the methods described above .It is now possible to use a finite element model directly in a multibody simulation using a modal superposition technique known as ponent mode synthesis (CMS).Using modal superposition, the deformation of a structure can be described by the contribution of each of its modes. Normally, a very large number of modes are required to accurately capture the deformations at points where constraints are applied to the structure. CMS was developed to alleviate this problem. It bines normal modes with constraint modes. These constraint modes, or static shapes, capture the deformation of key areas of the structure without having to maintain an excessive number of normal modes. As a result, they are putationally more efficient. The CMS procedure adopted in the ADAMS code is based on a modified version of the CraigBampton approach. In this method the structure is considered to 14 have interface points where constraints and forces are applied, and each interface point can have up to six degreesoffreedom: three translations and three rotations. The motion of the structure is then described by a bination of two sets of modes: constraint modes for the interface points, and fixed interface normal modes. A constraint mode is calculated for each degreeoffreedom of an interface point, and it describes the static shape of the structure when that degreeoffreedom is given a unit deflection while keeping