Concept Level Performance Improvement in Frame with Light-Weighting

Abstract

Amongst passenger vehicles, SUVs are mainly constructed Body-on-Frame as it has better off-road capabilities. Frame design must take into consider the vehicle architecture, geometric and functional interfaces, crashworthiness and local and global stiffnesses. The stiffness of the frame directly affects critical vehicle dynamics & NVH characteristics. The vehicle frame contributes to roughly 10% of total vehicle weight. Hence, Light-weighting of the frame will result in significant weight reduction of the overall vehicle. Weight of the vehicle is a critical parameter in fuel economy, material cost and dynamics. The frame design is a critical platform architecture development as it has direct linkage to architectural dimensions and the platform modularity definitions. Currently, the frame is designed in 3D CAD and taken through a CAE loop. The frame design is iterated until it meets the pre-defined performance targets. This results in extended duration for the architecture freeze and subsequently the entire product development process. To speed up the frame design process, a method is proposed to optimize the frame design with respect to the global stiffnesses and weight at the concept stage, even before the first level CAD is prepared. The paper focuses on parameterizing the ladder frame of a vehicle, running structural simulations with a parametric beam model, correlating with the actual values and finally generating a frame concept model from optimization to be taken further for detailed engineering. The ladder frame is parameterized based on the geometry and sections of the side-members and locations and sections of the cross-members and 46 parameters were generated. The simulations were run in a commercial software where the frame was modelled as beam elements and the joints are modelled as rigid joints. Loads & constraints are applied as per the stiffness test protocols and global stiffnesses are simulated. The results from the simulation were correlated with test results as well as 3D CAE values for internal benchmarks with 90% accuracy. A Design of experiments was run which resulted in a mathematical model which was used to perform optimization. The optimization generated a concept frame with 20% weight reduction and a 50% increase in Bending and Torsional Stiffness. The results matched with 3D CAE simulation results and the frame taken forward for detailed engineering. Extremely low simulation time make it possible to run multiple iterations and perform optimization very quickly. The sensitivity and direction of goodness of each parameter which is an output from the simulation helps in further detailing the design. As this new process enables front-loading of CAE during the concept stage which accelerates the process of product development. Keywords: Frame Design, Global Stiffness, Light-weighting, Parametric Model, Concept Development