Conceptual Dynamic Analysis of a Vehicle Body

Abstract: The noise, vibration and harshness (NVH) and body dynamic performance of automotive vehicles is highly dependent of the components included in the body structure and compartment, since they add mass, stiffness and damping to the overall structure. Today, the noise levels in the compartment are predicted using complex and detailed computational models during both early and late stages of the vehicle development process. However, detailed information regarding the vehicle structure is limited during the concept phase, which makes the predictions unreliable. This dissertation investigates if simpler and more robust measures for the vehicle body could instead be used to describe the NVH performance in the concept phase. Three different measures of the vehicle body without trim items was investigated: 1) eigenfrequencies of global bending, torsion and yaw modes, 2) global static bending and torsional stiffness, and 3) a mobility index which reflects the vibrational velocities of the structural frame. In order to decide on appropriate measures, the correlation between the NVH performance of the vehicle body with trim items and the results of the simpler measures is evaluated using linear regression. To evaluate the NVH performance of the complete vehicle body a road noise index representing the broad-band acoustic pressure due to a load resulting from the interaction between the car and road surface was created. This road noise index was used as a measure of the NVH performance of the vehicle body with trim items. These measures are calculated on a finite element (FE) representation of the vehicle. The correlation was first investigated on a selection of vehicles currently in production by Volvo Cars. Also, a case study was performed on one of the vehicles by modifying its structural properties. It was concluded that the mobility index offers the best correlation out of the early measures investigated, and is a possible robust and simple alternative measure usable in the early concept development stages.