Body Cross-Sectional Stiffness Criteria for the Optimal Development of the BIW Weight and Torsional Stiffness 2021-01-0797

Body-in-white plays a key role in protecting passengers in the event of collision between vehicles, and also endures external forces during cornering in a vehicle. Stiffness of body-in-white is the basic characteristic of a car body, and it is closely related to the full-vehicle-level performance such as body durability, ride and handling, etc. There have been many attempts to correlate body stiffness to full-vehicle-level performance, and studying the relationship between torsional body stiffness and durability has been the popular topic among others. In general, it is believed to be true that bodies with high torsional stiffness exhibit good durability performance, and in many cases this assumption seems to be verified. However, not all cases are true to this assumption. In this paper, relationship between torsional body stiffness and body durability has been closely studied. After obtaining torsional analysis results from finite element analysis, the new method of generating body cross-sections under torsional loading is applied. Torsional angles for respective cross-sections at different locations are calculated along the longitudinal direction of body-in-white, and shear strains along successive cross-sections are found. The stiffness evaluated from this methodology is called body cross-sectional stiffness with respect to body-in-white. These stiffness evaluations are also carried out for trimmed-bodies of different body types which include five-door-vehicles, sport-utility-vehicles and sedans. In addition, procedure which can calculate cross-sectional stiffness of a full vehicle using the stiffness value of the trimmed-body is proposed. In this procedure, numbers of factors are considered to compensate the body stiffness of the trimmed-body. They are factors influencing stiffness such as roofs, tailgates and high voltage battery packages, also factors related to loadings derived from the Belgian block road driving, and lastly factors related to targets which normalize different durability mileage targets for different vehicles. Full vehicle cross-sectional stiffness is compared with actual durability test results in order to investigate correlations between them. The optimal target for the body cross-sectional stiffness is proposed based on this correlation. Finally, the strategies for optimizing weight and body stiffness during the body development phase are presented.