sizing in conceptual design at BWM

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PAPER SERIES 2004-01-1657

Sizing in Conceptual Design at BMW

Ingo Raasch

BMW AG

Reprinted From: CAD/CAM/CAE Technology and Design Tools

(SP-1858)

2004 SAE World Congress

Detroit, Michigan

March 8-11, 2004

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Sizing in Conceptual Design at BMW

Ingo Raasch

BMW AG

Copyright ? 2004 SAE International

ABSTRACT

In the early stages of conceptual design the available geometric data are very coarse and the lifespan of a design idea is very short. The structural evaluation and improvement of a design has to take both facts into account. Its focus is on the total vehicle and its performance. This can be estimated by a modeling technique, which is adequate for the lack of geometric details. Static and dynamic global stiffness as well as some aspects of crash and NVH have to be considered. Optimization will lead to the proper sizing and some indication of the potential of the structure. In order to maintain high quality standards this approach has to be supported by specialized CAE tools and extensive rules on modeling techniques and analysis procedures.

INTRODUCTION

Concept analysis models with beams and shells have a long and sometimes controversial history at BMW /1/. Their usage for sizing in conceptual design was most successful in low-key projects and for vehicles, which did not have a direct predecessor or a reference car. Optimization is an important function in finding the

optimal outer dimensions and wall thickness of the beams.

SITUATION

This presentation is concerned with the very early phase of the development of a car, in which the topology and the sizing of the structure is under consideration. It is assumed that the new car still has considerable freedom in its design and morphing of existing structures cannot create the new one. At this point in time there are no or very little CAD data available.

Sizing in the present context means determining the necessary dimensions of structural beams while fulfilling the necessary structural requirements. Only very few details, e.g., at joints, are considered, but it is assumed that these will be taken care of later in a best practice manner. This process is supported by linear Finite Element analyses with fairly coarse models, which can capture the overall structural behavior of the vehicle.

As the lifespan of a design idea in this development period is fairly short, it is important to evaluate it within

this time frame. This means the analysis, its preparation

Fig. 1 Initial Topology

2004-01-1657

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Fig. 2 Section definition

Fig. 3 Definition of parts in a section

and the necessary modifications in the model have to be completed in a very short time period.

MODEL PREPARATION

The car body with its trim, and if necessary the whole or parts of the driveline, is modeled to simulate the behavior of the total vehicle. All beams of the car body are

modeled with FEM-beams and all closures by FEM-shell elements. The initial topology of the structure might be given as the midsection of the total vehicle (Fig. 1) and/or some parts of the reference cars. In addition there may be some sketches and simple oral suggestions.

For the generation of the analysis model there are strict rules on element usage, size, and connection. In general only the right hand side of the body-in-white is modeled. It is mirrored along the midplane for simplicity of the analysis. The model is structured in sections and parts with unique

property IDs, as most of the optimization capabilities in

MSC/NASTRAN are defined via properties (see Fig. 2).

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