Torsional Stiffness Analysis of Car Bodies

Expressions are provided for calculating the torsional stiffness of a car body passenger compartment. The two main reaction systems which contribute towards the torsional stiffness, i.e. the couple due to shear forces in the roof and floor and the rectangular frame formed by the sill torque boxes and the end bulkheads of the passenger compartment, are considered separately. This enables the behaviour to be studied qualitatively as well as quantitatively. It is shown theoretically and diagrammatically that the first system is dependent on the bend stiffness of the members forming the door apertures. Simplifying assumptions permit the expressions developed to be sufficiently straightforward for them to be acceptable for repeated use in the design office to optimize the distribution of material. Alternative approaches are suggested for cases where greater accuracy is necessary. The basic expressions relate to a simplified structure but a technique is supplied for assessing the contribution of extra members. The effect of the horizontal offset of the wheel centres from the ends of the passenger compartment is discussed and the influence of suspension and structure geometries on the effective offset is mentioned. Ways in which the deflections of the passenger compartment affect the front and rear end structures are discussed qualitatively.