Hillier’s Fundamentals of Motor Vehicle Technology

A modified construction is needed when the roof cannot be fully utilized as a compression member. This occurs on convertible (drop-head coupé) models, and in situations where either a sunshine roof, or very thin door pillars are used. In these cases the required strength is achieved by using a strong underbody frame. In addition, extra stiffness is given to the body-shell parts that are subject to torsion.

A body-shell is normally constructed in one of two ways: it is either made by spot-welding the panels, pillars and pressings together so as to form a strong box, or by building a space frame (Figure 1.43). The latter structure gives a skeleton of high structural strength on to which is attached the steel, aluminium or glass- reinforced plastic (GRP) body panels, doors, roof, etc. Of the three materials used, steel is the most common for vehicles made in high volume; this is because production costs are lower once the initial outlay on expensive body jigs and robots has been recovered.

The safety of a modern car is improved by enclosing the driver and passengers in a rigid cell. At the front and rear of this safety compartment are attached sub- frames; these are designed to concertina on impact as shown in Figure 1.44. The crumple zones of the body are intended to absorb the shock of a collision and, in consequence, reduce the rate of deceleration