2. Stressed skin

The next logical step for chassis development was the stressed skin design. This is more difficult to construct than a spaceframe with the accurate folding, forming, drilling and riveting of sheet steel or modern composite materials. The lessons learnt in the aircraft industry do not usually apply directly in automotive practice. The loads on aircraft are widely distributed – the lift that holds a plane up, for example, is spread over the entire area of its wings. On a race/sports car, the loads are much more concentrated, being focused on the suspension mounting points.

Even when a method is developed to accept forces and spread them over a load bearing skin, it becomes extremely inconvenient to make any modifications and may even require a major redesign. Analysis of the stresses in stressed skin construction is more difficult.

The continuous surface considerably complicates access for repair or replacement of the cars mechanical components. This may also explain why stressed skin construction was virtually unheard of in racecars before the modern mid-engined configuration. The majority of mid-engined racecars end their stressed skin construction at the back of the cockpit, with either a space frame or the engine itself forming the remainder of the structure. For all these drawbacks, stressed skin construction can potentially outperform any other form of racecar construction in terms of torsional stiffness.

3. Load Cases

A chassis is subjected to three load cases: bending, torsion and dynamic loads.

The bending (vertical symmetrical) load case occurs when both wheels on one axle of the vehicle encounter a symmetrical bump simultaneously. The suspension on this axle is displaced, and the compression of the springs causes an upward force on the suspension mounting points. This applies a bending moment to the chassis about a lateral axis. (See Fig. 6.)

Fig. 6 Bending Load case [Ref. 2]

The torsion (vertical asymmetric) load case occurs when one wheel on an axle strikes a bump. This loads the chassis in torsion as well as bending. (See Fig. 7). It has been found both in theory and in practice that torsion is a more severe load case than bending.

Fig. 7 Torsion Load case [Ref. 2]

The dynamic load case comprises longitudinal and lateral loads during acceleration, braking and cornering. These loads are usually ignored when analysing structural performance.

A torsionally stiff chassis offers a number of advantages:

1. According to vehicle dynamics principles for predictable and safe handling the geometry of the suspension and steering must remain as designed. For instance the camber, castor and toe angles could change with torsional twist or the steering geometry could change causing “bump steer.”

2. Once again according to vehicle dynamics principles a suspension should be stiff and well damped to obtain good handling. To this end the front suspension, chassis and the rear suspension can be seen as three springs in series as shown in Fig. 8. If the chassis is not sufficiently stiff in torsion then any advantages gained by stiff suspension will be lost. Furthermore, a chassis without adequate stiffness can make the suspension and handling unpredictable, as it acts as an undamped spring.

Fig. 8 Chassis and suspension as springs

3. Movement of the chassis can also cause squeaks and rattles, which are unacceptable in modern vehicles.