A chassis is an underlying supporting structure where some vehicle components are mounted. It is similar to the skeleton of a human. In a vehicle, a traditional chassis gives the vehicle structural strength. It also has a platform to mount the engine, wheels, transmission, and all the other mechanical components. The vehicle body is also bolted onto this frame. Originally made of wood, vehicle chassis were soon changed to an open steel ladder-frame structure. This is easier to manufacture and is longer lasting (FIGURE 1).
Body-on-frame is the term used when a vehicle body is mounted on a rigid frame or chassis. It was the preferred way of building passenger vehicles. Manufacturers did not need to retool the structural components to release new models of vehicles with different body styles. However, by the 1960s, most manufacturers switched to vehicle designs that integrated the bodywork into a single unit with the chassis. The vehicle body became part of the vehicle structure rather than just an external skin. This is the unibody design, or single-shell design (FIGURE 2). The unibody design is constructed of many steel sheet metal panels. They are precisely formed in presses and spot-welded together into a structural unit.
The unibody design was first used in aircraft and then spread to automobiles. This is because with less of a chassis component, it was quicker to manufacture and lighter in weight. The lighter weight meant less cost in both material and labor. Another benefit of being lighter was that the vehicles became more fuel-efficient.