Vehicle Structure: How A Car Body Is Built

Everything bolted to a car has to be mounted to something. This is what that something is made of, and why it is shaped the way it is.

Every fitment in or on a motor vehicle has to be mounted to the bodywork or structure. By and large the bodywork performs various functions aside from the aesthetics; it provides a weather resistant environment for those inside and also facilitates the fitment of equipment to make that environment very comfortable indeed. Everything from the latest in multi-adjusting seats, through climate control, road noise suppression, a whole glut of airbags and the most up to date in satellite driven route finding, not to mention mind blowing in-car entertainment systems. From a purely engineering point of view all this equipment has to be securely fixed if it is to function as the manufacturer intended.

Producing a structure that can handle the forces

The forces generated by not only the weight of the vehicle itself plus occupants, their luggage, fuel and hundreds of items of ancillary equipment, but also the side effects of the vehicle simply being driven along, must be correctly calculated and distributed throughout the structure. This can only be achieved by producing that structure from a wide range of material specifications. This means close attention to not only the type of material used but also its dimensions and shape; what may be the ideal profile for one body panel will not necessarily mean that it would be correct for another.

The key to the chassis

The two main structure types are separate chassis and monocoque (sometimes referred to as integral construction). The monocoque, although probably the most common in car and light commercial production, has been in existence long before the internal combustion engine. Take a look at the construction of the type of trailer pulled by a horse: the drawbar is fastened to the body, the body is either flat or adapted to carry a load requiring side panels, and the rear may have a moveable tailboard for unloading. It has no separate chassis. The main reason for the success of this type is that it suits modern production methods including the ubiquitous computer driven robot.

The separate chassis construction is not as old as the integral type but does date back many years. Its strength lies in the adaptability of a simple frame: make a powered frame with the essentials fitted — transmission, steering, brakes and so on — and then bolt on the body which suits your particular needs, be it multi seating, flat back, tipper or tanker. In the world of heavy commercial vehicles it is normal practice, when required, to cut a chassis and add in extra length for the fitting of a particular or unusual piece of equipment. This can include extra axles to spread the vehicle load.

Crash structural engineering

Common to both structure types in the design and production of modern motor vehicles is the requirement to protect the occupants from crash injury. This is not done by simply making the whole vehicle a rolling bank vault — that would be prohibitively expensive and the result would be something akin to a main battle tank, complete with fuel consumption to match.

The answer lies in the very careful engineering of certain areas: the uprating of the mounting points for the seats and seatbelts, the fitting of side impact protection bars, and the strengthening of the A, B and C pillars. It can also include the incorporation of deliberately weak areas — the much vaunted crumple zones. These zones are specifically designed to deform under impact, folding and compressing and thereby absorbing as much crash energy as possible before it reaches the driver and passengers. Very often, a car involved in a head-on collision will display a completely destroyed frontal area but will have a relatively untouched passenger compartment.