Inside-Out: Auto Body, Exterior and Interior

Text by Dmitry Orlov

Form and Function

The visual design of vehicles, just as with every other aspect, has changed drastically over the years. Our better understanding of speed, the elements, physics and computers has affected how cars are envisioned and produced.

The Pagani Huayra illustrates the importance of aerodynamics in a wind tunnel test.

As cars got faster and more powerful, aerodynamics started to play a role. As manufacturing and engineering capabilities developed, so did the design of cars, allowing for more complex shapes and features. Computer modeling and design made things easier to put into reality. Simulation technology allowed designers to see how things worked with safety, dynamics and overall look before a single part was even made.

Body and Structure

The overall body, frame or structure of a car, depending on design is the core of a vehicle. It connects all the components, houses the drivetrain, protects from the elements and most importantly carries passengers and cargo. It needs to be rigid to support stress, weight and to securely tie together all the components. Moreover, it must resist and soften the impact of a crash to safely protect the passengers.

Crash testing is crucial to creating the safest vehicles possible.

Additionally, it needs to be as light as possible to optimize fuel economy and performance. Over the years, various designs have been used and each has its benefits and drawbacks.


The oldest structural vehicle design is the frame, which typically consists of two parallel, connected rails which the suspension and power train are attached to. The rest of the body, or the shell, sits on the frame.

Frames are often found on trucks because of their overall strength and ability to sustain weight. The downside of the frame setup is that it is usually heavy, making it difficult to use a refined suspension for good handling. Also, the frame tends to take up a lot of valuable space and forces the center of gravity to go up. While this is great for off-road or heavy-duty applications, it is not good for performance and agility.

Safety is also compromised in a frame vehicle because the rails do not give under impact; this lack of absorption passes more energy from the collision into the cabin and to the other vehicle or object involved in the crash.


Vehicles with a unibody integrate the frame into a sheetmetal body. Computer modeling and design have allowed engineers to maintain the strength and rigidity of a frame yet incorporate it all into the sheetmetal body shell. Suspension components, as well as the power train, mount directly to the unibody.

This design yields great reductions in weight and allows for a more compact, yet spacious configuration of vehicles. Safety is also increased because crumple zones are engineered into the unibody. These zones are designed to give under impact, working as absorbers which reduce the forces passengers will experience. Rigidity is compromised slightly because the whole assembly is made of sheet metal, which is often spot welded together. Various manufacturers can choose to strengthen the unibody with better assembly (seam welding) or by adding bracing or other components to increase rigidity.


Subframes are used in unibody setups and serve as a strong mounting interface between the drivetrain, suspension and the unibody itself. A car’s suspension takes a constant beating from unfavorable road conditions; therefore, the subframe is constructed to be stronger than the unibody to handle this. Strength and rigidity in the unibody can be increased by bracing certain components and mounting points.

The firewall is located between the passengers and the engine bay and is designed to prevent a potential engine fire from spreading into the cabin. Since the accelerator, brake and clutch pedals are mounted to the firewall, it is especially important in vehicles with a frame as a structural piece to prevent flex between the frame rails.

Body panels are typically constructed from steel or aluminum sheet metal and give a vehicle its unique shape. The body panels represent the first line of defense in the event of a crash and must have an acceptable level of strength with as little weight as possible. Fiberglass and carbon fiber are strong, lightweight materials also used to make body panels.

While roll cages are not found in passenger vehicles, for performance and racing applications, they are a standard requirement. Besides protecting the driver, roll cages offer unparalleled chassis rigidity and strength, which translates to more predictable handling characteristics. A roll cage is usually constructed from steel or chromoly bars welded together and to the chassis itself.


As mentioned above, the body panels serve as the "skin” of the car and dictate its overall shape and appearance. The advent of new technologies has enable automakers to create body panels that are both strong and light, to increase safety while also increasing efficiency. The principle "form follows function” is particularly evident in body panel design, as pieces change due to aerodynamic requirements, as well as the incorporation of lights, grilles and ducting. In modern vehicles, fiberglass-reinforced plastic and carbon fiber have begun to replace traditional steel and aluminum panels to further reduce weight.

Full carbon fiber bodywork lies underneath the matte black wrap on this HPD NSX.


The importance of aerodynamics is reflected in exotics and racecars, where the entire vehicle is designed to have the best possible aerodynamics for safety and performance. These same considerations are taken into account in the construction of everyday passenger cars, albeit to a lesser extent. The better a car cuts through the air, the better its performance and overall efficiency.

Formula 1 and Indy Cars are the embodiment of "form follows function."


For high-speed driving, downforce is absolutely critical and must be maintained at all times to keep a car firmly planted on the road. To achieve this, the amount of air that is allowed to pass underneath the car must be limited; the underside of most performance cars is usually as flat and smooth as possible to reduce turbulent flow. Front lips or "chin spoilers” are often affixed to the lower edge of the front bumper to reduce under-car airflow. Diffusers are typically a race-only component, but occasionally appear on street cars; the function of a rear diffuser is to smooth out airflow from underneath the car.

Carbon fiber front lip or "chin" spoiler on an E46 BMW.

This STi is wearing a rear diffuser made from carbon fiber.

Aside from the purely aesthetic examples, rear spoilers or wings are functional pieces that are designed to increase downforce at high speeds, which keeps the back of the car and the rear wheels stuck to the pavement.

A massive wing adorns the decklid of this R35 GT-R.


Airflow must be directed to the intake so the engine breathes the coolest, densest air possible. Sufficient airflow through the front bumper to the radiator is also crucial to keep water temps nice and cool. On many cars, ducts are installed to draw in air to cool the brakes.

Great example of ducting on a Porsche 996 turbo.

In many cases, ducts will deliver air via hoses to cool the brakes.


Glass makes up a large portion of a vehicle’s exterior and contributes to its structural integrity. Windshields, in particular, are made of a special type of laminated glass that resists shattering to protect passengers during a collision. If the windshield does break, it turns into mostly-harmless pieces instead of long, jagged shards. The frame of a windshield also acts as a roll bar in the event of a rollover. Just like other body panels, windows need to be as light and strong as possible.


The interior is another important aspect of vehicle design, as it is how the driver and passengers interact with the car. From the seats to the steering wheel to the sound system, the interior has a huge impact on the overall driving experience. Seatbelts and airbags ensure passenger safety during a collision, while new technologies such as back-up cameras are integrated into the multimedia systems of modern vehicles. The climate control system provides a comfortable environment inside the cabin, while seat heaters and air conditioned seats are now optional in many luxury cars.

While more subtle, all of these components are just as important to vehicle operation as the engine or transmission because they are responsible for the safety and comfort of the driver and passengers. Modern-day technology has allowed for the creation of cars that are faster, more efficient, more reliable and incredibly safe.

That's a Wrap...

This article wraps up the first, most basic level of the Building Blocks series. The next round of articles will go through each of these automotive systems again, but take it one step further and discuss things in more detail. In the meantime, make sure you check out our supplementary articles, which are designed to augment the information presented in the main Building Blocks articles.

Fully Integrated: Automotive Electrical Systems

Stopping Power: How the Brake System Works

Putting the Power Down: Wheels, Tires, Suspension & Steering

Power into Propulsion: How the Transmission & Driveline Work

RPM to MPH: How Car Engines Work


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