Body Structures:

A car's body structure and the exterior surface have four main functions:

1. Protect the occupants and cargo
2. Connect the major components and manage stress between them
3. Provide an appealing product image
4. Make the car aerodynamically sound for efficiency and reduced wind noise. 

Below are the types of body structures used in a variety of vehicles.

Unibody: this is the most cost-effective option for mass-produced cars, minivans, and SUVs. It is made from steel and aluminum panels that are stamped into shape and spot-welded together. Exterior panels are made form metal or plastic, depending on the impact requirements.

Body on Frame: This is common in trucks and SUVs, where heavy loads and rough terrain require extra strength. The engine, suspension, and main body are all attached to the frame on rubber mounts. This improves ride quality, noise and vibration. On the other hand, this increases the vehicle weight and step-in height. This also gives trucks poor torsional rigidity, which is important for good handling.

Main body (above) and Steel Frame (below)

Space Frames: In low-production high performance cars, high stiffness and low weight are critical. The space frame acts as a skeleton to which the mechanical components and exterior are attached. These can be made from steel/aluminum tubing, extruded aluminum, or plastic composites.

• Extruded Aluminum: The thick sill sections (the sections beneath the doors) add a lot of rigidity to the frame, improving handling. This is ideal for mid- or rear-engine cars that don't need a tunnel across the length of the car for drive shafts. 

Audi Concept Frame

• Carbon Fiber: This "tub" is used on some exotic sports cars for high strength and very low weight. Additional metal structures are attached to the front and rear to complete the frame. 

• Steel or Aluminum Tubing: This is a "backbone" frame that looks similar to the body-on-frame layout used in trucks, but there is an important difference. There is a tunnel structure running across the center of the frame, improving torsional rigidity. Additional panels close off the floor and stiffen the structure. Exterior panels are made of plastic. This is used in front-engine Rear-wheel drive cars that need a tunnel for the drive shafts and transmission. 

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