The body of a modern – produced car is built on the monocoque  ( single – shell) principal , in which the roof , side panels , and floor are welded into a single4 integral unit . This bodyshell protects and supports the car’s internal parts steel and glass are used to construct the bodyshell , creating a unit that is both light and strong . Its lightness helps to conserve energy , while its strength protects the occupants . Modern bodywork is designed with the aid of computers , which are used to predict factors such as aerodynamic efficiency and impact -resistance . High – technology is also employed on the production line , where robots are used to assemble , weld , and paint the body .