Frames And Unitized Bodies


The suspension system must provide proper steering control and ride quality. Performing these functions is extremely important to maintain vehicle safety and customer satisfaction. Some vehicles, such as rear-wheel-drive cars, sport utility vehicles (SUVs), and trucks, have a frame that is separate from the body. Other vehicles have a unitized body that combines the frame and body in one unit, eliminating the external frame.


Unitized body design

  • In a unitized body, the body design rather than a heavy steel frame provides strength and rigidity.
  • All parts of a unitized body are load-carrying members, and these body parts are welded together to form a strong assembly.




The frame or unitized body serves the following purposes:

1. Allows the vehicle to support its total weight, including the weight of the vehicle and cargo.

2. Allows the vehicle to absorb stress when driving on rough road surfaces.

3. Enables the vehicle to absorb torque from the engine and drive train.

4. Provides attachment points for suspension and other components.


  • The unitized body provides a steel box around the passenger compartment to provide passenger protection in a collision.
  • Modern automotive bodies use a wide variety of materials, such as hydroformed aluminum rails, magnesium roof frames, and carbon fiber-balsa wood composite floor panels.
  • In most unitized bodies, special steel panels are inserted in the doors to protect the vehicle occupants in a side collision.
  • Some unitized body components are manufactured from high-strength or ultra-high-strength steels.
  • The unitized body design is typically used in small- and mid-sized front-wheel-drive cars.
  • A steel cradle is mounted under the front of the unitized body to support the engine and transaxle. Rubber and steel mounts support the engine and transaxle on the cradle.
  • Large rubber bushings are mounted between the cradle and the unitized body to help prevent engine vibration from reaching the passenger compartment. Some unitized bodies have a partial frame mounted under the rear of the vehicle to provide additional strength and facilitate the attachment of rear suspension components.