vehicle frame, also known as its chassis,  is the main supporting structure of a motor vehicle to which all other components are attached, comparable to the skeleton of an organism.

The main functions of a frame in motor vehicles are:

  1. To support the vehicle's mechanical components and body
  2. To deal with static and dynamic loads, without undue deflection or distortion.
These include:
  • Weight of the body, passengers, and cargo loads.
  • Vertical and torsional twisting transmitted by going over uneven surfaces.
  • Transverse lateral forces caused by road conditions, side wind, and steering the vehicle.
  • Torque from the engine and transmission.
  • Longitudinal tensile forces from starting and acceleration, as well as compression from braking.
  • Sudden impacts from collisions.

Types of frame according to their construction:

  • Ladder-type frame
  • X-Type frame
  • Offset frame
  • Offset with cross member frame
  • Perimeter Frame

Ladder frame

Named for its resemblance to a ladder, the ladder frame is one of the simplest and oldest of all designs. It consists of two symmetrical beams, rails, or channels running the length of the vehicle, and several transverse cross-members connecting them. Originally seen on almost all vehicles, the ladder frame was gradually phased out on cars in favor of perimeter frames and unitized body construction. It is now seen mainly on trucks. This design offers good beam resistance because of its continuous rails from front to rear, but poor resistance to torsion or warping if simple, perpendicular cross-members are used. The vehicle's overall height will be greater due to the floor pan sitting above the frame instead of inside it.


It was specifically chosen to decrease the overall height of the vehicles regardless of the increase in the size of the transmission and propeller shaft humps since each row had to cover frame rails as well. Several models had the differential located not by the customary bar between axle and frame, but by a ball joint atop the differential connected to a socket in a wishbone hinged onto a cross-member of the frame.

The X-frame was claimed to improve on previous designs, but it lacked side rails and thus did not provide adequate side-impact and collision protection. This design was replaced by perimeter frames.

Perimeter frame

The Chevrolet Corvette has used a variation of the perimeter frame since 1963, but its fourth-generation variant to its current generation as of 2016 has elements of the perimeter frame integrated with an internal skeleton that serves as a clamshell. Similar to a ladder frame, but the middle sections of the frame rails sit outboard of the front and rear rails just behind the rocker and sill panels. This was done to allow for a lower floor pan, especially at the passenger footwells, to lower the passengers' seating height and therefore reduce the overall vehicle height in passenger cars. This became the prevalent design for body-on-frame cars in the United States, but not in the rest of the world until the uni-body gained popularity. It allowed for annual model changes introduced in the 1950s to increase sales, but without costly structural changes. There are no perimeter frame passenger automobiles sold in the United States after the Ford Motor Company phased out the Panther platform in 2011, which ended the perimeter frame passenger car in the United States.

In addition to a lowered roof, the perimeter frame allows lower seating positions when that is desirable, and offers better safety in the event of a side impact. However, the design lacks stiffness, because the transition areas from front to centre and centre to rear reduce beam and torsional resistance, and is used in combination with torque boxes and soft suspension settings.

Space frame

The first true spaceframe chassis were produced in the 1930s by Buckminster Fuller and William Bushnell Stout (the Dymaxion and the Stout Scarab) who understood the theory of the true spaceframe from either architecture or aircraft design. In a (tubular) spaceframe chassis, the suspension, engine, and body panels are attached to a three-dimensional skeletal frame of tubes, and the body panels have little or no structural function. To maximize rigidity and minimize weight, the design makes maximum use of triangles, and all the forces in each strut are either tensile or compressive, never bending, so they can be kept as thin as possible.


A subframe is a distinct structural frame component, to reinforce or complement a particular section of a vehicle's structure. Typically attached to a unibody or a monocoque, the rigid subframe can handle high chassis forces and can transfer them evenly to a wide area of relatively thin sheet metal of a unitized body shell. Subframes are often found at the front or rear end of cars and are used to attach the suspension to the vehicle. A subframe may also contain the engine and transmission. It is normally of box steel construction but may be tubular.

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