Vehicle Frame Construction

Vehicle Frames

A vehicle frame in the construction of a car is the main and principal supporting structure of a motor vehicle. This is the component to which all other elements and parts are attached to. Until the 1930s, almost every car had an internal structural frame which was separate from its body. This design is known as body-on-frame. Over the years and continued advancement in technology, nearly all passenger vehicles have adopted a unibody construction design, this referring to their chassis and bodywork being integrated or infused into one another. Nearly all Lorries, buses, and some pickups continue to use a body on frame design


The main functions of a frame in motor vehicles are;

  • To give the vehicle’s mechanical components and body support to handle static and dynamic loads, without undue deflection or distortion. (Weight of the vehicle body, passengers, and cargo load)
  • Vertical handling and twisting experienced by going over uneven road surfaces. Transverse or cross lateral forces caused by various road conditions, side wind, and vehicle steering.
  • Handle rotational force or torque from the engine and transmission. Longitudinal tensile forces from engine start and subsequent acceleration as well as compression from braking.
  • Handle and absorb sudden side impacts and collisions.

There are different types of a frame according to the construction:

  • Ladder frame
  • X-Type frame
  • Unibody
  • Perimeter Frame

Ladder frame

The design resembles that of a ladder. Being one of the simplest and oldest of all designs, it is made up of two symmetrical beams, rails or channels which run the length of the vehicle, and several transverse cross-members connecting them. The ladder frame was slowly phased out on cars in favour of perimeter frames. This design offers good beam resistance because of its continuous rails from front to rear of the vehicle. On the downside, it provides poor resistance to warping when simple perpendicular cross-members are used.


The term unibody describes the frame of the vehicle as a one-piece frame and body structure combined. As a result, the design is lighter and firmer than one having a separate body independent from the frame. In a fully integrated body structure, a whole vehicle is a load-bearing unit that handles all the loads experienced by the vehicle, i.e., forces experienced from driving as well as cargo loads.

X-type frame

This is the design that was used for the full-size American models in the 1950s and 1960s in which the rails from alongside the engine crossed in the passenger compartment, each continuing to the opposite end of the cross member at the extreme rear of the vehicle. It was designed 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.

Perimeter frame

Like a ladder frame, but the middle sections of the frame rails sit outboard of the front and rear rails just behind the sill plates. This allows for a lower floor pan, thus lowering the passengers’ seating height and in turn reduces the overall vehicle height. In addition to a lowered roof, the perimeter frame offers better safety in case of a side impact. Unfortunately, the design lacks proper rigidity or stiffness.