What Is Car Frame?- Definition, Types, And Repair Cost

What is a Car Frame?

A Car frame, also historically 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.

Until the 1930s, virtually every car had a structural frame separate from its body. This construction design is known as body-on-frame. By the 1960s, unibody construction in passenger cars had become common, and the trend to unibody for passenger cars continued over the ensuing decades.

Nearly all trucks, buses, and most pickups continue to use a separate frame as their chassis.

Functions of Car Frame

The main functions of a frame in a motor vehicle are:

  • To support the vehicle’s mechanical components and body
  • 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.
Car Frame

Types of Frames

  • Ladder frame.
  • Backbone tube.
  • X-frame.
  • Perimeter frame.
  • Platform frame.
  • Subframe.

1. Ladder frame

Named for its resemblance to a ladder, the ladder frame is one of the oldest, simplest and most frequently used under-body, separate chassis/frame designs. It consists of two symmetrical beams, rails, or channels, running the length of the vehicle, connected by several transverse cross-members.

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 large 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.

2. Backbone tube

A backbone chassis is a type of automotive construction with chassis, that is similar to the body-on-frame design. Instead of a relatively flat, ladder-like structure with two longitudinal, parallel frame-rails, it consists of a singular central, strong tubular backbone (usually rectangular in cross-section), that carries the power-train, and connects the front and rear suspension attachment structures.

Although the backbone is frequently drawn upward into, and mostly above the floor of the vehicle, the body is still placed on or over (sometimes straddling) this structure from above.


3. X-frame

This is the design used for the full-size American models of General Motors in the late 1950s and early 1960s in which the rails from alongside the engine seemed to cross in the passenger compartment, each continuing to the opposite end of the cross member at the extreme rear of the vehicle.

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.

4. Perimeter frame

Similar to a ladder frame, but the middle sections of the frame rails sit outboard of the front and rear rails, routed around the passenger footwells, inside the rocker and sill panels. This allowed the floor-pan to be lowered, especially the passenger footwells, lowering the passengers’ seating height and thereby reducing both the roof-line and overall vehicle height, as well as the center of gravity, thus improving handling and road holding 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 unibody gained popularity. For instance, Hudson introduced this construction on their 3rd generation Commodore models in 1948. This frame type allowed for annual model changes, and lower cars, introduced in the 1950s to increase sales – without costly structural changes.

5. Platform frame

This is a modification of the perimeter frame, or of the backbone frame, in which the passenger compartment floor, and sometimes also the luggage compartment floor, have been integrated into the frame as loadbearing parts, for strength and rigidity. The sheet metal used to assemble the components needs to be stamped with ridges and hollows to give it strength.

Platform chassis were used on several successful European cars, most notably the Volkswagen Beetle, where it was called “body-on-pan” construction. Another German example is the Mercedes-Benz “Ponton” cars of the 1950s and 1960s, where it was called a “frame floor” in English-language advertisements.

6. Spaceframe

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 limited or no structural function. To maximize rigidity and minimize weight, the design frequently 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.

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.

How To Tell If Your Car Has Frame Damage

It doesn’t take too much for a car to end up with frame damage. Here are a few symptoms to watch out for:

  • Visibly bent or damaged
  • Alignment is off
  • Wheels don’t track correctly
  • Panels don’t line-up
  • Uneven tire wear
  • Unusual sounds
  • Uneven suspension wear

Each of the above symptoms typically corresponds with a specific type of frame damage.

  • Sagging/Twisted Frame: If your frame is sagging or twisted, it’s likely to leave gaps in between the different body panels. If ignored, a sagging or twisted frame can cause your tires and suspension components to wear faster.
  • Mashed Frame: A mashed fame is most common after a collision, where the frame itself crumples in on itself. Look for distortions, or wrinkles in the body panels, like the hood, fenders, or side rails.
  • Sway Damage: Sway damage typically occurs when a vehicle gets hit in a corner. The problem with sway damage is that your car won’t be able to drive in a straight line, meaning it’s a hazard to yourself and those around you. Additionally, if you continue to drive a car with sway damage, there’s a good chance it wreaks havoc on the transmission.

Car Frame Damage Repair Costs

The truth of the matter is that there is no set range for frame damage repair costs. It all depends on the severity of the damage.

Take a dented rear quarter panel, for instance. If it’s as simple as removing the dent, repairs may run between $500-$1,000.

However, if the damage is severe, repairing it may require that entire sections be replaced, which means cutting off the old, and welding on the new. Then, industrial machines are used to stretch your car back to its original shape.

Sound expensive yet? Depending on the extent of the issues, frame damage repair costs can run as high as $10,0000. Because of this, insurance companies usually total out the vehicle rather than going through the effort of fixing it.