Body-on-frame is a type of automobile construction technique where the car body is mounted to a separate frame. The framework consists of the car's chassis, along with most of its mechanical components and drivetrain. The first mass-produced automobiles consisted of body-on-frame constructions, with the framework typically being made of wood. While welded steel eventually replaced wood, body-on-frame construction fell out of favor to monocoque construction, where the framework is eliminated by a rigid body. Today, however, body-on-frame construction is occasionally used because of its design flexibility and durability.
In the early days of mass automobile manufacturing, body-on-frame construction was favored because it could easily accompany the frequent body design changes of the period. Since the chassis and mechanical components were separate from the actual body of the car, automobile manufacturers could easily change the design of the car's body without having to worry much about the chassis or the drivetrain. This resulted in lower design costs, stronger automobiles, and less time necessary for design, which ultimately proved beneficial for many companies. A single chassis design also allowed automobile manufacturers to use the same chassis for different cars, which was far easier from a design standpoint.
Starting in the early to mid 1930s, automobile construction technology changed slightly with the introduction of monocoque construction. Monocoque construction was used mainly to reduce overall weight, which helped to optimize the performance of the automobile employing it. Over the next couple of decades, monocoque construction became increasingly popular. By the early 1960s, most automobile manufacturers had completely switched to monocoque construction, with body-on-frame being reserved for more heavy-duty vehicles. This is still the trend today, with body-on-frame construction being used only for larger vehicles, or some passenger vehicles, such as the Lincoln® Town Car.
There are numerous advantages to body-on-frame construction. One of the most notable of these advantages is the fact that they allow for design flexibility. As different types of cars can use the exact same chassis and drivetrain, all that really needs to be altered is the car body, which is a far simpler process than having to redesign the automobile's chassis and mechanical components. Another advantage is the increased strength and durability. Unlike a monocoque body, the car is supported by a separate, rigid frame. Not only does this increase the automobile's strength, but it also increases its longevity, as it is not as prone to rusting through, as happens in monocoque construction.
Body-on-frame vehicles do, however, have their disadvantages. The framework adds a great deal of extra weight to the vehicle, which can drive down fuel efficiency and overall performance. Along with this, the rigidity of the automobile eliminates what would be a crumple zone, making framed vehicles very dangerous in accidents. This problem has been overcome by the installation of front and rear "clips," which can absorb impacts.