Bodies in white: A short guide to vehicle chassis

It's very likely when riding a car, all of us wonder how they are built. 

The answer to that question is far more complex to answer in a single article, but to summarise, cars are built by bolting together various components to a single platform, otherwise known as the chassis. Modern automobiles are built using many types of chassis, each with its advantages and limitations. With these in mind, here is a quick summary of the most common vehicle chassis.

Body-on-frame

Application: Trucks, SUVs

The oldest type of chassis still in use, the body-on-frame design predates the automobile itself and traces its lineage back to horse-drawn carriages. 

As the name suggests, the body-on-frame system contains two major components, the passenger-carrying body, and the load-bearing frame. Mechanical components such as the drivetrain, suspension and other heavy items are placed onto a metal frame. Said frame is then covered with a body, containing the cabin, lights, cargo boot etc.

Body on frame designs feature excellent load carrying capabilities and the tough frame is well suited for unpaved terrain. They are also extremely versatile, as virtually any type of body can be bolted onto a frame, be it for passenger or commercial use. Conversely, they are expensive to manufacture and quite heavy, and the separate body and frame design make for poor crash safety. 

Because of its limitations, auto manufacturers have mostly moved away from using body on frame designs on most vehicles. At present, trucks and off-road capable SUVs are the main users of the body on frame chassis, as it is essential for such vehicles to have the capability to carry heavy objects and cross rugged terrain.

Platform frame

Application: Microcars, electric vehicles 

A variation of the body on frame design, and somewhat of a halfway point between body-on-frame and unibody, a platform frame incorporates the bottom of the passenger cabin into the chassis of the vehicle. 

Historically, platform frames were used on smaller economy cars, as they were lighter and cheaper to produce than the heavy full-sized frames. The most famous example of such a design would be the Volkswagen Beetle, whose "body-on-pan" construction laid the groundwork for one of the most iconic cars of all time, the birth of Porsche, one of the most beloved sports car brands among auto enthusiasts. 

Platform frames were mostly phased out by the 60's, but has seen a recent resurgence with the rise of electric vehicles. EVs like Tesla's Model S use a platform chassis, with the battery cells being integrated into the "platform" under the passenger cabin.

Unibody

Application: Sedan, hatchbacks and many more

The most common type currently in use, a unibody or "unitised body" design is when the body and chassis are merged together. 

The load-bearing frame of the car is built into the bodywork, forming a single structure that is lighter, stronger and safer than any body-on-frame platform. 

Compared to frame chassis, Unibodies are cheaper to make, less material intensive, fuel-efficient and thanks to their integrated frame, provide far superior crash protection. This is done by absorbing the energy of the collision into the car frame, a method that while severely compromising the frame, ensures the occupants are left relatively unharmed. 

A major limitation of the unibody design is its lack of versatility, as because of the combination design, it is not always feasible to change the bodywork to serve a multitude of roles. But because of the provided safety and other advantages, unibody has become the industry standard for automobile manufacturing and is likely to remain so for the foreseeable future. 

Space frame

Application: Racing and other specialised vehicles 

A space frame chassis is where the suspension, engine, and body panels are attached to a three-dimensional skeletal frame of tubes. Basically, such frames are built with tubular steel pipes that are connected in a triangular formation to maximise strength. 

Space frame designs maximise rigidity and minimise weight, but provide limited interior space and are very laborious to make. Such chassis are used in very specialised vehicles, such as race cars and all-terrain vehicles. 

Their highly custom nature also made them very popular among bespoke and boutique carmakers, whose limited production nature justifies the time and effort needed to put together such platforms.