Two types of construction have dominated the car industry in the United States since the invention of the automobile: body-on-frame and unibody. Body-on-frame construction entails mounting the body of the car on a chassis carrying the powertrain, while unibody construction involves manufacturing the frame and body of the vehicle as a single piece. Today, almost all cars and most heavy-duty vehicles such as trucks feature unibody construction, but that wasn't always the case. Here's a breakdown of the differences between body-on-frame and unibody vehicles.
Body-on-Frame vs. Unibody: What's the Difference?
The first American cars ever made featured wooden chassis similar to the buggies that had come before them. Ford continued this tradition with the Model T, as body-on-frame vehicles were easy to manufacture on an assembly line. The chassis of cars in the 20th century consisted of steel ladder frames featuring two symmetrical frame rails connected by cross-members. Because the body was separate from the chassis, automakers could experiment with the design of the body without having to alter the whole car. By the end of the 1930s, unibody, or monocoque, designs – which were more efficient to construct and came with other benefits – started to appear. The Citroen Traction Avant and Opel Olympia were the first popular unibody cars. Trucks and sport utility vehicles continued to use separate frames predominantly until the 21st century, and many still do.
Two modern passenger cars that use body-on-frame designs are the BMW i3, an electric vehicle, and the BMW i8, a plug-in hybrid sports car. Modern body-on-frame SUVs include the Chevrolet Suburban, Chevy Tahoe, Nissan Armada, Lexus LX, Lincoln Navigator, Jeep Wrangler, Ford Expedition, Cadillac Escalade and Toyota 4Runner. Some body-on-frame pickup trucks include the GMC Canyon, Ram 1500, Nissan Frontier and Jeep Gladiator.
Over time, many SUVs, such as the Ford Explorer and Jeep Cherokee, have switched to unibody. The Honda Ridgeline is one unibody pickup truck, and Hyundai plans to release a similar vehicle. The popularity of unibody frames now encompasses just about every type of passenger vehicle. That includes crossover SUVs, which increasingly featured unibody designs after the release of the Toyota RAV4 in 1994.
The Benefits of Body-on-Frame Design
Despite its dwindling popularity, body-on-frame construction enjoys some benefits over unibody construction. For one, body-on-frame vehicles tend to be better off-roaders, as they withstand twisting forces better. Off-roading means driving on rough terrain instead of paved roads. When driving off-road, body-on-frame vehicles are also more resistant to dirt, mud, snow, and stones, since they're typically mounted higher than unibody vehicles. Also, body-on-frame vehicles have a greater hauling and towing capacity than unibody ones, due to their sturdy foundations and tolerance for torsional flexing.
Another benefit of body-on-frame vehicles is that they're cheaper to build and repair. Because the body and frame are separate, you can replace one or the other if it gets damaged rather than both. Damaged body parts such are much cheaper to fix than an entire vehicle shell, making body-on-frame vehicles ideal for people who are hard on vehicles, such as police officers or livery drivers. Also, body-on-frame vehicles make less noise on the road. That's because rubber pads around the attachment bolts separate the chassis from the frame, insulating passengers from the sound of the tires. Higher elevation from the ground also protects body-on-frame vehicles somewhat better from the moisture of the road, which causes rust.
The Drawbacks of Body-on-Frame Design
Despite their benefits, body-on-frame vehicles are far from perfect. For example, their torsional flexing hurts their handling and road grip, and they're heavier overall than unibody vehicles, which cuts into their fuel economy. The high ground clearance of many body-on-frame vehicles — and resulting high center of gravity — also makes them harder to drive.
A significant drawback of body-on-frame vehicles is that they lack crumple zones. Crumple zones are regions of vehicles that absorb kinetic energy during a crash, so the passengers don't absorb it instead. They're a significant safety feature of modern cars and regularly save lives. According to a National Center for Biotechnology Information study, occupants of body-on-frame SUVs are more likely to die in accidents than occupants of unibody SUVs. Finally, although body-on-frame vehicles are better for off-roading, they sometimes produce a rougher ride on normal roads.
The Benefits of Unibody Design
Unibody vehicles have come to dominate the market for many reasons. In addition to their superior safety features and better fuel economy, they're easier to design. Computer-aided design software allows automakers to mold the shells of cars like clay to achieve precise contours. Sophisticated manufacturing processes can pump out these shells quickly, even if they consist of many materials that must be welded or riveted and parts that must be attached. Since unibody vehicles are much more common than body-on-frame ones, finding one to buy is also easier. Also, the unibody's low center of gravity makes rollovers less likely and offers a smoother ride. Many unibody SUVs handle as smoothly as cars.
The Drawbacks of Unibody Design
Despite their perks, unibody vehicles don't handle twisting forces as well as body-on-frame vehicles. As a result, hauling and off-roading are more difficult with unibody vehicles. Unibody SUVs are also more expensive to repair since damage to part of the SUV is more often damage to the whole.
Although research points to unibody SUVs being safer than body-on-frame ones, differences in the average model age between the two types of vehicles may be a confounding variable. Body-on-frame vehicles skew older, and many unrelated improvements in vehicle design have taken place since the shift to monocoques – as well as the advancement of driver assistance technologies that have become available on body-on-frames and unibody vehicles..
Which type of vehicle is right for you? That depends on how you plan to use it. If you want cheaper repairs, superior towing capacity and better off-roading capabilities, you might prefer a body-on-frame vehicle. If you want better handling, greater fuel economy and a safer ride, a unibody vehicle might be more to your liking.