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What is Body on Frame?

By Autolist Staff | April 10, 2019

Modern sport-utility vehicles come in a wide variety of sizes, styles and trims. Underneath all those different models is one of two kinds of frames. Many newer SUVs are unibody vehicles. Most of these are based on passenger car designs. Traditional SUVs, as well as a few prominent current models, feature body-on-frame construction, as the first SUVs were closely related to pickup trucks of the time. This article discusses the differences between body on frame and unibody SUVs. It also highlights the benefits of body-on-frame SUVs for uses such as off-road driving and pulling heavy loads. Lastly, it looks at the benefits of unibody frames and why these frames have come to dominate the market.

What is Body-on-Frame Construction?

Body-on-frame design means the vehicle frame and body are two different pieces. Typically an existing pickup ladder frame is used as the base, with the body designed to fit the frame. Originally all vehicles were made using body-on-frame construction. From the 1930s onward it became primarily used on trucks. From their origins as military vehicles, SUVs were traditionally built using truck frames. That remained true until the late 1970s. From the 1980s onward manufactures shifted away from body-on-frame designs for passenger cars.

The number of body-on-frame SUVs is only a fraction of the total SUV market, with only about 15 model lines available. Most of these are larger SUVs, such as the American made vehicles from Ford and General Motors. GM makes the Cadillac Escalade, Chevy Suburban, Tahoe and GMC Yukon using the Silverado pickup truck frame. Ford produces the Ford Expedition and Lincoln Navigator using the F-150 frame. The upcoming 2021 Bronco is based on the Ford Ranger frame design, as are the internationally sold Everest and older Ford Explorers. The Jeep Wrangler features body-on-frame construction.

Toyota is a major automaker outside the United States that has body-on-frame designs. Here, it offers models such as the Toyota 4Runner, Sequoia and Land Cruiser, as well as Lexus GX and LX models. Mercedes G-Class SUVs are body-on-frame, as is the Nissan Armada and its companion Infiniti QX80.

What is Unibody Construction?

As the name suggests, unibody frames, are made with the frame and body combined into one piece. Unibody design SUVs are typically modified forms of passenger car unibody frames. Unibody frames were first used on a large scale in passenger cars starting in the 1930s and, by the 1970s, increasingly became the standard for refinement, quality and safety standards.

The first SUVs with unibody construction appeared in the 1980s, with the 1984 Jeep Cherokee is a notable example. The 1996 Toyota RAV4 was a first in that it was built off of a platform used for a sedan. By 2006 more than half of the SUVs manufactured had unibody construction. Today's SUVs, whether they have off-road pretentions or not, are predominantly constructed with unibody designs, including models from Jeep and Land Rover, as well as Acura and Volkswagen. Even the Honda Ridgeline pickup uses a unibody structure from the Pilot SUV.

Fuel economy and a smoother ride have been the dominant reasons why unibody SUVs have overtaken body-on-frame designs. The first unibody models appeared after the fuel shortages of the 1970s. SUVs as an entire class became a prominent vehicle in North America through the 1990s and 2000s. This rise coincided with regulations requiring better fuel economy in vehicles, which favors the lighter unibody frames.

Additionally, driver use of SUVs changed as the vehicles dominated the market. For many drivers, the ability to load the cargo area with groceries, pets or kids' gear—not off-roading capabilities or hauling capacity—was the primary reason for buying an SUV. That helped smoother handling and greater fuel economy become higher priorities in the SUV market.

##What are the Advantages and Disadvantages of Body-on-Frame Construction?##
The primary benefit of this type of construction is resistance to torque and twisting compared to unibody vehicles. That makes these vehicles well suited for off-roading and transporting heavy loads. Off-roaders typically can drive over rougher terrain in a body-on-frame vehicle.

Body-on-frame vehicles are also typically less expensive to manufacture. This is in part because manufacturers can mount different bodies on the same frames. Body-on-frame vehicles can also be cheaper to repair after collisions. The separate body and frame can allow mechanics greater access to damaged parts. Alternately, a severely damaged structure or body can be replaced in its entirety if one or the other remains intact.

The main disadvantage of body-on-frame construction is that it increases the weight of vehicles. That leads to reduced fuel economy. Body-on-frame vehicles also tend to have poorer handling and cornering than unibody vehicles. In general, the ride in body-on-frame SUVs is not as smooth as passenger cars or crossover SUVs.

What are the Advantages and Disadvantages of Unibody Construction?

Unibody construction gives SUVs greater handling and cornering for a smoother ride and allows them to be tuned to drive more like passenger cars. These SUVs are also lighter in weight, which results in better fuel economy. Additionally, as a majority of models are built on unibody frames, there are more choices for SUV buyers.

The primary drawbacks of unibody design are reduced off-roading ability. That is because the single frame does not handle twisting forces as well. Likewise, unibody vehicles may have less hauling and towing capacity than body-on-frame SUVs. Lastly, unibody SUVs can be more expensive to produce and repair.

Summary

SUVs come in both body-on-frame and unibody frame designs. Body-on-frame models feature a separate frame and body. The frame is typically used as the base for pickup trucks as well as SUVs. Unibody frame models combine the body and frame into one piece. This design mimics passenger cars.

Body-on-frame construction is better at resisting twisting forces. It is better for off-road driving and hauling heavy loads. Unibody frame construction results in a lighter vehicle, usually resulting in better fuel economy, handling, and cornering.

In general body-on-frame SUVs are best for buyers planning to drive off-road or frequently haul loads. Otherwise, the smoother driving and better gas mileage of unibody SUVs typically outweigh the benefits of body-on-frame construction. Drivers that use their SUV for a mix of activities must weigh better on-road handling and fuel economy versus better off-road handling and hauling ability.

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