What’s the Difference Between Unibody and Body-on-frame Construction?

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Within the past 10 years, popular midsize SUVs like the Ford Explorer have switched from body-on-frame to unibody construction. With the exception of a handful of models – like the Jeep Wrangler and Toyota 4Runner – the only body-on-frame vehicles left are pickup trucks and the few big SUVs that share these truck platforms. Pretty much everything else relies on unibody manufacturing.
Yet that began to change again last year. First, there’s been a recent uptick in interest for unibody pickups. The 2017 Honda Ridgeline, despite its unibody origins, was named North American Truck of the Year. More recently, Hyundai confirmed that it would launch its own small unibody pickup in the near future. But at the same time, body-on-frame vehicles are garnering new interest as well, and three new ones should be on the market by 2020: a small pickup based on the Wrangler and next-gen versions of Ford’s Bronco SUV and Ranger midsize pickup.

With unibody vehicles finding that kind of success in the truck segments, and body-on-frame choices also gaining in popularity, let’s take a quick refresher course on the differences between the two.


Body-on-frame Construction: It Is What It Is

The traditional method of assembling a car or truck is called body-on-frame construction for a simple reason. Essentially, this process starts with an underlying frame, and then the vehicle’s body goes on top of that. In the case of a body-on-frame pickup, the bed is mounted on the frame separately. Indeed, if you look closely at the side of a truck, you’ll be able to see exactly where the cab ends and the separate bed begins. (Shoppers should keep in mind that the new Ridgeline has vertical trim pieces in the same position, but purely as a pickup design cue.)

Turning to the frame itself, that generally features two long rails of high-strength steel that are connected by shorter steel cross-members. As a result, these are often called ladder-type frames.

That solid foundation remains important for towing and hauling, and it’s better able to stand up to extreme twisting forces, too. This is a major advantage for off-roaders travelling through river beds, over large rocks or logs and across other uneven terrain. On the road, however, body-on-frame vehicles are heavier, which means worse fuel efficiency. Also, the same rigidity that was so helpful on the trail creates a noticeably harsher and less forgiving ride on the pavement, especially with bumps and potholes.

There’s also some reason to believe body-on-frame vehicles aren’t as safe as their unibody counterparts. A study conducted by the National Center for Biotechnology Information reports that, “Occupants of compact unibody SUVs were … at lower risk of death compared to occupants of body-on-frame SUVs.”

(General Motors)

Unibody Construction: Frame + Body = Modern-day Vehicle

Deciphering unibody construction is only slightly more challenging, since it comes from the phrase unitized body. In other words, with these vehicles, the body and frame are considered one unit. Now to be clear, that structure, sometimes called a monocoque, is actually composed of individual pieces. For the Cadillac CT6, as one example, 13 different materials have been welded, riveted and screwed together to create its body structure, which also uses structural adhesive in some places.

A key benefit is that not all of those pieces have to be made of heavier metals like steel. In fact, as automakers continue to reduce vehicle curb weight and improve efficiency and performance, they’re also introducing body structures made with lightweight materials like aluminum and carbon fiber. This doesn’t affect safety, because unibody vehicles usually incorporate crush zones and other structures specifically designed to keep crash forces away from the cabin.

Along with their advantages in terms of weight, fuel-efficiency, safety and performance, unibody vehicles have gotten a further boost from the growth of computer-aided design and engineering. In the early days, putting fresh bodies on top of pre-existing frames was a simpler way to deliver a “new” vehicle. Today, though, technologies have taken much of the guesswork out of the more complicated unibody design, while also reducing the need for as many expensive physical models.

Tomorrow? Although those previously mentioned Jeep and Ford throwbacks are on their way, customers can expect unibody vehicles to continue dominating the United States marketplace.

By | 2018-02-13T20:51:07+00:00 September 7th, 2017|Car Tech|0 Comments

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