GM’s Ultium Trucks & SUVs Aren’t Really Unibodies Or Body-On-Frame. What Should We Call Them?

Energy
Wednesday, September 28th, 2022 9:00 am EDT

A couple months ago, when the Silverado EV was announced, I got into an interesting conversation with Jo Borrás, one of my colleagues here at CleanTechnica. In an article, I called the Silverado EV a unibody, like the Honda Ridgeline or the Ford Maverick. He told me I had made a rookie mistake calling it that, and that it’s really more of a body-on-frame design, but without a separate bed module like most body-on-frame trucks. This would make it more like a body-on-frame car, which mostly went extinct when Ford stopped making Crown Vics for police departments.

To people who only care whether a vehicle is an EV or an ICE vehicle, this distinction may seem kind of silly. Any EV is better than a nasty ICE truck, right? But, there are important differences between these platform designs. A body-on-frame vehicle resists twisting forces more, allowing it to do better at tasks like off-roading or towing. They’re also great for modularity and collision repair, as you can just unbolt badly damaged sections of vehicle and bolt another one on as long as the frame isn’t seriously compromised.

But, they’re also more expensive to produce than unibody vehicles and tend to be less efficient, which makes them a tough choice for EVs (which need as much range as they can get).

Unibody vehicles tend to get derided by enthusiasts as “Coke cans,” because their strength is more like that of an aluminum can. They’re more like an insect’s exoskeleton than a mammal’s backbone when it comes to structure. But, they’re cheaper to build, and unibody vehicles tend to weigh less than body-on-frame vehicles.

Pixar’s Wall-E struggles to figure out where to put the spork. Fair use, commentary.

But, this dichotomy hasn’t always held in the automotive industry. There have been some oddball vehicles that took elements of both body-on-frame and unibody design. Like when Wall-E couldn’t figure out whether to put the spork in the spoon cup or the fork cup, these vehicles have needed another cup to categorize them. A great example would be the Pontiac Fiero’s “space frame” design, which was later adopted by GM’s now-defunct Saturn brand.

GM’s Unibody Heresy With The BEV2 Platform

What started troubling me about this is that I’ve noticed some very interesting things about the way my Chevy Bolt EUV rides and handles. Unlike most unibody cars I’ve had over the years, the EUV feels a lot more solid. I can feel when the suspension bends, but I don’t feel the extra little bit of twist in my seat-of-the-pants torsion meter. While most unibody vehicles have become more rigid in recent years to be better at surviving crashes and protecting vehicle occupants, I could still feel a difference between them and trucks/SUVs.

So, I started looking into it. It turns out that the GM BEV2 platform the original 2017 Bolt EV and its newer cousins (the EUV, and Chinese market vehicles like the Buick Velite 7, Chevy Menlo) are all based on took a bit of a departure from unibody orthodoxy in the design phase.

They’re very clearly still unibodies and not body-on-frame or space frame vehicles, but they added an extra subframe that most unibody vehicles don’t have. For those unfamiliar, subframes are small sections of frame that bolt to a unibody vehicle’s body to contain things like the drivetrain and maybe the rear suspension. This helps spread the weight of the engine and suspension forces to a wider part of the vehicle and make everything more rigid. Subframes can also help isolate vibrations from the main body so you don’t feel them.

The extra subframe the Bolt EV and its cousins have is for the battery. GM has told multiple media outlets that this subframe contributes 28% of the car’s total torsional rigidity. This concentrates more of the rigidity in the lower part of the unibody+subframe assembly, giving it a slightly different feel than even rigidized modern unibodies.

Similar things have happened in the designs of other unibody cars that have a battery pack, such as the Nissan LEAF and to some extent Tesla vehicles, so this departure seems to be about the only way to do a unibody EV right. But, for some reason, the EUV just feels more rigid than those other vehicles, but again, only by a seat-of-the-pants feel, which can be difficult to calibrate.

The BT1 Platform Departs Further From Body-On-Frame & Unibody Orthodoxy Than These Other EVs

As I investigated more, I found an article from earlier this year at GM Authority that showed GM doing something even more radical. Their writers noticed that the GM BT1 platform didn’t seem to fit in either box, and defied categorization in ways far beyond the Bolt and its cousins. When asked point-blank what kind of platform the Silverado EV was built on, GM’s Chief Engineer, Battery Electric Trucks, Nichole Kraatz said:

“It is not a unibody and it is not a body-on-frame. We’ve designed a different type of architecture where we have a body that has a floor, but also, the Ultium battery structure is actually a good portion of the structure and those two are connected after the body exits the body shaft. So we’ve defined kind of a new category of vehicle that doesn’t have that traditional body-and-frame approach.”

When pressed further and asked what their teams call their platform, she said it’s internally called the “Ultibody,” a reference to the Ultium platform. A later advertisement for the truck gives a great cutaway video that shows the different body and frame elements that show how the vehicle is neither and both at the same time:

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In some ways, it’s just a unibody platform, but with the front, rear, and battery subframes tied together. But, with them tied together, it’s sort of a frame, even if not built in one piece like most truck frames. But, the body is providing a lot of the rigidity, making it a unibody. It really does defy categorization.

Is it a beefier Bolt EV with the subframes tied together and more frame parts? Well, yes. But actually, no.

Is it a Silverado with a multi-piece frame? Well, no. But, actually yes. But no.

Should we call this unibody-on-frame? Or are those just more subframes?

EVs Are Leading To A Very Different Automotive Future

The truth is, EVs have changed automotive design forever. What was once a very solid distinction turned out to only be driven by the needs of ICE engines. When engineers started finding ways to optimize for a battery pack instead of for a primitive pile of pistons (apologies to Dr. Smith from the old Lost in Space), new things started becoming not only possible, but desirable.

It seems GM’s journey from embracing front-drive unibodies for most vehicles took a left turn at Albuquerque when it came time to add batteries to a unibody platform, and the rest was history.

One thing’s for sure, the industry is going to keep producing interesting things as the EV transition goes forward.

Featured image: A screenshot from an online advertisement for the Silverado EV, showing its construction.

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