GM’s Ultium trucks and SUVs aren’t really unibody or body-on-frame. What should we call them?
A few months ago when the Silverado EV was announced, I had an interesting conversation with Jo Borrás, a colleague of mine here at Clean Technica. In an article, I called the Silverado EV a unibody, like the Honda Ridgeline or the Ford Maverick. He told me I made a rookie mistake calling it that, and it was more of a body-on-frame design, but without a separate bed module like most body trucks on frame. It would look more like a body-on-frame car, which mostly disappeared when Ford stopped making Crown Vics for police departments.
For people who only care whether a vehicle is an electric vehicle or an ICE vehicle, this distinction might seem a bit silly. Any electric vehicle is better than a mean ICE truck, right? But, there are important differences between these platform designs. A body-on-frame vehicle is more resistant to twisting forces, allowing it to do tasks like off-roading or towing better. They are also great for modularity and collision repair, as you can simply unbolt badly damaged sections of the vehicle and bolt on another 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, making them a tough choice for electric vehicles (which need as much range as possible).
Monocoque vehicles tend to be derided by enthusiasts as “Coke cans”, as their strength is more like that of an aluminum can. They are more like an insect’s exoskeleton than a mammal’s spine when it comes to structure. But, they are cheaper to build and unibody vehicles tend to weigh less than body-on-frame vehicles.
But this dichotomy has not always existed in the automotive industry. There have been bizarre vehicles that have taken both body-on-frame and unibody design elements. Like when Wall-E didn’t know whether to put the spoon in the spoon tumbler or the fork tumbler, these vehicles needed another tumbler to classify them. A good example would be the “space frame” design of the Pontiac Fiero, which was later adopted by GM’s now defunct Saturn brand.
GM’s Unibody heresy with the BEV2 platform
What started to confuse me about this was that I noticed some very interesting things about the way my Chevy Bolt EUV drives and handles. Unlike most unibody cars I’ve had over the years, the EUV feels much sturdier. I can feel when the suspension bends, but I don’t feel the extra little twist in my pants seat twist meter. While most unibody vehicles have become stiffer in recent years to better survive crashes and protect vehicle occupants, I could still feel a difference between them and trucks/SUVs.
So, I started to inquire. Turns out 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 a little deviation from monocoque orthodoxy in design. phase.
They are very clearly monocoques and not body-on-frame or spaceframe vehicles, but they have added an extra subframe that most unibody vehicles do not have. For those unfamiliar, subframes are small frame sections that bolt to the body of a unibody vehicle to hold things like the transmission and possibly the rear suspension. This distributes the weight of the engine and suspension forces over a wider part of the vehicle and makes everything stiffer. Subframes can also help isolate vibrations from the main body so you don’t feel them.
The extra subframe of the Bolt EV and its cousins is for the battery. GM has told multiple media outlets that this subframe contributes 28 percent of the car’s total torsional stiffness. This concentrates stiffness more in the lower part of the monocoque + subframe assembly, giving it a slightly different feel than even stiffened modern monocoques.
Similar things have happened in the design of other battery-powered unibody cars, such as the Nissan LEAF and, to some extent, Tesla vehicles, so this departure seems to be about the only way to make a good monocoque electric vehicle. But, for some reason, the EUV just feels stiffer than those other vehicles, but again, only through a seat-of-the-pants feel, which can be tricky to calibrate.
The BT1 platform deviates more from body-on-frame and Unibody orthodoxy than these other EVs
Investigating further, I found an article from earlier this year on GM Authority which showed that GM was doing something even more radical. Their editors noticed that the GM BT1 platform didn’t seem to fit into either box and defied categorization far beyond the Bolt and its cousins. When asked bluntly what kind of platform the Silverado EV was built on, GM Chief Engineer, Battery Electric Trucks Nichole Kraatz said:
“It’s not a monocoque and it’s not body-on-frame. We 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 part of the structure and those two are connected after the body comes out of the body tree. So we’ve defined a sort of new category of vehicles that doesn’t have this traditional approach to body and chassis.
When pressed further and asked what their teams called their platform, she replied that it was internally called “Ultibody”, a reference to the Ultium platform. A later commercial for the truck gives a great cutaway video that shows the various body and chassis elements that show how the vehicle is neither at the same time:
In some ways, it’s just a unibody rig, but with the front, rear and battery subframes tied together. But, with them strapped together, it’s kind of a frame, even though it’s not built in one piece like most truck frames. But, the body provides much of the stiffness, making it a monocoque. It really defies categorization.
Is it a beefier Bolt EV with the subframes strapped together and more chassis parts? Well yes. But in fact, no.
Is it a Silverado with a multi-piece frame? Well no. But, in fact yes. But no.
Should we call it unibody-on-frame? Or are they just more subframes?
Electric vehicles lead to a very different automotive future
The truth is that electric vehicles have changed automotive design forever. What was once a very solid distinction turned out to be driven only by the needs of ICE engines. When engineers started finding ways to optimize for a battery instead of a primitive stack of pistons (apologies to Dr. Smith from the old Lost in Space), new things started to become not only possible, but desirable.
It looks like GM’s path to adopting front-wheel-drive unibody for most vehicles took a left turn in Albuquerque. when is the time to add batteries to a unibody rigand the rest was history.
One thing is for sure, the industry will continue to produce exciting things as the transition to electric vehicles progresses.
Featured Image: A screenshot of an online advertisement for the Silverado EV, showing its construction.
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