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GM’s transition to an all-electric lineup is bringing a wealth of new technology and engineering to the fore, including the development of new vehicle platforms to underpin a deluge of new GM EV models. Among these is the GM BT1 platform, which, according to GM is considered neither a unibody platform, nor a body-on-frame platform.

During a recent interview regarding the new Chevy Silverado EV between GM Authority Executive Editor Alex Luft and GM Chief Engineer, Battery Electric Trucks, Nichole Kraatz, Kraatz was asked about the Silverado EV’s underlying architecture, which was confirmed to be the BT1 platform. When asked whether BT1 was considered body-on-frame, rather than a unibody architecture, Kraatz responded by saying:
“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 batterystructure 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 asked if there was a specific term to describe the BT1 architecture, Kraatz said that she “actually used the term UltiBody to my team the other day because we have yet to assign one.”

To note, the “Ulti” prefix is a reference to GM’s Ultium brand, which includes the GM Ultium battery and the GM Ultium drive motor tech.
The GM BT1 architecture underpins GM’s battery-powered pickup trucks and SUVs, including the ChevySilverado EV, GMC Sierra EV, GMC Hummer EV, and the forthcoming all-electric variant of the Cadillac Escalade.

Conversely, GM’s new BEV3 platform is used to underpin the automaker’s all-electric crossovers and sedans, including the Cadillac Lyriq, Cadillac Celestiq, Chevy Blazer EV, and Chevy Equinox EV.
 

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I watched a technical presentation for an engineering society by a GM propulsion engineer about Ultium. Engineering tech talk basically, lots of interesting details and thoughts from a team member (I forget his name but he shows up in Chevy promo videos talking about Ultium).

Anyhow it’s not a simple explanation - the old body on frame was an obvious approach from the old days. But a key part of Ultium is how the battery plays an integral part of the construction. In a simple way you can think of it as maybe frame rails with unibody architecture, but that simplifies it too much as the modules also provide torsional rigidity. My simplified takeaway was the horizontal frames protect the battery and vehicle, while the modules provide torsional support. It’s quite rigid, the ride and handling guys are thrilled with the kind of ride they can get with this architecture. But on top of that it is a unibody approach basically with the rest of the vehicle.

It’s a great approach with many benefits, but not something that is explained easily for marketing such as “body on frame”, so instead its called Ultium which encompasses much more than just the energy source. With ICE vehicles it makes sense to make a frame and stick components on that, loosely decoupled because the engine vibrates so much. But batteries are different, internally they require room for expansion as the pack goes through a charge cycle, but they don’t want any structural deformation beyond that.

tl/dr, in the new era of EV’s we need to find something else to obsess about :) Or maybe not, I think Ultium is already a generational step beyond the ‘skateboard’ design other OEM’s still use.
 

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I watched a technical presentation for an engineering society by a GM propulsion engineer about Ultium. Engineering tech talk basically, lots of interesting details and thoughts from a team member (I forget his name but he shows up in Chevy promo videos talking about Ultium).

Anyhow it’s not a simple explanation - the old body on frame was an obvious approach from the old days. But a key part of Ultium is how the battery plays an integral part of the construction. In a simple way you can think of it as maybe frame rails with unibody architecture, but that simplifies it too much as the modules also provide torsional rigidity. My simplified takeaway was the horizontal frames protect the battery and vehicle, while the modules provide torsional support. It’s quite rigid, the ride and handling guys are thrilled with the kind of ride they can get with this architecture. But on top of that it is a unibody approach basically with the rest of the vehicle.

It’s a great approach with many benefits, but not something that is explained easily for marketing such as “body on frame”, so instead its called Ultium which encompasses much more than just the energy source. With ICE vehicles it makes sense to make a frame and stick components on that, loosely decoupled because the engine vibrates so much. But batteries are different, internally they require room for expansion as the pack goes through a charge cycle, but they don’t want any structural deformation beyond that.

tl/dr, in the new era of EV’s we need to find something else to obsess about :) Or maybe not, I think Ultium is already a generational step beyond the ‘skateboard’ design other OEM’s still use.
The ultium platform, is the battery pack, inverters, drive units, and electronics, "ultium" has NOTHING to do with the structure, and nothing in the chassis design is shared between Hummer, Lyriq, BrightDrop, and Blazer/Equinox.

Platform used to mean Chassis (Epsilon, Zeta, Lamba, and GMT XXX) , now platform means Electronics, and Electronic architecture. BT1 architecture shares nothing in chassis or suspension with BEV3 or "BV1Hx" BrightDrop, but both are called Ultium. There is another coding coming in 2025 for the mid size EV pickups "G264", and for the EV vans "GMT620", all coming in 2026.
 

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The ultium platform, is the battery pack, inverters, drive units, and electronics, "ultium" has NOTHING to do with the structure, and nothing in the chassis design is shared between Hummer, Lyriq, BrightDrop, and Blazer/Equinox
Sorry but that’s incorrect, at least according to Andy Oury who is Engineering Technical Lead for HV batteries at GM - you made me dig up the name of who gave the talk. The battery is structural, that’s one of the big innovations here. Probably the most significant one maybe, as much of the rest of Ultium appears to be specification rather than a particular innovation*. Also incorrect that chassis design isn’t shared - the basis for the chassis is the battery in Ultium, of which there are standard configurations. Sorry I don’t have the slides from the talk but that’s straight from Oury, so take it up with him if you want to disagree whether the battery is a major structural component or not.

* the motor with it’s bar wire structure is very innovative, but that work was done in the late 90’s as a result of the EV1 R&D. The electronics and chips R&D came from other industries, software is the standard approach of using a MPSoC ARM with Linux. Anyhow GM has the longest track record of being in EV’s since the 90’s EV1, so taken together Ultium is very innovative IMO
 

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Sorry but that’s incorrect, at least according to Andy Oury who is Engineering Technical Lead for HV batteries at GM - you made me dig up the name of who gave the talk. The battery is structural, that’s one of the big innovations here. Probably the most significant one maybe, as much of the rest of Ultium appears to be specification rather than a particular innovation*. Also incorrect that chassis design isn’t shared - the basis for the chassis is the battery in Ultium, of which there are standard configurations. Sorry I don’t have the slides from the talk but that’s straight from Oury, so take it up with him if you want to disagree whether the battery is a major structural component or not.

* the motor with it’s bar wire structure is very innovative, but that work was done in the late 90’s as a result of the EV1 R&D. The electronics and chips R&D came from other industries, software is the standard approach of using a MPSoC ARM with Linux. Anyhow GM has the longest track record of being in EV’s since the 90’s EV1, so taken together Ultium is very innovative IMO
I am just going to tell you almost everything you said is wrong... Andy Oury is a friend of mine, and he designs batteries, cell to pack, has nothing to do with chassis, structure, suspension, or anything else.

The Hummer (and all BT1 trucks) does not share a single part of it's chassis or suspension (structure) with Lyriq, Blazer EV, Equinox.

The battery box is secondary structure, but not the cells and modules, are just like sacks of potatoes attached into the battery pack. The battery enclosure is built by Magna, not GM, and the Hummer EV is structurally sound with the battery pack removed .

As for the motors, you are wrong again, Bar wire motors are easier to manufacture, but wire wound motors are better in almost every other way, which is why most manufactures wire wind their motors incl Tesla and Lucid. Also Audi, Porsche,

As for your opinion of GM being innovative, I like GM and have a Hummer EV in my garage, but GM is not and I mean not innovative. GM EV efficiency trails most of the leaders for starters the Ultium Platform vehicles are all too heavy, inverter/motor efficiency is mid pack, and vehicle aerodynamics are not class leading. I could go on and on on systems the Hummer has that could be optimized and lightened without losing strength, to start with it has hundreds of bolts in the Structure that are too long, half to a full inch of threads hanging out, that is basic 101 level cost and weight reduction that could be done and not lose strength.

As for the infotainment in the Hummer EV, what ever GM is doing, it is wrong, that is the most lagy infotainment system in any car I have ever owned. It seems solid and does not crash, but many functions take up to 3 seconds to activate.

Back to platform, and what platform means... The Ultium Platform is the battery modules (pack sizes vary by vehicle), inverter/motors, and electronic architecture. Just look at a Lyriq and Hummer cutaway side by side and try to find me a single chassis or suspension component shared? Heck GM internally calls them 2 different structural platforms, Lyriq is "BEV3", Hummer is "BT1", BrightDrop EV600 is "BV1".

On your battery primary structure comment, if the battery pack can be removed without bracing the structure, that means the battery pack is not primary structure. Hummer, Lyriq, and BrightDrop Van can have the battery pack removed, and roll around without it. The battery pack in Ultium vehicles does provide some additional stiffening to the chassis, but the chassis is structurally sound even with the pack out.
 

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I am just going to tell you almost everything you said is wrong... Andy Oury is a friend of mine, and he designs batteries, cell to pack, has nothing to do with chassis, structure, suspension, or anything else. The Hummer (and all BT1 trucks) does not share a single part of it's chassis or suspension (structure) with Lyriq, Blazer EV, Equinox.
Well if true then you can talk to him about his presentation at the Charged EV conference where he discusses this point, and you seem to be getting excited and argumentative about nothing. The point from the talk was how the battery is part of the structure, I said nothing about suspension, and clearly outside of the battery the cars structure is different.

As for the motors, you are wrong again, Bar wire motors are easier to manufacture, but wire wound motors are better in almost every other way, which is why most manufactures wire wind their motors incl Tesla and Lucid. Also Audi, Porsche,
They do it because it’s cheaper to set up a mfg line, we’ve been winding motors that way since the 1800’s. For power delivery with large currents, and packing density, bar wires are better. This should be intuitively obvious based on Ohms law, and the fact that squares (or hexes) pack better than circles. This is all from a GM published technical paper from the ‘90’s so don’t argue with me about it.

Otherwise looks like you’re spoiling for a fight which isn’t worth the time, cheers
 

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Well if true then you can talk to him about his presentation at the Charged EV conference where he discusses this point, and you seem to be getting excited and argumentative about nothing. The point from the talk was how the battery is part of the structure, I said nothing about suspension, and clearly outside of the battery the cars structure is different.



They do it because it’s cheaper to set up a mfg line, we’ve been winding motors that way since the 1800’s. For power delivery with large currents, and packing density, bar wires are better. This should be intuitively obvious based on Ohms law, and the fact that squares (or hexes) pack better than circles. This is all from a GM published technical paper from the ‘90’s so don’t argue with me about it.

Otherwise looks like you’re spoiling for a fight which isn’t worth the time, cheers
Wrong again... You most likely misunderstood Andy's presentation. Going back to your earlier comment that a structural battery pack in innovative, not really, the Bolt pack also stiffens the structure of the car, as does nearly every BEV made.

On motors you are totally wrong. OEM's use Bar Wires because they are more simple to manufacture, it's as simple as that, there is no efficiency or power benefit, and as a matter of fact wire wound motors are more efficient because of the way current travels (on the outside of the wires) and so if you have more fine wires you build a more efficient field. I call this lazy engineering to use bar wound motors. ON GM's EV's they are just installing larger batteries to get range, the powertrains and vehicles are not overly efficient. Larger batteries make the whole package even less efficient, due to weight. Sandy Munro once said the same thing you did on motor winding on one of his videos, and about 100 motor engineers from all over the world including one of the top motor designers from Tesla corrected him. Sandy got educated.

Tesla doesn't use bar wire because they would sacrifice efficiency and power to do so. I don't care about GM's paper from the 90's, let's just look at the EV fleet and see whose motors are the most efficient and how they are made, as mass produced motors go, Tesla motors are the most efficient, and most powerful per weight. (Lucid's are better but not mass produced) Tesla Plaid drive unit is more powerful than GM's in the Hummer, revs higher, is lighter, and more efficient. Tesla Plaid motor uses amazingly strong magnets, and very tight clearances to reach its power and efficiency.

I don't see GM's Ultium platform as having any real benefits over competitors with more advanced technology. GM screwed themselves with the large cell format of the Ultium battery which to get to 800V which is required to be competitive going forward, you have to put 200 cells in series, and a pack like that in the Equinox EV only has 200 or so cells, so basically you have no parallel cells, meaning if a single cells fails the pack is bricked.

Then we get to pouch cells, I have mixed feelings, pouch cells stack tight helping with energy density, however if a single cell in a pouch battery runs away there is no way to stop it from propagating to other cells in the module, as they heat it all expands and soon you have a serious problem. Tesla latest design of battery packs in 3-Y have far more safety factor against thermal runaway on a pack level, if a single cell goes bad and heats it has a fuse that burns off, disconnecting that cell from the current collector (most times once disconnected the cell will calm down) but if the cell continues to heat the cell is thermally insulated from the other cells with heat conductive foam, which distributes that heat evenly to the other cells around hopefully dissipating the heat from the bad cell without superheating the neighboring cells. Talking to battery manufacturing experts they tell me pouch cells are difficult to manufacture perfect, and often have hot spots where the layers are folded on top of each other, and it's obvious if there is any manufacturing defect here, you have Bolt, Kona, total recall. I believe cylindrical cells might be a better solution all around, hence so many OEM's going that way, even VW and BMW recently announced a transition to cylindrical cells from the pouch cell s they use now.
 
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