When the BMW i3 was first presented, it had a Life Module and a Drive Module. The former was the cabin and the latter was the rolling chassis of the electric car. Sandy Munro recently discussed Tesla Cybertruck’s mega casting with InsideEVs to clarify how it may match an exoskeleton and told us the i3 and the electric pickup truck could have a lot in common when it comes to their structure. In that respect, the Cybertruck will be unique, as Munro stressed.

Elon Musk said at Tesla’s Q4 2020 Earnings Call that the electric pickup truck would require a high-pressure die casting machine with a clamping force of 8,000 tons to create the rear body of the pickup truck because “you’ve got a long truck bed that’s going to support a lot of load.”

Check Out How The Single-Piece Rear Casting Joins The Tesla Model Y

That would fit perfectly in a unibody vehicle such as the Model Y, . However, a stressed-skin structure vehicle was not supposed to need that due to how much stronger it is compared to a unibody vehicle. Theoretically, its own structures could handle everything and a rear casting would not be necessary.

Tesla was supposed use an exoskeleton to make the Cybertruck as tough as a body-on-frame vehicle without the weight penalty. Musk said this at the electric pickup truck's presentation:

“We moved the mass to the outside. We’ve created an exoskeleton. The way that trucks are normally designed, you have a body-on-frame, a bed-on-frame. And the body and the bed don’t do anything useful. They are carried like cargo, like a sack of potatoes. It was the way that aircraft used to be designed when we had biplanes, basically. The key to creating an effective monoplane was a stressed-skin design. You move the stress to the outside skin. Allows you to do things you can’t do with body-on-frame.”

This Engineering Analysis Explains The Tesla Cybertruck Exoskeleton’s Edges

He also told Munro personally that electric vehicles currently carry their battery pack as a sack of potatoes. The exoskeleton design would allow Tesla to put all of the Cybertruck's body to work structurally, avoiding carrying the body, the bed, and the battery pack as useless cargo. The rear mega casting, however, indicates that design could not deliver on all of Tesla's goals.

Munro told us more about the benefits the stressed-skin design could bring to the Cybertruck.

“What the exoskeleton does is to get rid of the requirements for internal longitudinals, stiffening ribs, and things like that because the structural skin would be doing all the work. What you are really doing is getting rid of roof bows, door surrounds, and things like that. They may still need an extra structure for where the hinges are. At the end of the day, you’re looking at a product that will still be self-supporting.”

Yet, it will have a massive single-piece rear casting, which does not fit the pure stressed-skin design definition. We asked Munro what role the mega aluminum rear part would play in the Cybertruck.

“The general premise would be to have the body just drop over the top of a rolling chassis. If I was doing it, I would drop a fully skinned body over the top of a rolling chassis like in the olden days. We used to do that all the time.”

He even had a suggestion about how Tesla could do that. It is here that the Cybertruck could share a lesson with the BMW i3.

“If I wanted to be really clever, I’d use the BMW way because they had the same sort of a deal. They put a body basically on top of a rolling chassis. They had two locating pins and a very, very strong urethane glue that we normally use for windshields and things like that.”

Summing up, what Munro believes will happen is this:

“It’s going to be slightly lighter than if I would have done a body-on-frame and slightly heavier than a monocoque design. The structural element of the outside skin is going to save quite a bit of weight. Having the frame made out of aluminum – instead of the steel it is normally made of – that’s also going to make it a little bit lighter and a little more nimble. It’s a little bit better than what you’d find with a body-on-frame and maybe a smidge heavier but a lot more rigid than if you have a monocoque design.”

Tesla Cybertruck Tooling Costs Will Be Just a Fraction Of Those Of A Regular Truck

In other words, the rear casting will be part of a frame for the exoskeleton body on the Cybertruck. If Tesla put a monocoque over a frame, that would be redundant in terms of strength and heavier than a regular body-on-frame pickup truck.

Considering the battery pack weight will already be a disadvantage for any electric pickup truck, Munro believes Tesla has solved for this with the exoskeleton for the body and a lighter frame made with mega castings.

It is not as revolutionary as a vehicle made solely with a stressed-skin design, as Musk's remarks made us believe. However, Munro thinks it will still be good enough for customers.

“With the frame the way it is, made out of cast aluminum, that will be a very, very strong product and that’s what you want when you’re going off-road. What you find is that when you have something less than that structure that you need for the carriage, for supporting the vehicle, it doesn’t work the way you want if you’re an off-roader. I believe that they’re probably going to have a cast structure underneath it and will drop the stainless steel body on top. I’ll be a happy camper when I buy one of those.”

Tesla Battery Day 2020 - Tesla Cybertruck

As you can see, being redundant in terms of strength would be more than welcome if it does not increase weight. The Cybertruck's new structure may have achieved just that. Munro just warned that aluminum and stainless steel are not the best friends, something that Tesla should really take into consideration.

“You’d have to isolate the aluminum frame from the stainless steel body because aluminum and stainless steel do not get along with each other. They cause galvanic action and they do it rather quickly. However, the actual marriage should not be that big a deal.”

Instead of the classic definition for car body designs, Tesla’s solution would be something we have never seen. Jokingly, we asked Munro if we could define that as a hybrid of an exoskeleton with a body-on-frame – it would be Tesla’s first official hybrid. The Obrist Mark II was an independent creation.

“You can say that it has an exoskeleton and that it is unique. Whenever I hear hybrid I think of an engine, a motor, and a battery. I’d skip that and just go say it is unique. Nobody’s got anything quite like it. It is a perfect combination of what someone who wants to drive off-road would want.”

Interestingly, Munro does not classify the Cybertruck as a pickup truck.

“I look at it as something that could fulfill some pickup truck needs, more like a utility truck, similar to my existing Jeep (Wrangler Rubicon). The difference is that I can get my quads and my bikes in it, but I don’t think I would be hauling two-by-fours or material for building a house. If I wanted something like that, then I’d move toward the Rivian.”


The engineer is also enthusiastic about the stainless steel body, but we'll tell you more about that in a future article from our 50-minute conversation with Sandy Munro. Don't miss it!

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