Is The Tesla Cybertruck Radical Enough? A Look At The Body Design Choices
Published on November 29th, 2019 |
by Paul Fosse
November 29th, 2019 by Paul Fosse
When the CyberTruck came out on the stage, I was as shocked as everyone with the look of the vehicle. I was impressed by the specs and intrigued by Tesla’s ability to innovate in a direction nobody expected, structural efficiency of the body and frame. I was one of the few people, however, who expected a more radical surprise, like an autonomous interior without a steering wheel or cold air jets to fly over rough terrain. We may still get those in the future, but today I’m going to dive into the choices the Tesla team made and even ask if they considered something more out of the box than they released.
Using A Body On Frame vs. Stressed Skin vs. Monocoque Structure
At about 0:45 to 1:00 in the video, Elon says that the Cybertruck has the same dimensions and same weight as the competition (Ford F-150). Listen to Elon at 1:30 above and you can hear him talk about studying the history of aircraft design. He implied that today’s pickups are the equivalent of a World War I biplane that nobody has thought of redesigning.
I’m a software engineer, not an aerospace engineer, but from my reading of the links above, it appears that early planes used frames and wires to stress the frames for strength and fabric for aerodynamics. Then they moved to single-wing designs with lots of internal triangles for strength, but they also started to use the skin (now called a “stressed skin”) of the fuselage and the wing to carry some of the load. Modern commercial plans like the Boeing Dreamliner use a lot of expensive carbon fiber to move a great deal of the strength of the airframe to the skin of the plane. This answer also helped me to understand this.
This design idea — using the skin of a plane or truck to do two things at once, both support the load of the vehicle and direct the air over the vehicle — saves a lot of weight overall.
Sites like Motor1 claim that the Cybertruck is 1,000 to 1,500 pounds heavier than the F-150, which contradicts Elon’s statement during the unveiling. That doesn’t mean it is right or wrong. I’ve seen no written backup to Elon’s statement, so he could have misspoken. On the other hand, the whole point of the radical design is to save weight, so why would it be way heavier with this radical design? They could just slap a battery pack into a Ford F-150 and it probably would have been 1,500 pounds more. I’ve emailed Tesla’s press team to try to get clarification on this point.
Exoskeletons & Why Ants Are So Strong
Next, I thought of all those times I’ve heard that ants can lift many times their weight. I thought, “they have exoskeletons, maybe that is the secret to the Cybertruck’s amazing specs.” Well, that was a dead end. According to the articles I read here and here, the reason ants are so strong and the reason the exoskeletons work for them is they are small. The problem come when you try to scale an exoskeleton from an ant size to a human or a truck size. Every time you double the height and width of the animal, they become 4 times as strong (they go up with the square of the height), but the weight becomes 8 times as heavy (because the weight contains the mass of the height, width, and depth, it goes up with the cube of the height). This is why only small animals have exoskeletons.
So, I guess this shows exoskeletons work for a given size, but as size increases, the advantages become disadvantages.
Now, the difference biological creatures have is they can have thin skin that is way lighter than sheet metal on a truck. So, the idea is, if you have all this heavy sheet metal, you might as well design it so that you get some structural support for all the weight and not just carry it around like a “sack of potatoes,” as Elon put it.
What about the Steel? Did They Consider Super Wood?
I thought I would look into the specs of the steel used by the Cybertruck. At the unveiling, Elon called it 30× cold rolled steel, but in this Motor Trend article (the writers got exclusive access to the design team, so I hope they got things right), they state that Tesla is using the same stainless steel as SpaceX is using for its coming Starship spacecraft. Frankly, that worries me a little, since this steel was picked for its excellent strength at high temperatures — for the spacecraft during reentry. Wouldn’t we want another variation for the low temperatures on Earth? The article states that this is 301 stainless steel, which I found here is used commonly on aircraft structural parts, trailer bodies, roof drainage, door frames, auto body trim, wheel covers, utensils, tableware, and conveyor parts. I also found a manufacturer spec sheet here. It includes more specs I don’t necessarily fully understand.
In researching the strength of the steel the team is so familiar with because of their relationship with SpaceX, did they forget to investigate the advances happening to make engineered wood strong and lightweight? I originally thought of the strong and light model airplanes built from Balsa wood, which has a high strength-to-weight ratio but has high volume that would be a problem. It also isn’t what I would call tough. I know Tesla is using a dense paper product for the dash (that looks like marble). Maybe the designers and developers should consider super wood for the body or even the windows, since it can even be made transparent. That article claims it is stronger than steel and even some titanium alloys. It might not meet the high temperature requirements of SpaceX, but it could help Tesla save some more valuable weight with the Cybertruck.
The super hardened steel body has a number of advantages over the painted body-on-frame trucks that automakers have been building for years.
- If properly designed, it can save weight (that’s why everything looks like a triangle).
- You get the added benefit of a bulletproof exterior.
- You avoid the capital and operational expense of a paint shop.
- You avoid the capital and operational expense of a stamping press.
- You make an exterior that is almost impossible to scratch, lowering warranty claims for bad paint and lowering insurance costs for your customers.
- It is much cheaper than carbon fiber or titanium and somewhat cheaper than aluminum.
But there are a few disadvantages, too:
- It is hard to bend, so they need to develop some new manufacturing methods.
- As Jennifer pointed out in this article, the Cybertruck gives up the modularity of the truck market that let you put a camper, a bucket, or other custom use on the back of the truck. This limits your sales somewhat.
Overall, I applaud Tesla’s courage in being willing to take a bold step forward in truck design and hope the company is able to continue to make every vehicle the best it can be, even it it means they have to learn to work with different materials and use different techniques.
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