About that Cybertruck Glass…

It’s been over a week now since Tesla unveiled their newest creation, the Cybertruck, and what we can now refer to as windowgate. If you’ve managed to escape the news cycle last week, here’s what happened:

• Elon unveils new truck
• Truck is very un-truck like. Cue tension as people try to figure out how they feel about it.
• Elon talks about its steel frame and bullet proof “armor glass”. A ball drop demo is used to show the difference between a normal piece of glass that shatters and a sample of the cybertruck glass that the ball just bounces off.
• Hammer time: Design lead Franz Von Holzhausen hits the front door with a hammer. Nothing happens!
• Time to ball: But then, Franz tries to show off the unbreakable glass by throwing a steel ball at it and the unthinkable happens — the glass cracks.
• And then to add salt to the wound, when Franz tries again on the back door window, it also cracks.

And now, we have Elon, standing in front of his newest creation, but with two extremely large cracks in the window for the rest of the show.

What happened?

A day later, Elon shared the following behind the scenes video showing Franz doing what looks to be the same demo backstage and the glass notable not breaking. He also claimed that hitting the car door with the hammer first weakened the glass. That’s valid, but what about the back window cracking? The back door wasn’t hit.

So I thought I would jump on the opportunity to dig into this because it’s been on my mind ever since it happened, and I love any chance I get to share how materials science really relates to you, the everyday person. The thoughts that follow are based solely on the third party information I could gather.

The behind-the-scenes demo that worked

Let’s start first with the fact that there’s a demo video showing the glass clearly not breaking. What I found really interesting about the setup shown in this video, though, is that if you look closely enough, it’s not the same demonstration as in the launch event. First, there’s the added tape and blue tarp that I assume were added just in case the glass really were to shatter, they could protect the door from the shards or a missed throw.

But then, there’s the fact that in the video, the door seems to open right after the ball hits it. You can even see the window actually retract a bit. This is a typical feature of frameless car doors — a window on a frameless car door will retract when opened so that when the door is closed again, the window pushes back up to create a tight seal with the weatherstripping. With this feature, it’s better to have the over structure of the vehicle to be stiffer to support the open glass window and ensure the window seals tightly and correctly. (Perhaps another good reason the body is steel.)

So herein lies a problem. If the door in this successful demo was allowed to move, then the glass had some space to flex and dissipate energy from the steel ball.

The Ball Drop demo that worked

And to go back to the ball drop demo (where, props to Elon for making mechanical testing look super bad-ass by having this Jason Mamoa look-alike with tattoos do it. I don’t look that cool when I do that.) and rewatch it, there’s again a couple of details to note. In the first drop, it’s not a very high drop so the impact force was most likely much lower than the final throw was, so for a bulletproof glass, I expect it to survive something like that. (Watch the ball drop test starting at about the 6 minute mark.)

But in the higher drop, watch closely what happens as the ball hits the glass. Not only are there only a few contact points on the glass, holding it down instead of a still continuous clamp, these point contacts immediately release once the ball hits the glass, again giving the glass space to flex and dissipate energy from the ball.

And finally, that one demo that didn’t work

Now this brings us back to the final demo — the final ball throw that resulted in the cracked windows seen around the world. Most things were the same: the steel ball, the glass material…the air. But what changed is the setup. Unlike the previous demos, the glass installed on the Cybertruck looks clamped down. There was no freedom of movement. Since it could no longer flex, all the energy from the throw went straight to the glass.

What does this mean?

All the prior demos were valid and true, but my personal opinion on what did it for the windows on the Cybertruck? The glass did what glass does: absorb all the energy. At the end of the day, bulletproof glass is still glass, and glass is brittle. In this case, the installed glass had no space to flex or bend, so it took all the energy from the throw, which ended up being more than the strength of the glass.

Looking at the crack pattern left by the steel ball on both sides reiterates this. The pattern is radial from the initiation site (where the ball contacted the glass) and, as many people have noted, bulletproof glass is actually designed to break like glass does, just in a smarter way. If the glass breaks, it means the energy from the projectile is absorbed and reduced, meaning the bullet/steel ball/hammer can’t penetrate as far.

So what does this mean for the Cybertruck? Well I don’t think it personally fits my style, but I think this demo only went awry thanks to how it was executed, but I don’t think this happened just because Franz hit the door with the hammer first though.

Wrapping it all up

And that’s where the catch really is with all material testing. If you try hard enough, you can break anything, and if you try smart enough, anything can look invincible. It’s all about making sure you’re actually testing under the conditions that are most relevant to your use case and being thoughtful about the outcome. I mean, if I were in the Cybertruck and someone were throwing steel balls at me, I think the glass did what I wanted it to do: deflect the steel ball away. I’m just left with some pricey repairs, but hey, I’m not dead.