This highlights how impractical Star Wars: The Force Awakens‘s soccer ball droid BB-8 is.

I’ll be honest – I initially wanted this article to be about why BB-8, the soccer ball droid revealed in the new Star Wars: The Force Awakens teaser, is a technically superior design to R2-D2. I figured that those of us who fear BB-8 may become the Jar Jar Binks of Episode 7 needed some consolation that there is more to this ridiculous ball of cute than a blatant attempt at making audiences go “awww.”

But the more I tried to prove that BB-8 is superior to R2-D2, the more I realized that… it isn’t. At all. The general argument is that R2-D2 has difficulty getting around rough environments like Tatooine on his stumpy little legs, and BB-8 can just roll around with much greater maneuverability and speed. Superficially, this makes sense, but we’re forgetting one key factor:

Traction.

Here are three reasons why the science of locomotion tells us that BB-8 is inferior to R2-D2.

1. Material Composition

Do you want your droid to be able to accelerate quickly? It better have a lot of traction.

Do you want it to be able to brake suddenly? Better have traction.

Do you want it to make tight corners without slip-sliding everywhere? Traction.

In the vast majority of cases in wheel design, engineers seek high traction between the wheel and ground. One factor that influences traction is the material composition of the wheel – or, in BB-8’s case, its entire body, which effectively functions as a wheel. When different materials rub against each other, they exhibit different amounts of friction – and more friction means more traction. The combination of rubber contacting other materials tends to result in greater amounts of friction than most other combinations, which is one reason rubber is such a great tire material.

We don’t get a good look at R2-D2’s wheels, but it’s safe to assume they are made of some form of rubber, as would be expected. But BB-8’s body is definitely not made of rubber – it’s metal. Or potentially some kind of plastic, which would be even worse than metal. I don’t care how much faster BB-8 can move – at least R2-D2 could stop himself from crashing into things.

Point: R2-D2, for having better surface friction.

2. Points of Contact

An oft-touted trick among off-roaders is to deflate your tires slightly when driving on particularly loose and soft surfaces like sand, gravel, or snow. Why does this help? Because by letting out some air, a tire becomes flatter, increasing its surface area.

It may seem intuitive to think that a greater surface area will generate more friction, but that’s actually not the case. The force of friction remains the same, because as surface area increases, the amount of weight at any given point decreases to a corresponding degree. What does happen is the snowshoe effect: by spreading out the weight over a greater area, the wheel won’t sink as deep into the snow. The deeper it sinks, the more resistance a wheel is met with, and the more likely it is to get stuck.

A sphere is shaped perfectly to get itself stuck in sand. As a geometric figure, a wheel can be approximated to be a cylinder, and a cylinder is tangent with (just touching) a plane (the ground) along a straight line. A sphere, however, is tangent with a plane at only a single point – its the least amount of surface area that can possibly be touching the ground before it begins to sink.

R2-D2

But wait – it gets worse for BB-8. If an off-road vehicle does get one of its wheels stuck in sand, it has three other wheels (assuming four-wheel drive) to haul the load and get the vehicle moving. It can afford to lose traction on one or more wheels and still keep going. But BB-8 consists of only a single wheel – if he gets stuck, he’s screwed. R2-D2, on the other hand, has three legs, and each leg seems to have at least two wheels on it. One wheel spins out or gets stuck? Many more to compensate!

Point: R2-D2, for having better weight distribution and multiple redundancies.

3. Fallback Options

If BB-8 gets stuck in sand, he can keep spinning and spinning in place forever without freeing himself. If R2-D2 gets his wheels stuck in sand, guess what? He can still walk! All he has to is execute his little hobble maneuver, move a few paces forward, then resume rolling. Or, if the surface proves too unfriendly for wheels, he can just keep hobbling.

And I don’t want to hear any of that jet thruster nonsense – you know, those little side rockets that R2-D2 had in the prequel trilogy. First off, if BB-8 has those as well, then that still doesn’t make him superior to R2, just on par with him. And secondly, using those rockets is obviously an incredibly energy inefficient form of locomotion, otherwise why even roll at all? Why not just fly all the time?

Now, I will concede that it is possible that BB-8 has some form of retractable legs to get himself unstuck. The teaser showed us very little. But I’m willing to bet that no such feature will appear in the movie – and that the movie will grossly misrepresent how maneuverable a self-propelled soccer ball can be.

Point: R2-D2, for being able to simply walk into Mordor.


By now, some of you you may be saying, “Oh yeah? Well you can’t be right, because they built a working prop of BB-8 that actually rolls around!”

First of all, they did, and that is awesome. I could have sworn the BB-8 in the trailer was CGI, but I’m all for practical effects that are well-executed. That said, a prop is a far cry from the real thing. A prop only needs to perform under very specific conditions – conditions that are engineered to make the prop look as legitimate as possible. According to Making Star Wars, the prop was “allowed to roll on two tracks that [were] either obscured or digitally removed.” And if you’re rolling a ball on tracks, then you’re engineering conditions that render my three aforementioned arguments moot while also confirming that there was a need to roll the prop on tracks to account for some unmentioned weaknesses. You know, like not actually being practical.

But hey. At least it’s not Jar Jar.

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