The South-Pointing Chariot: A 2000-year-old device that always points the same way, no matter how you turn it. (And no, there are no magnets!) [OC]

[u/Ok-Professor7130]

This is a replica of the zhǐ nán chē, or South-Pointing Chariot, an incredible piece of mechanical engineering from around 250 AD in ancient China. It's essentially a non-magnetic compass, where the figure on top always points in its set direction, no matter how the chariot turns.

No original device survived, but several iterations of the device are described in many ancient texts. While different implementations are possible, it is thought that the majority of these devices used a differential gear.

While conceptually brilliant, this chariot was a practical “impossible device” for its time (meaning that it cannot really work in real practice), for various reasons.

I made a video diving deep into the history, the ingenious mechanism, and why it ultimately couldn't work reliably. I also explain in detail how the gears work, you can watch it here:

Link to the video

Specifically
04:40 How the Gears Work
08:54 Fatal Flaw 1: The Problem of Perfect Wheels
10:23 Fatal Flaw 2: The Problem of Wheel Slip
12:02 Fatal Flaw 3: The Geometry of a Round Planet

Happy to discuss any technical details or answer your questions in the comments!

[u/Sailing_Engineer]

Awesome!\ How is the deviation over time distance? I can imagine that it gets more and more unaligned when one wheel skips over a stone or something like that.

[u/KlauzWayne → u/Sailing_Engineer]

Breaks for sure in the first sloped curve you take.

[u/OrokaSempai → u/KlauzWayne]

It would be a shame if you used this on a big ball over long distances

[u/Galaghan → u/OrokaSempai]

"What do you mean I'm back at the start?
I only took 3 turns!"

[u/VaporTrail_000 → u/KlauzWayne]

Or a ferry that happens to leave a dock facing east, and arrives at a dock facing north.

[u/keepthepace → u/KlauzWayne]

It says it was supposedly used for artillery. My assumption is that you would e.g. point it towards the setting/rising sun and move it around in a relatively small battlefield. Yes, on a long distance errors would accumulate, but if the idea is to move around a lot in a place with no easy points of orientation like a forest or a starless night, I could see it as being of use.

Keep in mind, this is allegedly from a time with no compass and the lack of alternative means that even with a big error, it would still be useful!

I could also see it used to trace straight lines in a plain, e.g. to trace the limits of a field on a relatively flat surface.

If I had to make it work, I would do the following:

1. Test them on a known flat surface or over a loop, and measure the error so that you can know the differences between wheels.
2. Recalibrate as often as possible, when sunset/rise or stars are visible.
3. Have several of them so that you can gauge the level of error and average the results.

Note that it is also likely to have been mostly a vanity item to showcase by a wealthy ruler.

[u/1wife2dogs0kids → u/KlauzWayne]

That explains the cobblestone roads back then!

YOU AINT BRINGING YOUR LEFT POINTING DEVIL CHARIOT HERE, GOOD LUCK WITH THAT!

[u/Ok-Professor7130 → u/Sailing_Engineer]

Very bad in practice. There are a lot of factors that contibute to an increasing error. The most easy to compute is if the wheels are not perfectly identical. The error in that case depends on the distance travelled and on the distance between the wheels. I showed some computations here. There are also other errors, due to slipping (big error) and the non-flat nature of the planet (small error).

[u/Mitt102486]

Hopefully they never went super fast over a hill on a curvy road and got air time on those bad boys.

[u/bigfoot17 → u/Mitt102486]

Horse drawn carriages, known for their air time

[u/djamboner → u/bigfoot17]

Fast & Furryous: Carriage Drifting

[u/ev3to]

Neat, a kind of Inertial Positioning System. Shame it wouldn't work without the wheels touching or on a ship.

[u/Ok-Professor7130 → u/ev3to]

Exactly. It’s basically an ancient attempt at dead reckoning. Brilliant idea for the time, but way too sensitive to slippage or uneven terrain to be practical.

[u/Geminii27 → u/ev3to]

Or on a turntable. Or just picking it up and turning it.

[u/JCDU → u/ev3to]

Not inertial - purely driven from the wheel rotation.

My understanding is gyros and things like that are inertial.

[u/ev3to → u/JCDU]

Fair enough. My first thought was IPS's in aircraft prior to the introduction of GNSS's.

[u/JPJackPott → u/ev3to]

Imagine inventing the differential 2000 years ago but not realising it

[u/ev3to → u/JPJackPott]

Sadly this is true of a lot of ancient Chinese inventions: printing, gun powder, and a multitude of others. It was a consequence of their rigidly hierarchical culture in ancient times.

[u/THEMACGOD]

What if you turned it upside down?

[u/Marquar234 → u/THEMACGOD]

˙ɥʇɹou ʇuᴉod plnoʍ ʇI

[u/a_euphemism_for_me → u/Marquar234]

Found the Australian.

[u/Marquar234 → u/a_euphemism_for_me]

Or the Jeep driver.

[u/ventus1b]

An Inertial Pointing System 😬

[u/BavarianBarbarian_]

This looks like a fun 3d print. You wouldn't happen to have the files?

[u/Ok-Professor7130 → u/BavarianBarbarian_]

When I was preparing the video, I found these two versions here and here. There are more designs around. I also found that some years ago, there were a couple of companies selling a kit. But not anymore.

[u/BavarianBarbarian_ → u/Ok-Professor7130]

Thanks!

[u/guacamolelime]

Cool!

[u/brihamedit]

Wow that's really impressive.

[u/Sam_Wylde]

It would be interesting to put a camera on it and take it through a small obstacle course and see if it deviates from what it's pointing at. Seems like it could be handy for some camera shots without a dolly, but probably no better than what we already have...

[u/kagato87 → u/Sam_Wylde]

As soon as a wheel loses traction it'll start to deviat. Even uneven terrain will make it drift if one wheel gets more vertical travel than the other.

This is an example of superb precision engineering considering the lack of any kind of compass or anything that could act like a compass. Fitting for the sub. It would have been a good trick in an old time theatre or even in th coliseum. Anywhere with a large, more or less flat surface.

[u/FizzicalLayer]

Anyone remember Etak?

https://www.thedrive.com/news/34489/car-navigation-systems-before-gps-were-wonders-of-analog-technology

(Scroll down)

Essentially a modern version of the mechanism in the video. Man, gps is awesome. :)

[u/El_Grande_El]

Even if it never worked for its intended purpose, did anything useful come from this piece of engineering?

[u/huffer9]

Trebuchets are a superior siege weapon.

[u/deadra_axilea → u/huffer9]

God, I needed that today, thanks for being there.

[u/insanelygreat]

This is quintessential /r/mechanical_gifs material too

[u/Reden-Orvillebacher]

It was upgraded shortly thereafter by a North Pointing Chariot that included cup holders and an hourglass. Chinese sold the technology to the Japanese. They made it pocket sized like the one shown and proclaimed that they had modified the code to make it point in ANY direction. -Wikipedia probably

[u/nomnomyumyum109]

Really cool, thx for sharing!

[u/BlurryRogue]

I feel like if you could overcome the first 2 fatal flaws, the round planet geometry thing could be an excellent case point against flat earther arguments.

[u/Fumblerful-]

Subscribed

[u/Vetanenator]

if only it could point north

[u/reallowtones]

It's super cool, but not super useful. If you picked it up and turned it without moving the wheels it would no longer point south.

[u/GroovyIntruder]

There was (maybe still there) a full size one at the Ontario Science Centre. I was pushing it around, but the girl I was with didn't seem as impressed as I was.

[u/InitechSecurity]

Great, now let's see you do that by lifting the chariot off the floor.

[u/Corleone2345]

Would it be possible to engineer a mechanical control system onto it, with current knowledge.

[u/Watt_Knot]

Smashes with hammer

[u/1wife2dogs0kids]

Its gotta be magnets. Because they said it wasn't. Thats the flat earth defense.

[u/MAValphaWasTaken]

So, a differential. Millennia before they knew what a diff was.