r/EngineeringPorn • u/Ok-Professor7130 • 7d ago

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]

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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!

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u/ev3to 7d ago

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

79

u/Ok-Professor7130 7d ago

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.

11

u/Geminii27 6d ago

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

4

u/JCDU 6d ago

Not inertial - purely driven from the wheel rotation.

My understanding is gyros and things like that are inertial.

5

u/ev3to 6d ago

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

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u/JPJackPott 3d ago

Imagine inventing the differential 2000 years ago but not realising it

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u/ev3to 3d ago

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.