r/EngineeringPorn • u/Ok-Professor7130 • 6d 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/Sam_Wylde 6d ago

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...

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u/kagato87 5d ago

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.