Rasomuro.org - Il rumor bianco del Web - 22 marzo 2009

Building a south-pointing chariot

An amazing application of the differential gear

While looking for some information about differential gears for the construction of a truck with the Lego Mindstorms set, I wondered when such simple but extremely ingenious piece of mechanical gear was first devised in the history of man. I didn't expect it to be used anytime sooner than engine-powered vehicles, where it is almost necessary.


I discovered that the Chinese knew it almost 2000 years ago, and it was used in a most peculiar way: to differentiate the movement of two wheels of a chariot so that, with some additional gearing, a pole would always point to the same direction. It was referred to as the "South-pointing chariot".

Not only it is fun to see and to build, it also helps understanding how differential gearings work (which has always looked like pure wizardry to my eyes). While there are many descriptions of how to build it with Lego (or Meccano) parts, none of them would fit to my available set of pieces: a Lego Mindstorms set, plus a Lego Technic 8287 kit (which only provides the differential gear). Moreover, many realizations just aim at a compact and good-looking object, while I preferred a sparse design with all the inner workings clearly visible.

How it looks like

Here are some photos:

Front (or rear?) view Rear (or front?) view Bottom view

How it works

Besides referring to the Wikipedia article, here is a video clip that provides some hints. Anyway, once you have built it, its inner workings become clear!

How to build it

Let's start with the differential.

Then, let's move to the left wheel axle.

Now it's time for the right wheel axle.

An internal gear to transmit the motion to the south-pointing vertical pole.

Structural elements for the frame.

Let's start putting things together!

Once these pieces are put together, adjust the elements until axles don't slip horizontally; however, provide that everything turns freely. If you turn the two wheel axles (the grey protruding ones) in the same direction, the differential box should not turn.

It's time for some structural elements of the vertical pole!

Let's build the indicator pole, now.

Now the vertical element must be joined to the rest.

The back wheel, now.

The anchor for the back wheel.

Finally, let us join the back wheel...

...and the two side wheels.

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