The south-pointing chariot: how an ancient cart pointed south with gears

Imagine a cart with a little figure on top, its arm pointing south. You push the cart round corners, in circles, across a courtyard, and the arm keeps pointing south. There is no magnet anywhere. The cart works it out from its wheels: when it turns, one wheel rolls further than the other, and gears turn the figure back by exactly the amount the cart turned.

That is the south-pointing chariot, one of the cleverest machines of the ancient world, and you can build one from Technic-style parts.

A legend and an engineer

Chinese stories credited the chariot to legendary rulers thousands of years ago, but the best-known builder is the engineer Ma Jun, who is recorded as rebuilding one around 250 AD after other scholars said it was only a myth. Later engineers built more, and some descriptions of how the gears were arranged survive. Nobody knows exactly what was inside the earliest ones. Historians think some may have used a differential, the same gear box that lets a car's wheels turn at different speeds in a corner, and that is how most modern rebuilds work, including ours.

It is worth being clear about what it is not: it isn't a compass. It doesn't sense south. You aim the pointer south at the start, and the gears *keep* it aimed by cancelling every turn the cart makes. That makes it an early example of what engineers now call dead reckoning, working out where you're facing from how you've moved.

How the gears find south

The South-Pointing Chariot build (level 5, about 75 minutes) is the hardest in the set, and its build page lists the RC Racer and the Bevel-Gear Windmill as the builds to do first. Here's what happens inside it:

1. Drive straight. Both 56 mm wheels roll the same distance. The left wheel turns one side of the differential; the right wheel turns the other side *backwards*, through three 12-tooth corner gears. The two cancel, so the differential's body, the carrier, stays still.
2. Turn left. The right wheel rolls further than the left. Now they no longer cancel, and the carrier turns by half the difference.
3. Turn the pointer back. The differential's 28-tooth ring drives a 12-tooth corner gear, then an 8-tooth gear drives a 40-tooth gear that carries the pointer. Altogether the pointer turns 7/15 as much as the carrier, backwards against the cart's turn.

Why 7/15? The build's 56 mm tyres are 7 studs across, and the two wheels are 15 studs apart. The gear ratio has to equal wheel size ÷ track width, and the build file checks exactly that: 7/15 = 7/15. Get it wrong and the pointer over- or under-corrects on every turn.

Try it before you build. Drive a quarter turn, a circle and a figure of eight, then swap in the wrong wheels and see what happens.

The surprising part: errors come and go

Put the wrong wheels on and drive a full circle, and the pointer ends up about 107° off. Now drive a figure of eight with the same wrong wheels, and the pointer comes back to south. The error only builds up with the cart's *total* change of direction, so a loop to the left undoes a loop to the right.

Slipping is different. If you lift the chariot and turn it, or a wheel skids on a smooth floor, the gears never see that part of the turn and the pointer is carried round with the body. Small slips add up, which is probably why long journeys were never the chariot's strength. The build's own play notes say the same: slow, smooth pushes work best.

Test your own chariot

1. Aim the pointer at something fixed, like a window.
2. Push the chariot slowly in a big circle round the room. Does it still point at the window?
3. Try a figure of eight, then a tight turn on the spot.
4. Try a slippery floor and a carpet. Where does it drift most?

If the pointer drifts the same way on every turn, check that the corner gears mesh fully and that the wheels are pushed right onto their axles.

Where to go next

The chariot's brain is a differential. What is a differential? explains how one lets two wheels turn at different speeds, and the bevel gears worksheet explains why the right wheel's three corner gears reverse its turn. The Differentials page has the short version.

Check yourself