🧭 South-Pointing Chariot: step-by-step build

Push it anywhere — the pointer keeps pointing the same way. No magnets, just gears!

Almost 2000 years ago, Chinese inventors built a chariot with a little figure that always pointed south, however the chariot turned. There was no magnet inside — only gears! When a cart turns left, its right wheel rolls further than its left wheel. A differential secretly measures that difference, and the gears turn the pointer back by exactly the same amount the cart turned. For it to work, the gear sizes must match the wheel size and the distance between the wheels perfectly — engineers call that a calculation. This is the hardest build in the book. Take your time, and then take it for a drive round the room!

Level 5 of 5 (a big challenge)
About 75 minutes
34 steps, 101 parts
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Series: Mechanical Wonders

What you will learn

The Differential: Lets the two driven wheels spin at different speeds in a turn.
Bevel Gears: Around the Corner: Slanted gears send spinning power around a corner.
Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.
Triangles Are Strong: A triangle cannot be squished — a square can.

Build these first

🏆 RC Racer: A real remote-control car: motor drive, differential, servo steering and headlights. (level 5 of 5, about 75 minutes, 38 steps)
🌬️ Bevel-Gear Windmill: Turn the handwheel at the bottom — the sails spin at the top! (level 3 of 5, about 25 minutes, 16 steps)

Parts you need

1 × Open frame 5×11
10 × Blue long pin
8 × Split pin block
8 × Red axle-pin
4 × L-beam 2×4
14 × Black pin (grippy)
3 × Beam 7
2 × Beam 5
2 × Beam 11
1 × Axle 6
20 × Half bush
1 × Differential
2 × Axle 3
3 × Axle 5
4 × Axle 4
1 × Open frame 5×7
2 × Beam 9
1 × Axle 9
1 × Axle 7
2 × Corner gear (12 teeth)
2 × Grey corner gear (12 teeth)
1 × L-beam 3×5
1 × Bush
1 × Tiny gear (8 teeth)
1 × Giant gear (40 teeth)
1 × Thin beam 4
2 × Big tyre wheel
1 × Cross block
1 × Small white wheel

Steps

Lay the white 5×11 frame flat, long sides pointing left and right. It will ride high above the wheel axles.
Left hanger: push blue long pins DOWN through the 1st and 3rd holes of the frame's left end and slide two split pin blocks up onto them from below, one under the other.
Right hanger: the same at the right end.
Push a red axle pin into each left block, pointing in, and press the black 2×4 L-beam onto both, its long leg pointing forwards.
Push black pins into the L-beam's 1st and 2nd holes, pointing out, and press the left 7-hole bearing beam onto them, standing on its edge.
Right side: two red axle pins into the right blocks, pointing in, and the second 2×4 L-beam.
Back posts: push blue long pins up into the 2nd and 4th holes of the frame's back side and slide two split pin blocks down onto them, one on top of the other.
The same in the 8th and 10th holes.
Push a red axle pin into each of the four blocks, pointing back.
Press a 5-hole post onto each pair of red pins, holes facing back.
Push a black pin into the top hole of each post, pointing back, and press an 11-hole rail onto them.
Left wheel: push a 6 axle through the left bearing with a half bush on the outside, and a second half bush on its inner end, snug against the beam: it can turn but not slide.
Slide the differential onto the inner end of the axle, big ring towards the left wheel, against the half bush, and push a 3 axle into its other side.
Push a 5-long axle up into the frame's back side, 5th hole, and a 4-long axle into the front side, in the middle. Slide a half bush down each axle onto the frame.
Slide a second half bush onto each axle, near the top, and press a 5×7 frame flat onto their tops: the pointer deck. The half bushes hold the axles tight between the two frames; one more half bush on top of the longer axle holds the deck down.
Push black pins into the two front holes of the left bearing beam, pointing in, and press a 9-hole shaft onto them.
Centre bearing: slide an 11-hole beam's first hole onto the 3 axle, against the differential, long end forwards. Push the yoke, a 9 axle, through the shaft's front hole and the beam's last hole. Add two half bushes on its free end: one against the beam, one 3 holes out.
Push a 5 axle through the shaft's 5th hole, two half bushes and the beam's 7th hole: one half bush just inside the shaft, one against the beam. Now the centre beam cannot swing or slide.
Idler bearing: push blue long pins down into the frame's front side, 7th and 8th holes, and press two black 2×4 L-beams flat onto them, one on the other, long legs pointing forwards.
Right side: take the second 7-hole bearing beam and push black pins into its 1st and 2nd holes, pointing in.
Push black pins into the beam's two front holes, pointing in, and press the second 9-hole shaft onto them.
Push black pins into the shaft's 3rd and 4th holes, pointing in, and press a 7-hole beam onto them, its first hole in line with the bearing beam's middle hole: the inner bearing.
Push a 7 axle through the bearing beam's middle hole, a half bush, the inner bearing and a half bush, with one more half bush outside: two bearings, and it cannot slide.
Slide a corner gear (A) onto the inner end of the axle, against the half bush.
Slide the right side in: the shaft goes over the yoke and the black pins into the right L-beam. Slide a corner gear (C) onto the 3 axle's tip, against the centre beam. Hold the idler corner gear (B) between A and C with a half bush on top, under the L-beams' 4th holes, and push a 4 axle down through both L-beams, the half bush and B.
Lock the idler axle on top of the L-beams with a half bush: now B can neither ride up nor drop out of mesh.
Pointer tower: lay the grey 3×5 L-beam flat and push black pins down through the two holes of its short leg.
Hold a half bush under the 2nd hole of the long leg and push a 4 axle up through the bush and the hole. The bush stops the axle sliding up out of the tower.
Press the pointer tower onto the deck: the two pins go into the deck's front side.
Hold the grey corner gear (pinion) under the deck, on the differential's ring, with a bush on top of it. Push a 5 axle down through the corner of the L-beam and the deck into both, and put the tiny 8-tooth gear on top.
Slide the giant 40-tooth gear down the tower's axle next to the 8, push the thin red pointer on top, lying over the 8, and lock it with a half bush. The pointer now keeps both gears from lifting off.
Push the two big wheels onto the axle ends.
Caster: hold a cross block under the back of the frame, round hole sideways, and push a 4-long axle down through the frame into its cross hole. A half bush on top so the axle can't drop out.
Push a 3-long axle through the block's round hole with a half bush behind it, and add the small wheel: it rolls freely.

Play time

Point the arrow at the window. Now push the chariot slowly in a big circle round the room — does the arrow still point at the window? Try a figure-eight. (Slow, smooth pushes work best: if a wheel slips, the pointer drifts.)

Real-world inspiration

South-pointing chariot (ancient China, rebuilt by Ma Jun c. 250 AD) — https://en.wikipedia.org/wiki/South-pointing_chariot