🏹 Counterweight Trebuchet: step-by-step build

The heavy battery box drops, the long arm whips up — and your bush flies across the room!

Long ago, castle attackers built giant throwing machines called trebuchets. There was no motor and no spring: just a huge heavy box of stones on the short end of a long arm. Lift the heavy end up and you store energy. Pull the trigger, the weight falls, and the long end swings up super fast and flings the ammo. Our counterweight is the AA battery box — with the batteries in it, it is the heaviest thing in the kit! Please only fire bushes and never at people or pets.

Level 3 of 5 (medium)
About 40 minutes
17 steps, 52 parts
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Series: Mechanical Wonders

What you will learn

Counterweight: A weight on the other side of a pivot that balances a load.
Levers: A bar on a pivot multiplies your push.
Stored Energy: Energy can be saved up in a spring and let out later.
Triangles Are Strong: A triangle cannot be squished — a square can.

Build these first

🏗️ Worm-Gear Crane: Crank the worm gear to lift the boom — then let go. It stays up by itself! (level 3 of 5, about 30 minutes, 15 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 × H frame 7×11
8 × Blue long pin
6 × Split pin block
8 × Red axle-pin
2 × Beam 13
1 × Beam 11
1 × Beam 15
3 × Black pin (grippy)
1 × L-beam 2×4
1 × Thin beam 3
1 × Axle 10
12 × Bush
2 × Half bush
1 × Axle 8
2 × Beam 5
1 × AA battery box
1 × Axle 3

Steps

Lay the H frame flat, pointing sideways. Its two long rails are the base of the trebuchet.
Right rail: push blue long pins up into holes 8 and 10 and slide two split pin blocks down onto them, one on top of the other.
Push a red axle pin into each block, pointing out, and press a 13-hole tower onto both. Two pins, so the tower can't tip.
Trigger post: push blue long pins up into holes 4 and 6 of the same rail and slide two more split pin blocks onto them.
Push a red axle pin into each of these blocks, pointing in, and press the black 11-hole trigger post onto both.
Left rail: push blue long pins up into holes 8 and 10 and slide two split pin blocks onto them.
Throwing arm: take a 15-hole beam. It swings, so it is a moving part.
Push two black pins into holes 12 and 13 of the long end and press the L-beam "spoon" onto them. It holds the ammo.
Catch plate: push a red axle-pin (pin end first) into hole 8 and a black pin into hole 9 of the arm, from the spoon side. Press a black 3-hole thin beam onto them so its free cross hole sits over hole 10.
Push a 10-long axle through hole 5 of the arm (the pivot) and slide a bush on each side.
Slide two more bushes onto each side of the pivot axle.
Bring the arm in from the left side and push the front end of the pivot axle through the top hole of the right tower. Lock it with a half bush.
Push a red axle pin into each left block, pointing out. Slide the second 13-hole tower onto the other end of the pivot axle and the two red pins. Lock the axle with a half bush.
Push an 8-long axle through the short end of the arm (hole 1) and slide a bush on each side, against the arm.
Hang a 5-hole link on each side of the axle and lock each one with a bush on the outside, right at the axle's end. The links still swing freely.
Stand the AA battery box (with batteries!) on its end between the links. Push a blue long pin through the bottom hole of each link into the middle hole of the box's top ear.
Pull the long end of the arm down until the catch plate's cross hole sits behind hole 10 of the trigger post. Push a grey 3-long axle in from the back through the post into the cross hole and put a bush on its back end as a handle. Push a red axle-pin into the tip of the spoon and put a bush on it as ammo.

Play time

Pull out the trigger axle and see how far the bush flies. Measure it with your feet! Take the batteries out of the box — does it still throw as far? Try firing a pin or a half bush.

Real-world inspiration

JK Brickworks Trebuchet — https://jkbrickworks.com/trebuchet/ ; Rebrickable "The Trebuchet" MOC-52242 — https://rebrickable.com/mocs/MOC-52242/EDGE%20OF%20BRICKS/the-trebuchet/