How did trebuchets work?

At the height of its power the trebuchet was an unstoppable force, crushing fortifi cations, buildings and men alike

(Image credit: ChrisO)

The setting: King Richard’s crusades in the latter half of the 12th Century. A counterweight trebuchet’s arm and sling swing up to a vertical position, releasing its housed projectile at a ferocious speed and power. The result? Twelve men crushed by a single, giant stone.

Built originally in the middle ages to bombard and besiege enemy fortifications and troops, the counterweight trebuchet was used as recently as 1600, the equivalent of today’s long-range artillery cannons. Capable of launching objects of tremendous weight – including stone boulders, burning animals and even diseased human corpses used as an early form of biological warfare – over large distances, the counterweight trebuchet still to this day is a phenomenal feat of engineering.

Differing from the far earlier traction trebuchets, which relied on torsion and manpower, the counterweight trebuchet earned its reputation through the simplicity of its central design concept, relying on gravity alone to propel its ammunition further and more consistently than its predecessors. By increasing projectile propulsion by relying on the mechanical advantage principle of leverage, rather than that of torsion and manpower, for centuries the counterweight trebuchet laid waste to city walls and fortifications worldwide.

A counterweight trebuchet consists of five basic parts; the frame, guide chute, beam, sling and counterweight, which in order to propel an object must all work in harmony. Supported by the frame – which provides the all-important raised position needed for this gravity-reliant system – the counterweight is dropped which in turn rotates the beam and attached sling. The sling, which is guided along the chute until critical acceleration is achieved (therefore keeping the projectile in the sling), then reaches a near vertical degree before releasing its contents. 

While historians still argue to this day about the precise details of early counterweight trebuchets – such as their numbers, design and origin – one thing is definitely clear; it was most certainly a weapon of epic proportions, giving whoever wielded it a considerable advantage in any siege.

The firing mechanism

(Image credit: Onno)

1. Counterweight

Momentum for the trebuchet’s beam and sling comes courtesy of its heavy counterweight

2. Sling

Once the counterweight is dropped the beam accelerates quickly, dragging its sling with it along a guidance chute.

3. Boulder

At maximum extension the sling leaves the ground under the huge generation of upward motion.

4. Release

At a near vertical degree, critical acceleration is achieved and the load is released.


This article was originally published in How It Works issue 5


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