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One of us is 7, so obviously we made a trebuchet. Actually we have made several in the last year or so – the current version goes sufficiently high and far  to only be useable outside.

Trebuchet, about 69 cm to the axis

We used FischerTechnik, which allowed us to easily experiment with the design ourselves by adjusting each component.

This website was helpful in detailing what to aim for in the design.

Parts of the Trebuchet

The design features we found useful were:

• The main frame needs to be quite sturdy – we used quite a bit of bracing
• The optimum ratio for the length of the weight arm to the projectile arm is 1:3.75
• The arm needs to be strong but light
• The length of the sling (we used linen thread) should be about the same length as the projectile arm
• The counterweight needs to swing freely
• The mass of the weight should be about 100x the mass of the projectile
• The angle of release (affected by the pin angle) should be about 45^o
• There needs to be a smooth surface for the projectile to travel along before it is launched (we used some cardboard to smooth the transition over the base)
• Solid ground underneath helps (e.g. not carpet)

Trebuchet frame, with lots of bracing

Weight – full of metal screws and weighing about 400 gm, to match our projectile of approx. 4 gm. There is a hinge connecting this weight to the end of the arm so that it swings freely.

Release pin for the sling, on an angle. Different angles produce very different results.

One of the projectiles – this was the ‘medium’ weight version.

It is interesting to adjust various parts and see the effect. For instance,

• small weight + 60^o pin = 4.1 m range
• heavier weight + 60^o pin = 3.7 m range
• small weight + 30^o pin = 8.3 m range
• medium weight + 30^o pin = 8.7 m range

Drawing back the projectile, about to launch

The projectile is difficult to see in a video, so we fired it at dusk with LEDs attached to the projectile:

Trebuchet fired at night, with red LEDs attached to the projectile and showing its path.

And here is a video:

It would be nice to be able to provide a general description of the physics of how a trebuchet works, but it turns out to be rather more complicated than it initially appears. There is an explanation by Donald Siano here if you really want to get into it.

Physics explanations aside, they are quite satisfying to make and to fire. I expect our next (better, bigger) version will require trips to a park so as to be able to launch projectiles without losing them into the neighbours place.