Building a medieval siege engine for fun and profit.

The completed machine.

The completed machine.

So, a little background. It was decided one evening that it would be a good idea to build a trebuchet. I have no idea where this idea came from. It probably had some influence from the Pumpkin Chukin’ show on Discover. But it might have predated that, I’m not sure. I made the mistake of mentioning this plan to my 14 year old son, who immediately latched onto the idea and never let me forget about it. Every weekend it was “when are we going to build the trebuchet, dad?”

Eventually the time was right, and my brother and father came over to help. I was able to find very little information in the interwebs on construction of a trebuchet larger than a few inches. The only hard data I was able to get was a 4:1 ratio on the throwing arm, and 100 lbs of counterweight for each pound of projectile. I used this to figure out the length of the throwing arm (governed largely by the lumber I had in the rack) and built the frame around the arm.

Material used were determined by what I had on hand. 2 x 4 pt for the throwing arm, 4 x 4 pt for the uprights, 4 x 8 pt for the skids, 2 x 3 for the bracing. The pivot arm was made from 3/4 steel rod, with the arm centered on the bar with 1 1/2 inch PVC pipe as spacers to keep it centered. I used a milk crate for the counterweight basket, and an old metal downspout for the projectile chute. A leg from a pair of blue jeans was used to make the sling.

If I had to do it over again, apart from building it BIGGER, the only thing I would change would be the milk crate. It just could’ hold up to the stresses involved, or hold enough weight for the projectiles we were using (candle-pinbowling balls). I think a plywood box would be just the thing for this.

After the trebuchet was complete, it was time to test it out. After quite a lot of trial and error, we found a suitable trigger mechanism and sling configuration. This being New England, we started using rocks as projectiles. This, it turns out, is a bad idea. The irregular shape of rocks causes them to release unpredictably from the sling. The rocks went every which way – slammed into the ground in front of the trebuchet, straight up in the air, and even backwards. Very dangerous, and slightly hilarious at the same time. Finally, I visited our local bowling alley, and they were able to donate a few chipped candle-pin bowling balls. These worked perfectly! They launched very consistently and landed in nearly the same spot every time. The only problem was that the milk crate could only hold about 100 lbs of counterweight in the form of hauling chains, and the balls weighed about 4 lbs or so. This combo launched the balls about 150 feet. I never did get around to building a bigger counterweight basket.

I received a lot of comments from the neighbors, and showed it off to all my friends. It was a fun project. Eventually my son brought it to school where they were doing a physics lesson and built different kinds of catapults. Someday we will build a bigger one!

Laying out the foundation of the machine and making the throwing arm.

Getting Started: Laying out the foundation of the machine and making the throwing arm.

Ctrl-Z: 'Dammit, why did you put that nail there?' There was probably a better way to do this.....

Ctrl-Z: ‘Dammit, why did you put that nail there?’ There was probably a better way to do this…..

 Building the throwing arm. : 'Are you going to help, or just take pictures?'

Building the throwing arm. : ‘Are you going to help, or just take pictures?’

Some assembly required: It starts to take shape. Note the committee style work arrangements.

Some assembly required: It starts to take shape. Note the committee style work arrangements.

The second upright goes up: Finally got the help motivated.

The second upright goes up: Finally got the help motivated.

The throwing arm goes in: Note the PVC pipe to keep the arm centered.

The throwing arm goes in: Note the PVC pipe to keep the arm centered.

Adding bracing: We didn't spend a lot of time making it pretty.

Adding bracing: We didn’t spend a lot of time making it pretty.

Below are a few movies of the completed machine in action:

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