Big Trebuchet! (2008)

Left to right: Felix, Sam, Mike, Me, Chris, Matt, Paddy and Laura, David, Richard. And Chuck. (Note that the counterweight hasn't been attached yet)

Mike had a plan. Mike wanted to build a large trebuchet. Mike also has a field. A bit of organising later and 10 of us had built a trebuchet in Mike's garden. I can't take much credit for the design as Mike made all the plans. I only contributed a few ideas, designed the sling and helped design the adjustable sling-pin mechanism.

We were able to fire water balloons about 20 metres. If we'd had more time to tweak the sling and release angle we probably could have managed a bit further.

How It Works

(Click the photos for larger versions)

A trebuchet is a long arm, mounted on a double A frame, with a sling on one end and a heavy weight on the other. When the arm is released, the sling is flung round, detaches at about 45 degrees and releases the pay load.

Our sling was fairly crude. We sliced up a tarpaulin bag, folded it double, gaffa taped the edges, cut some holes and tied some rope in a U shape.

The sling was the major cause of problems when launching; quite often the balloon would get caught in the sling and not release, or burst as it was released. Once, the knots came undone mid-fire.

If we ever improve the trebuchet, I think the sling will be at the top of our list of things to do.

One end of the sling is tied to the arm, the other end forms a loop (which has an orange balloon attached in the photo) which detaches from a peg (we used a screw with the end cut off) while the arm is moving. For optimum range the sling needs to release the payload at 45 degrees.

So we could easily adjust the angle of the pin, we screwed it into a piece of wood separate from the rest of the arm, which was then held to the arm with only one bolt. With the bolt tightened, it was part of the arm, but if we loosened the bolt we could change the angle of the pin.

In practice, tiny adjustments of the pin angle had a very large effect on the range of the trebuchet.

In order to safely fire the trebuchet we used a remote trigger mechanism. This consisted of a bolt pushed through 3 carefully aligned eyelets (two on the frame, one on the arm), with some rope tied to the bolt. When everyone was clear of the treb, someone would pull the rope and the bolt shoot out.

We first thought that the thread on the bolt might cause it to get stuck, but it always came out easily with a good tug.

If the sling was crude, then I don't know what the counter weight was. We put a bag of sand in a rucksack, another bag of sand on top and what looked suspiciously like a metal ten-pin on top of that. Tied and taped together it held surprisingly well!

We think it weighed about 50kg. I have no idea how the axle held the weight.

We found, literally found, an iron (or maybe steel) bar to use for the axle. I think it might have once been a fence post. Metal piping with a slightly larger diameter than the bar slipped over the top was used to keep the arm roughly central. The bar stayed still and the arm rotated around it.

Despite some nasty creaking sounds, and some considerable bending when fully loaded, the bar didn't break.

Before being fired the sling rested in a trough. This was so it wouldn't get caught on the frame as it was flung. We used the top of an old wooden bench, although we later covered it with a tarpaulin to reduce friction (this was quicker than sanding the bench).