Everything is going according to plan: Trebuchet, Part II

Hello, and welcome back! This is the part where we get to the interesting stuff, the real nitty gritty- talking about the actual build of the trebuchet, alongside what it took to dial it in.

I talked briefly about the math that went into building a trebuchet in the last post, and a lot of that comes into play here. This was what mainly determined the size of the trebuchet. We ended up throwing a projectile about the size of a grapefruit (spoiler: it’s because it was a grapefruit), so around one-two lbs., around 150-200 feet each time. To do this, we needed an eight-foot-long throwing arm, a sling that was around 6 feet long, and an 8-foot-long throwing arm, along with a 100-pound counterweight.

We used 2x4s, a sheet of plywood, and galvanized steel pipe for the majority of the build. I pre-measured, drilled, and cut everything at my house, save for pre-drilling the holes for screws. Somehow, I got all of it to fit into the back of the car, and I didn’t need a truck to get it to campus.

The only part that I assembled in advance was the counterweight arm. This was because I had to drill a 3/4” hole through 3 separate pieces of plywood, and I don’t have a drill press, so doing this would’ve been pretty much impossible without the three being screwed together. I had to sand the middle of it flat, because my drill bit only went part of the way through and it left a high spot in the middle. Yes, as expected, this would be a problem later.

All of the pieces laid out. Wouldn’t it be convenient if IKEA offered siege machine kits?

On assembly day, the team- Matthew, Jack, Noah, Connor, and I- got to Harris early and started work. The whole thing didn’t really take that long to put together. We just laid the pieces out like a puzzle, drilled pilot holes of the appropriate size, and screwed it together. It helped that most of us brought our own power tools, too. It probably happened a little quicker than the medieval way would’ve had it, but power tools have that effect on building pretty much anything.

We assembled the sides before the base, opting to use a combination of manpower and a table to prop things up and put the inner braces together. This is because the sides had to be pretty much perfect- the holes had to hold the pivot bar, and if they didn’t line up, it would be a lot of sanding to fix it and a likelihood that the pivot bar might move during launch or stress the sides at the pivot point. That’s pretty much a “redo everything” situation, and that’s less than ideal. Luckily, everything came together great! The holes for the pivot bar and the counterweight arm fit perfectly. Unfortunately for us, our first mistake was checking to see that the pivot bar fit.

The pivot bar is the steel bar that holds the throwing arm, and it’s called that because it’s the pivot point for the trebuchet. To check the alignment of the holes, we worked to push it through both- a process that was much different from the one we did to fit the throwing arm and counterweight arms together. Those bars moved smoothly, and we had no trouble taking them in and out. The pivot bar, on the other hand, fit a little too well. We didn’t put the throwing arm on because we assumed we would just push it in there to make sure it fit before pulling it back out. It took all four of us pushing and pulling and twisting it through the holes, alongside a little sanding with a sanding bit on the power drill. This should’ve been our sign to stop and re-assess. We were super satisfied when we finally got it, of course. That was before we tried removing it.

We had to get an angle grinder to get it out.

A phone call and some safety squints later, it was 5:30. We had long run out of pizza, I had to make the first of MANY trips to Lowes, and we decided to call it a day there.

The next day, we met to replace the pivot bar and try to get the whole thing assembled, save for the weights. We were still looking into different options at that point, keeping in mind that $1.5 million was just out of budget and dealing with the heartbreak caused by a lack of tungsten cubes. Luckily, everything came together well. We needed to do a lot of sanding, on both the base and on the throwing arm, but in the end all the parts fit together with just enough friction to work smoothly. We put together the quick release mechanism, too; three eye hooks, a carabiner, a piece of paracord, and a square steel bar made it easy to launch safely. The reason we used a square bar is because the quick release bar would be under a ton of tension when it was fully loaded and ready to launch, and a round bar made for full contact and more friction. This way, there’s only two points of contact to cause friction.

At this point, it only needed a few finishing touches. By that, I mean it needed the entire sling and also the weights. We measured out two pieces of paracord, making sure they were the right kind to withstand the force of the launch, and cut a sling made out of canvas fabric large enough to fit our projectiles. We took another eye hook and screwed it into the end of the throwing arm on the underside before driving a nail into the end of the arm at an angle. From here, one end of the sling gets a steel ring tied onto it, and the other gets tied to the eye hook. This way, if the length of the sling and the angle of the nail is right, the ring slips off the nail and releases the sling and the projectile just before the highest point in the throwing arm’s arc.

The fabric part of the sling was a little difficult; we tore through a few before we engineered one that held up. We just stacked the canvas four or five times before sewing around the outside of each hole for the ties, reinforcing the weakest parts. The weights, which we got off of Facebook Marketplace for $50, were originally barbell weights. We used these because they were the perfect size to slip on and off of the steel bar used on the counterweight arm so we could remove the extra weight for travel. We also had to make another counterweight arm because the first one was a little too long. Finally, it was time for our first launch!

We decided to try launching at the riverfront park in the late evening because there was a big open area and not many people were there at that time of day. We bought a few melons (we were originally going to try launching those, not grapefruit) and loaded the trebuchet up to head down to the riverfront. We got there with plenty of daylight and good weather, unloaded the trebuchet, and prepared for our first launch.

I want you to guess what happened after that.

This video is muted because if it wasn’t I couldn’t post it here. The very first time we tried to launch the trebuchet, the whole thing broke. Like, miserably bad. We completely destroyed the throwing arm, the counterweight arm had cracked, and we needed to replace both and figure out a way to reinforce them, fast. I ran to Lowe’s again while we were losing daylight, trying to figure out what we could do about it.

I came to the conclusion that it would be best to completely rebuild all the damaged parts and reinforce them with 20 gauge galvanized steel foundation straps. If you don’t know what that is, it’s a 2” wide thin piece of steel that’s strong enough not to snap but light enough to warp with a hammer. For our purposes, it’s what would wrap around the ends of all the 2x4s used to make the new throwing arm alongside those used to reinforce the counterweight arm. We were able to get all of them together on-site, but at that point we’d eaten a few of our projectiles and lost the daylight we could’ve used to do another test.

That’s all there’s room for this week. Come back soon to find out how we tuned it in, our final specs, and most importantly, videos and pictures of the trebuchet at our Talons & Trebuchets event and the feast! Thanks for reading!