Eli Kirk

The Background Stuff

As much as I love coding, there are times when it feels that the days blend together. Create an AJAX form here, build a new site template there, figure out how to make it all run across all the major web browsers, and at the end of the day you find yourself standing in the grocery store aisle using boolean logic in your head to compare price vs. flavor vs. nutrition on several boxes of cereal.

So when the e-mail showed up in my inbox announcing the second annual gourd-smashing “Slash and Bash” activity for the company, I was excited. Who doesn’t want to let out some stress by making large round vegetables splatter? Until PEETA provides ample proof that plants scream when being picked, cooked, smashed, or cut in half with a sword, we need to enjoy it.

Then I noticed a call for some brave volunteer who was willing to step up and build a “pumpkin catapult.” Being the geek that I am, I grew up drawing castles and reading books about dragons and armies that fought with sword, shield, pike, and bow. War machines were familiar to me before I was a teenager, enough so that I could tell them apart even if I couldn’t describe exactly how each was powered. Building such a device was a long-standing dream of mine. Also being the geek that I am, I knew right away that a catapult was not what we needed. We needed something that could expend much more kinetic energy for the same rough size. What we needed was a trebuchet. It had been a long time since I had worked on a larger wood project, so I called my Dad (who has a modest wood shop in his garage) and he agreed that it sounded like fun.

The “Boring” (aka “Technical”) Stuff

With a trebuchet, the force to throw an object comes from a long arm that is mounted up high. One end of the arm has large counterweights attached to it, historically a large wooden bucket full of boulders, rocks, and dirt. The arm is “loaded” by being pulled down to the ground through the use of a rope or chain and fastened in place. This lifts the counterweight up high and stores potential energy. The projectile is put into a pouch with one side securely tied close to the top of the trebuchet swing arm. The other side has a loop that hangs loosely on a peg on top of this swinging arm. When the tension is released, the counterweight drops forcefully to the ground, turning the potential energy into kinetic energy, and bringing the opposite end with the projectile up into the air. The loose pouch loop slips off the arm at about 90 degrees, and by the time the arm reaches about 120 degrees the projectile clears the pouch. The pouch adds length to the rotation of the swing arm without extending the arm itself. The longer the lever (or arm), the faster the end is moving, throwing the projectile further and at a higher speed. Watch this 4-minute video of a full-size trebuchet being built and fired for another explanation (worth watching—they throw a piano).

The Build Stuff

You would think that using design templates for page styles and diagrams to map out web functions would teach me that I should have drawn up detailed plans, but I suppose I figured that I had enough of an idea in my head I didn’t need one. Surprisingly enough, I did okay.

I’ve been asked a series of questions about how the trebuchet came together, what dimensions I used, etc. I did most of the building during the month of October, but I’ll try to remember the details and get them all in here.

The first dimension I needed to determine was the base size. From the base, I could decide roughly how high we could safely go and how long to make the swing arm. Since I would be transporting the trebuchet to the Slash and Bash in a 1997 Ford Ranger, I measured the bed of the truck and figured my base size from there. I think it ended up at about 32”x84”, which would allow the trebuchet to sit in the truck snugly with the tailgate down. I decided to use 2×12 boards for the base, which was a touch arbitrary other than they were large and would probably hold up to more stress than I was going to give them. I knew that to throw a 5- to 10lbs pumpkin 50 feet, I would need well over 150 lbs. of counterweight. That meant that the frame would need to be able to hold up under repeated stress.With the base built and reinforced, it was time to cut and attach some supports that would hold the fulcrum. It seemed like 2×10’s were just a little too big, so 2×8’s were my choice for this. Since the trebuchet was being transported in a pickup, I didn’t want to drive around with the arm and fulcrum (the point at which a lever sits) still up on the supports. I needed a way to be able to partially disassemble the machine for transport. In addition, I didn’t want to make the uprights so tall that it would tip the whole machine over, so with these two factors in mind we made it a little over 6’ tall. We took 2×8′s to 6’ and attached them with screws in the center of the base on opposite sides of the box. Again thinking of the repeated stress, we cut 2×4’s and reinforced these sides. There was no rhyme or reason to the angle, as long as it formed a triangle, so we ran them from the inside corners up.

The base and uprights have been cut and attached. This was the end result on the first night of building.

Picture on night number two.

For the fulcrum, I wanted it to be plenty reinforced to be able to hold the stress, but still light, so I bought a chain link fence end post and cut it to size with a metal grinder. The nice thing about chain link fences is that the companies make a few nice trim pieces such as end caps. We ground the ends down smooth and got rid of the rough spots with a metal file, drilled a hole in the caps and pipe, and then screwed them in with small machine screws, counter-tapped so they would sit flush with the caps.The swing arm was constructed out of a single 2×8, 8’ long. Originally we didn’t do much to the board itself other than using a hole saw the same size as the fulcrum pipe to cut a notch where it would rest on the fulcrum, but to reinforce and keep the wood from splitting, we boxed in the end that the weight would be attached to. The weight was attached to the swing arm with a ⅜”x8” eye bolt, and the boxing proved to be more effective than we would realize, but I’ll get to that below. On the opposite end, we used the rod portion of a gate latch and then ground down the knob at the end to prevent the sling from catching when it slipped free during firing. For the ratio on where to put the “contact point” where the swing arm would rest on the fulcrum, I went online and found that 1:6 seemed to be popular, which put my measurements at 1’3” on the counterweight side and 6’9” on the projectile end. Last, we cut away some extra wood for weight and added another smaller eye bolt to hold the stationary side of the projectile pouch.

To attach the swing arm to the fulcrum, we added a 2×4 “side wing” on either side of the arm using Simpson® stair brackets. These are heavy-duty L-shaped brackets with holes for screws that are typically used on stair cases for reinforcing, but they worked great here. We then used two U-bolts (named for their letter “U” shape), bolted into the “wings.” Using a U-bolt on either side of the swing arm helped to stabilize it and prevent it from twisting on the the fulcrum when firing.

The projectile pouch was made from two cheap basketball nets woven together with 40lbs-weight clothes line. The ends were pulled together and held with 280lbs-rated spring link clips.For the counter weight, I played with the idea of getting a pre-threaded pipe from the hardware store, hanging it from the eye bolt with two C-bolts (“C”-shaped bolts that have sides that screw closed), and then putting various weights on the end to adjust depending on the weight of the pumpkin. The trouble with that philosophy is that it wasn’t cost-effective. Even at Wally World the individual weights used in weight lifting ran about $1 per pound, and more for the larger weights. E-bay or Craigs List could have helped in this regard, but I found a great alternative. While browsing at the hardware store, I got the idea of putting a t-junction on the pipe, put some 4” pipes in the t-junction, lower the t-junction into a standard 5-gallon bucket (like an upside-down letter “T”) and then fill said bucket with concrete. This proved to be a simple and cost-effective solution, as each bucket held 80lbs of concrete. Some spare cast iron weights from my father’s shed were slid over the pipe to prevent them from falling off, which added an extra 40lbs, for a total of 200lbs.

Picture of the trigger mechanism.

To hold the arm down under tension, we tied a thick rope around the swing arm so it would be at 90 degrees to the back of the base when the arm was in a “loaded” position. Using three circles cut from 1×10 birch, we constructed a trigger mechanism that was used in the middle ages on larger trebuchets. We started by removing two 90-degree pie-shaped pieces from the outer circles. The middle circle was cut similarly, but an extra few inches was cut from the left side. When glued and screwed together, this created a sandwich effect that allowed the rope to slide past the middle circle and hold onto the outer two circles with a knot. The other cut in all three pieces was for a lever that held the circle in place under tension. A hole was drilled into the lever and a wooden dowel was glued and bolted into the hole. This would allow the operator to stand clear of the swinging arm. The dowel was pulled down, the lever rotated up a few degrees, and as it cleared the wooden circle, the stress of the counterweight caused the circle to turn, releasing the rope holding it down.

To fire, the (1) lever is pulled to the right, which forces the (2) trigger lock up, allowing the (3) trigger wheel to spin freely and release the tension on the (4) knot, which in turn releases the (5) rope, firing the trebuchet.

The final anti-stress measure was something I picked up from a History Channel special on siege weaponry: wheels. Like many people, I had always assumed that the purpose of wheels on an trebuchet or catapult was purely for transportation, but that is only a part of their function. The addition of wheels would allow the extra kinetic energy to be released by causing the trebuchet to swing back and forth as the counterweight lost momentum. This was actually one of the more expensive parts of the build, at about $20/wheel (they were spare wheels for hand trucks). After the Slash n’ Bash was over I found solid wheels for general use at Cal-Ranch for closer to $8/wheel. If I were to do this over again, you can bet that those would have been the wheels I would use. The wheels were attached to the base with 3/8″x6″ carriage bolts, using fender washers on either side of the wood.

The Fun Stuff

Click on the images for an enlarged view

Loaded and ready to go - myself with my father and his dog.

Our "test projectile" in flight

Emily Franson prepares to fire the war machine

Scott Smith and I load the trebuchet

At the Slash and Bash event, the trebuchet garnered some attention. Due to the limit of 200lbs on the counterweight, the pumpkins couldn’t weigh much over 15 lbs. We tried firing a few larger pumpkins, and it worked, but they often didn’t fly far enough to break open solidly. Still, the machine shot orange projectiles into messy piles for an hour and half. Before I said that boxing in the end of the swing arm was a smart idea. As the Slash and Bash came to a close, we tried firing a larger pumpkin that caused a little too much strain on the opposite end, because the two concrete-filled buckets crashed down to the ground after firing. Upon examination, we learned that the eye bolt holding the weight had stretched and eventually snapped, shearing it fairly clean at the board. We didn’t imagine the wood actually holding up better than the bolt. It’s not the end of the machine, though. A larger bolt could easily be put in just above it.

In total, the trebuchet cost around $430 and took my father and I about 50 or 60 total man-hours, but it was fun to work on another project with him. According to Lance, next year Eli Kirk will be building another machine—a little larger, a lot more weight. Who knows? Maybe in the future we’ll have a team to drive a competition “Punkin’ Chunkin’” machine to the World Championships in Delaware.