Building a Giant Trebuchet

This trebuchet was built entirely just for an event that me and my buddy Zack participated in early this year. At the event we premiered a "documentary" following us building, testing, and refining this trebuchet entitled "Your Doing it Wrong". Although slathered in facetiousness and irreverence, I feel like it still held some value where we discussed how money shouldn't be your focus in a creative pursuit. Further - don't follow trends, do things for the right reason, provide a real value, etc. The typo in the title was entirely on purpose and intended to be ironic given the authoritative nature of it, that carried through to our outfits and signed motivational photos that everyone who attended our talk walked away with as our gift (your welcome). Trebuchet facts -- the thing is 16' tall when deployed, pretty massive for something that we built in a garage. Most impressive launches were a watermelon that went ~200ft and a grapefruit that went ~450ft. So, how exactly does a trebuchet have to do with any of that? That's a hard question to answer, but I'll explain it the best I can...

So we started with a budget of $4,000 to build this beast ($2k for each of us). Really, that money was intended to go towards the class we were teaching, but where's the fun in that? (à la Mr. Casey Neistat, but with sarcasm) We figured we could use the trebuchet to teach a lesson to the world, or if that doesn't stick, we still got to throw a watermelon hundreds of feet, so..... win, win? The first day was spent planning and sketching the thing out and then the next day we went for a little shopping spree picking up tube steel, pipe, solid rods, and a utility trailer.

First step was to assemble the trailer. We actually decided to build the entire trebuchet right on the top of a utility trailer. One reason was to make the thing easy to move around and transport (a thing that trailers are surprisingly good at). We also figured that we'd have to build some sort of base platform for the thing anyway. We figured out that it was actually more economical when you factor in time and materials to just assemble a cheap trailer instead of fabricating a custom platform that we then have to figure out how to move around. And the last factor is that, because of physics, it's actually more efficient at launching things when the whole trebuchet is allowed to move horizontally during the launch.

The fabrication part starts by cutting the tube steel down to size on the horizontal bandsaw. We used 3x2" tube steel and got 1/8" wall thickness for the throwing arm and the columns and then used thinner gauge stuff for everything else.

We started with the throwing arm and figured we'd get that sorted out first in case we decide to change some dimensions along the way. The arm ended up being 11' long, but the key is that the ratio between the lengths of the arms on either side of the pivot point be 3.75:1 for maximum optimization. We first drilled holes for the pivot arms to pass through -- 1.5" rod for the main pivot and 1" rod for the pivot that will hold the weights.

To hold the pivot rods in place and also help eliminate as much friction as possible, we decided to use bearings at these 2 points. We picked up some flange bearings which are cast iron plates that hold a really beefy bearing in place and you can bolt that bearing to your work piece. A smaller hole was drilled on either side of the first holes to line up with the flange.

This is the pivot rod on the end of the arm where the weights will mount. This is the first test fit to make sure everything is aligned, with 2 bolts holding the flanged bearings in place on either side. Are these industrial grade bearings overkill? You'd better believe it, overkill is kind of my brand.

The weights will be held on to the end of the arm by hanging below with some steel bar stock. We found a couple of pieces 4" wide and 1/4" thick and cut them to length and drilled holes out so that it would slip over the 1" rod that we just installed.

Lower on the bar stock we drilled larger holes to accept this, a 2" pipe which fits almost perfectly inside of the weight plates that we're using to counterweight the trebuchet. Zack ground off the mill scale and ground down the outer surface a little bit and that was enough to let us insert it into the weights.

Then we test fit the whole set up with a single weight in it. Fairly simple really and worked just as expected. Also, just as we hoped, we can't feel any resistance in the weight pivot point at all... I think bearings are our new obsession. We went to a industrial supply place to find the bearings and they had a book literally the size of a phone book for all the different bearings. Remember phone books?

Anyway, with the arm complete, we start working on the A-frame assembly that will form the sides of the trebuchet that hold everything up in the air, a key feature of a trebuchet. We cut some of the thin gauge tube down to length for the "beam", which ended up being about 9.5' long so that we could extend it past the back of the trailer some and also center it over the axle. We clean off the paint on the trailer and the mill scale on the steel to prepare for welding.

Zack is the better welder of the duo, so he starts by tacking the beam in place to the cross members of the trailer and then welds them in place on each side. The trailer is surprisingly ridged already (especially for the price), but this really solidifies everything together. I also do my part as primarily a woodworker and reinforce the joints with some wood glue, as one does.

Nabisco, not a sponsor.

Next we worked on the columns. To hold the main pivot rod in place at the top of the columns, we drilled a hole all the way through so that the rod passes through and then welded a nut to the top so that we can thread a bolt down into the rod and pinch in in place. This was we can disassemble things as needed, but the bolts will hold it firmly in place while we siege various... Tim Horton's? Don't underestimate the power of friction.

We clamp the column in place and measure to make sure that it is plumb before we tack it in place. We then cut some cross-members to 45 degrees at each end and tack those in place too, making sure to grind off the mill scale before each weld.

With everything satisfactorily (I checked, it's a word) tacked in place, plumb, and parallel, Zack welds everything in place permanently. The bottom dimension was set thanks to the beams, but at the top of the columns we clamped a cardboard tube temporarily to ensure the columns were held parallel.

I supervised.

We install the arm through the pivot point and everything is moving as it should. Next we just need to figure out the trigger mechanism so that we can lock the arm in place while it's under load and then release it with the trigger, and key feature of a trebuchet. We ended up deciding to cut the back/bottom corner of the beams off at a 45 and weld on a piece 1.5x.5" tube spanning between the beams. This gave us a very secure mount for the trigger and also gave us lots of options if we wanted to modify it later to optimize it.

Now for the trigger we wanted a fairly simple simple release pin mechanism, but adding flair to stuff is kind of our thing, so we added the correct amount of complexity by pulling out the forge. From some round rod, Zack was able to forge a few eye bolts (those things that are ~$1 at the hardware store? yeah, those).

The eye bolts will be what holds the pin in place in the pin mechanism, but also we'll use a couple of these to hold the winch and pulley in place later that we'll use to pull the arm down to load it. Zack knocks out 3 of these impressively fast, faster then a trip to the hardware store (that we definitely weren't at earlier that morning).

Now for the "pin" part of the pin mechanism (a key feature of a pin mechanism) we flattened out some wider diameter steel rod and then split the end of it so each side could be bent over.

With a little bit of persuasion from the hammer, anvil, and Zack's Wheaties that morning, we're able to form one side into a ring, and the other side into the pin. The plan being that this whole arm will actually pivot making things easier to load since the pin will stay attached to the trebuchet. The ring allows us to tie a rope around and pull at a safe distance.

Once done forging and everything has cooled down, we pick the precise position on the cross tube where the launch pin thingy wants to be mounted and pivot around. First we weld the ring in place on the arm and then we do a couple of dry runs to make sure the pin pulls out with no problem (don't want no trebuchet babies) and then we drill through the arm of the pivot and also the cross tube where we can install a bolt and hold it in place with a pair of nuts so that one nut locks against the other and still allows everything to pivot freely.

Last part of the release/reload is the mounts for the winch that will help us reload. On the arm, I weld one of the eyes where our pulley will be mounted. Then on the cross brace on one side I weld the other eye. The winch is welded to the opposite cross brace so that it passes through the pulley mounted in the arm and then clips to the other side.

At this point we can finally put some weights on this beast and do a couple of test runs. Even with just 90lbs on there, this thing is pretty scary. We also made sure the winch was firmly mounted in place at this point because that was the only thing from keeping this thing from launching in to the affectionately named "poop pipe" in the ceiling (because honestly who doesn't have a sewer pipe up ahead ready to unload at any moment).

At this point, the trailer is pretty much just resting on the axle, so we need to stabilize it. We decided to just get a simple trailer jack for each of the 4 corners and weld it in place. We clean up the paint at these areas and weld the plates from the trailer jacks at the 4 corners of the trailer.

The jacks can then be installed with their retention rings and we can deploy these things so that the trailer doesn't keep tipping every time we try to climb on it. These did a lot to keep things steady while also being easy to pivot and lock up out of the way during transportation. We're also hoping that the large plates on the bottom will slide along the ground easily during launch (spoiler, they did).

Now the real tricky geometry needs to be figured out, which is all in the release mechanism/sling. We decided to make the release mechanism out of a pin welded to a small pipe. Our theory being (by "our" I mean "my") that we can use this to adjust angle so we can tweak it later until we get it releasing exactly where I want it.

We drill a hole through the top of the arm and bolt the release pin in place temporarily to make sure that it works. I said "my" theory earlier because Zack was adamant about the fact that the inside face of the tube steel arm would not exert enough friction to hold the pipe in place during launch. I won't say who was right, but let's all move onto the next step knowing that this is how the release pin remains to this day... (it was me, I was the one who was right, I'm not proud about it or anything)

The sling turned out to be a bit trickier because friction is simple (looking at you Zack). We mocked up a few different sling designs with our prototype projectile tennis ball and some paper. Once we were satisfied with how they felt, we moved on to the real thing. (not that this design failed, it cupped the ball too much and held it so it wouldn't launch)

We used the paper mock up's as templates and then cut and sewed some slings out of waxed canvas that Zack had kicking around at his shop.

Adding some eyelets on either end of the sling gave us a place to mount some carabiners where we can tie some rope and, boom, we have our sling. The length of the sling is equal to the length of the long end of the arm (so in our case, 8.5'). One end of the rope of the sling is tied in place permanently at the release pin and the other rope has and eyelet on the end that slips onto the pin and releases during launch, releasing the projectile.

Some test throws showed that the sling will definitely work. If anything, Zack puts more force on this thing then the trebuchet ever will, so we know we're good (lies, it's all lies).

Next I finally get to be a woodworker again! I won't tell you how much I had to beg and plead Zack to be able to add this one, but I just reminded him about the release pin and the power of friction and he let it go (this definitely is how it went, don't question me, I also couldn't go into the military because I'd definitely punch the drill Sargent in the face!!). Getting back on track, I cut these spacer blocks 3" tall to match the height of the beams, but added a taper on one end so that it formed a channel in the middle. I predrilled holes in the trailer and then screwed the blocks in place.

I then rip down some blanks to form the angle and cut a few more to width and screw them all into place, giving us a platform to both stand on to work on the trebuchet, but also a channel to guide the projectile straight while it picks up speed.

And that's it today is our day that we get to test fire this baby!... our trebuchet is our baby, we're not launching an actual baby.

Loaded up with 90lbs and a tennis ball, or first launch goes a mind blowing 33 feet! Luckily with our slo-mo camera footage we're able to see exactly what's going wrong and we're able tweak a few things. The release pin was much too steep, so we straighten that out (the optimum angle ended up being about 15 degrees off the axes of the arm). We also made some new slings since the first one held the ball too tightly. And we also noticed that the sling was hitting our pin mechanism on the trailer while launching, so we added some blocks under the back of the wood channel to direct the projectile up a bit during launch.

After all these tweaks we were able to get tennis balls and oranges a couple hundred feet just before we lost our sunlight for the day. But that was convenient because we needed to get serious before the real launch, make the trebuchet look pretty and ourselves look pretty to match.

The next day we loaded up on projectiles at all the local thrift stores. Lots of sporting goods and a few other thing caught our eye, then we picked up a bunch of produce at the grocery store. Even found a bowling ball for just a few dollars, but never got to try that since we feared for the safety of the people a few towns over where this thing definitely would have landed.

Loaded up with plenty of projectiles, we're ready, launch day is finally here! Worth noting that we painted the trebuchet yellow to dress it up, but while at the thrift stores also decided that we ought to be dressed up for the occasion. I went with Iranian porn set bouncer and Zack went with auctioneer of used mobility scooters. Maybe we should have coordinated better, but I have a feeling it didn't matter much.

So we loaded this fine baby up with more weight then she's ever seen and went to town. Some notable launches:

Our first launch of the day which went 250', but we later shot one twice that distance (we had to rotate the trebuchet first to shoot into Zack's neighbors property since we ran out of space on Zack's property)

One of the few projectiles that actually went backwards (because of the weird shape, it slipped out of the sling), but we were thrilled with the results anyway because of the resulting footage.

Probably my favorite launch honestly. The watermelon not only launched over some tall trees, going 189' while flying a perfect spiral, but it landed right in the neighbor's tiny pond where it exploded and caused a massive cannonball like splash.

We didn't think this launch would be as spectacular as it ended up being, we just wanted to show off how filthy rich we are. (if only we knew)

Really I just like the slo-mo footage of this launch showing how the trebuchet really just whips projectiles around and out of the sling. This went almost 250 feet and actually left a large juicy divot in the ground.

Here's a map of all of our furthest launches with each projectile (you can see the trebuchet on the far right side just below the fence). We definitely had more capacity in this thing and just plain ran out of real estate. Sometime in the future when we can launch again, with more tweaking and more room to launch I'm confident we could go at least twice as far.

I hope you enjoyed the build, and I hope that you also enjoy my first "documentary" feature film that was the motivation for this project. It has illegitimately received many awards and is available in 3D and IMAX at exactly 0 theaters. But it is available on YouTube and I hope you'll watch it if you're looking to kill some time and maybe even l̶a̶u̶g̶h̶ exhale out of your nose a few times.

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