Hello! My name is AJ, and I am currently a freshman at Ardrey Kell High School in Charlotte, North Carolina. In this project, we will learn how trebuchets work by building one!

When I first heard about trebuchets while watching an episode of Discovery Channel, I was puzzled.

"A trebuchet? What even is a that? Is it some sort of catapult?"

That's what I thought when I saw the trebuchet launching a stone boulder hundreds of feet into the air. Trebuchets, similar to catapults, launch objects into the air (obviously). Both these contraptions were often used in medieval warfare to attack castles, and have since been commonly attached to that stereotype. The key difference is however, that a catapult uses elastic energy in a form of rubber to launch an object, while a trebuchet uses gravitational energy in the form of a falling weight to launch an object.

In the Middle Ages, trebuchets were huge 60 foot tall machines that hurled 400+ pound stones at castles. As cool as building one of those would be, it wouldn't fit in my backyard :) So instead, we will be building a scaled down wooden trebuchet that can launch tennis balls across your yard.

I designed the trebuchet in Fusion 360 - feel free to explore the design.

Trebuchet

Supplies

All of the needed materials can be found easily at a hardware store or possibly laying around in your garage. The wood dimensions mentioned in this list are common sizes, so you can find them pre-cut on shelves at Lowes or Home Depot. However, I would recommend that you save some money by buying a long 2-by-4 and then cutting it yourself with a hacksaw.

That being said, here is the full list of materials:

Frame:

2" x 4" x 24" wood (2x)
2" x 4" x 12" wood (2x)
1" x 4" x 24" wood (2x)
Box of wood screws (at least 4 inches long)

Arm:

27" long 1" PVC Pipe (1x)
End caps for PVC pipe (2x)
3" M6 bolt and nut (does not need to be exact size/length - use what you have already)
23" long 0.75" threaded rod and 4 bolts
Tuna can (or any other bowl shaped can)
Eyelet screw

Other materials needed:

10 pound weight (Or any other compact heavy object)
Aluminum tent stake (or any skinny metal rod)
Couple yards of rope
U-shape metal bracket (or any other similar piece - look at images in Step 4 or CAD)

Step 1: The Frame

As with anything in life, a good foundation makes way for success. In order to have a good trebuchet, the base (foundation) has to be flat. This is really easy to do, and just requires an extra 5 seconds to check if everything sits flat before you put a screw in.

If you have not already, cut out the wood into the lengths shown in the picture below. Make sure to also drill a .75" hole 3 inches from the end of the uprights now, as it will be much more difficult to drill these holes once the uprights are installed. I would recommend stacking both wood pieces on top of each other, and drilling them both in one go. This ensures that the holes would be parallel to each other once installed. This image shows the the lengths of the wood, and where to drill the 2 holes.

Then, lay out the 24" long and 12" long 2-by-4s in this pattern.

Then, use fasteners to screw the 4 pieces of wood together. 2 screws are used here, but adding more wouldn't hurt. If you do not have a drill, wood glue will suffice.

Once you have this base part done, its time to move onto the uprights. Making sure that these posts are parallel to each other and perpendicular to the base is critical in ensuring that the arm can move straight and freely. Use a ruler to measure 3" from the end of the base, and mark it. Do the same on the other side. Once you have done that, fasten the uprights to base. It may be easier if you have someone else hold the upright in place while you screw it in. It is also important that you use a minimum of 3 fasteners, as these posts will be bearing the full load of the arm and weight.

Install the other upright on the other side too.

You may have noticed in the CAD model that there are additional diagonal bracings. These make the uprights stiffer, but are not required. Just screw in a piece of scrap wood on each side if you wish (look at the cover video for reference). They also make the trebuchet look better in my opinion :)

Optional:

Add a cardboard piece over the wooden base. This is not completely necessary, but it makes it look better and also creates a storage area underneath for the weights and rope when not in use.

This is what you should have by the end of this step (except for optional stuff):

Onto the next step!

Step 2: The Arm

The arm transfers the downward movement of the falling weight into the upwards motion that launches the projectile. The immense shearing force that is applied onto a rigid arm would splinter it, and a flexible arm would result in energy being lost. This is why PVC pipe was chosen as the arm material, as it is decently rigid in short lengths, but still has a tad bit of give.

First, cut the PVC pipe to a length of 27" if you have not already done so. Mark a line 6" from the end of the PVC pipe, and drill a .75" hole.

We also need a way for the shorter side of arm to hold the weight. Simply drill a small pilot hole at the center of an endcap and thread in a eyelet screw. Friction-fit this endcap to the side closer to the .75" drilled hole.

Next, we need to add a stopper bolt to the other end of the PVC to act as part of the remote launching system. The majority of this system will be installed in the 4th step. First, drill a hole on one endcap just wide enough for your bolt to slip in. Push the bolt through the inside so that head of the bolt is in the underside of the cap. Then, tighten the bolt in place by threading a bolt flush with the top of the endcap. Proceed by installing this endcap assembly to the PVC pipe. (On the side further away to the .75" drilled hole)

Step 3: Mounting the Arm

The arm needs some way to pivot freely, which is done by mounting it to a threaded rod that rotates in the holes drilled into the uprights. Minimizing friction is key here, as it will slow down the movement of the arm and reduce the launching distance.

To begin, slide your .75" thread rod through only one of the holes. Do not push it through both holes, as we need to bolt on the arm first. Then, thread on one bolt. You do not need to worry about the position of the nut yet, as it will be moved in a later step.

Add the arm to the threaded rod. Push the threaded rod through the hole in the PVC pipe, with the shorter side pointing up and the longer side pointing down. Then add another nut to lock the arm into place.

Push the threaded rod through the second hole. Move the nuts around so that the arm is in the center. Then, prevent the arm from sliding out by threading a bolt on the outside of the threaded rod. Do not fully tighten these bolts against the uprights, as the rod needs to be able to move freely.

Step 4: Remote Launch Mechanism

Safety First! A 10 pound weight moving really quickly poses some obvious dangers to someone reaching into the trebuchet. In order to prevent a need to be near the trebuchet in order to launch it, we need to create a remote launching mechanism that we can trigger from a distance. There are several ways to do this, but I found that the simplest way was to have a metal pin prevent the arm from moving, and then pulling that pin out with rope to release the arm. This mechanism may be tricky to understand, so please watch this video before you build it.

To begin, screw in the metal bracket at the back of the trebuchet. It needs to be roughly centered in line with the arm.

Then, find a thin rod that fits within the holes of the bracket. It can really be anything that is long enough to pass through both holes of the bracket, such as a pencil. I used a skinny leg from an broken mini tripod, which was perfect as it had a hole at the end of it. This hole is used to tie the rope securely to the rod. If the rod that you choose to use does not already have a hole at the end of it, drill a hole or bend the end of the rod to create a hook. Here is a picture of the mini tripod leg I used:

Next, we need to tie the rope to the rod. Pass a few inches of the rope through the hole, and triple-knot it to secure it.

Step 5: Finishing Touches

Almost there! The projectile needs to be held in place until the arm reaches a vertical position.

Simply do this by screwing in a tuna can to the PVC pipe. I would recommend attaching it no more than a few inches from the end of the PVC pipe.

Last but not least, we need to add the weight. Please be careful while doing this step, as you could really easily hurt yourself if you mishandle the weight. I would recommend doing this step once you have moved the trebuchet into an open place, and are ready to launch it. Make sure that the firing pin is preventing the arm from moving while you tie the weight.

Simply have someone hold the weight close to the hook, and then triple-knot the weight to the eyelet.

Step 6: Launch It!

Take the trebuchet outside, push down the arm, insert the trigger rod, load a tennis ball, take a few steps back, and pull the string! Watch as the tennis ball flies across your yard.

I hope you had fun and learned something new in this project! Feel free to mess around with trebuchet - add a sling, change some dimensions, add more weight, paint it, do whatever. You can really optimize the launching distance further if you do a little bit of research and find out how different variables affect the launch distance.