World's Simplest Trebuchet (Walking Arm Trebuchet)

This is the world premier for our new trebuchet design. Not only is this trebuchet simple, it is extremely efficient. It has many of the advantages of a floating arm trebuchet, but with fewer parts, less friction, and a unique projectile launch path. As you can see in my son's video, above, the projectile never swings backward beyond its starting point, reducing the danger to bystanders behind the trebuchet. Using this design last fall, my 8 year old son set the all-time record for best design at the Vermont Pumpkin Chuckin' Festival. His 20 pound, 41 inch tall trebuchet threw a 3 ounce ball 266 feet. My 500 pound, 10 foot version threw a 5 pound cantaloupe over 700 feet, but it went so high and so far that the spotter never saw it pass over him on its way into the woods. You can read about these trebuchets' success here on the VT Pumpkin Chuckin site, and you can see mine in action in the video above. [**update: I just added a better view of the 500 pound treb in action on Saturday, 9/28/19. The 5 pound rice-filled soccer ball flew about 870 feet and rolled to 975'. Unfortunately, at the VT Pumpkin Chuckin' event yesterday, all of the real fruits were crushed during the launch. My son won the grand prize again.]

The instructions provided here show how to make a 20 pound, 41 inch version, like my son's. The dimensions were chosen to adhere to the lightweight division rules at the VT Pumpkin Chuckin' Festival. As you will see, my son did all of the work himself. I did help out by holding some things in place and showing him how to complete the steps.

I have made other versions of this trebuchet with longer arms, mostly for throwing the excess apples that fall in our yard. While they would not be legal at our state competition, they can really whip an apple.

[**Update: In response to one of the replies, I just made a small, 3-D printed version and added a video. I was going for a desktop version, but it seems more at home on carpet. This is a rough version 1.0, but I attached a .stl file if anyone wants to print one and doesn't have CAD capabilities (scroll down, below the supply and tool lists). I used pieces of a large paper clip for the spikes and the finger. For good measure, I sharpened the spikes with sandpaper. I used fishing line for the sling and the tuning string. As you can see in the video, there's no pouch on this version. The sling is just one loop of fishing line, and it is permanently attached to the projectile. They fly off together. The proportions are different than those of the larger versions, which may be why I needed to bend the finger forward on this one to get a good release angle.]

Wood:

1. One 1/2"x3"x36" pine board
2. 1 foot of 2"x4" lumber
3. Three 0.75" x 36" square dowels
4. Several popsicle sticks (at least two, plus extras)

Hardware:

1. 1/2" x 6" lag screw
2. Heavy 1/2" washer
3. Three 3" nails
4. Two 2" drywall screws
5. Eight 3" drywall screws
6. One 1/4" x 2 1/2" hex bolt
7. One 1/4" x 1" hex bolt
8. Four 1/4" washers
9. One 1/4" hex nut
10. One 1/4" wing nut

Miscellaneous:

1. Several Feet of thin cord or twine (at least 10 feet). We used duck decoy anchor cord.
2. 2 cable ties
3. 10" square of fabric (we used ripstop nylon).
4. 17.5 pounds of iron weights (we used one 10lb, one 5lb, and one 2.5lb). Any weights can be used, but they will probably require modifications to the design. My daughter made one of these with concrete from the hardware store. Generally, denser weights are best, so iron is preferable to concrete, sand, or rock.
5. (optional) 3" long piece of 3/4" PVC pipe.

Necessary Tools: Pencil, Saw, Hammer, Power Drill/Driver, Assorted drill bits, 3/4" (19mm) Wrench, Phillips head driver bit, Scissors, Bolt cutters (or something else that will cut the head off of a nail)

Optional Tools: Center punch, Large file, Torch and vise grips (for heating and holding a hot nail), Slow motion video camera (for fine tuning trebuchet performance), Sharpie for drawing on fabric

These diagrams show the names I will be using for the trebuchet parts in the following steps. The triangle spikes, which serve to keep the triangle from slipping when it hits the ground, are most visible in the front and side views.

Make the arm from the 1/2" x 3" x 36" pine board.

1. Drill a 1/4 hole 20" from one end, in the middle of the board, as shown in Fig. 1. This will be the finger end of the arm.
2. Taper the arm from a full 3" at the axle to 1" at the finger end of the arm (Fig. 1). We used a bandsaw to cut the board, but a hand saw would work.
3. Determine the length of the counterweight end of the arm. Fig. 1 shows the counterweight end of the arm extending 14 3/4" inches from the axle hole. That was the length that worked for us. That distance may need to be different for your arm, depending on the height of your stack of weights, because the weights need to clear the triangle, as you can see in the second diagram. To determine how long to make the counterweight end of the arm, measure the height of your stack of weights and subtract that number from 17 1/4" inches. That should give you nearly an inch of clearance for the counterweight to swing through the triangle. If you allow too much clearance, then your weight doesn't fall as far, so you're giving up energy.
4. Using the length that you just calculated, measure from the center of the axle hole in the opposite direction of the tapered end of the arm. Make a mark and cut off the board.

The point here is to build a strong triangle with a base 20 1/2 inches from the axle. The triangle needs to be wide enough so that the counterweight can safely swing through it without hitting the triangle legs or base, and it needs to be strong enough to withstand extreme forces during the launch.

We tried to make the procedure simple for this instructable, so we made an equilateral triangle with plenty of extra room for a swinging counterweight. Feel free to make the triangle a little narrower. The final photo, above, compares the triangle from this instructable to the triangle from my son's winning trebuchet from last year.

1. Lay the three 36" square dowels on a floor and arrange them in a triangle, as shown in Fig 3.
2. When you cut the arm, you produced a couple pieces of 1/2" thick scrap. Use one of those pieces to represent the thickness of the arm. Lay it on top of the triangle legs as shown, and use it to mark the locations where you will cut (Fig 3).
3. Fig. 4 shows a close-up of these cutting marks ("cut here").
4. Now mark the axle hole (Fig 4). This mark should be perpendicular to the cut marks that you already made, and, therefore, perpendicular to the arm.
5. Without moving the legs of the triangle, shift the triangle base toward the axle mark until the bottom of the base is 20 1/2" from the axle mark (Fig. 5).
6. Check to make sure that you still have an equilateral triangle. The legs should be the same length. If they're not, adjust the base. Make sure it's still 20 1/2" from the axle mark. As you can see in Fig 5, our legs ended up being about 23 3/8" inches, when measured as shown.
7. Making sure to keep the legs in this position, mark the legs and the base for cutting, as shown in Fig. 6.
8. Using the appropriate marks as guides, cut off both ends of the triangle legs.
9. Similarly, cut off both ends of the base.

1. Use a 1/4" drill to drill the axle holes, parallel to the lines that you drew on each leg. Each hole needs to stay as close as possible to the center of the leg, and it should closely parallel the line that you drew. You can drill from the easy side or from the hard side. The easy side is the side where the hole will be perpendicular to the wood. To be extra sure that he drilled in the right spot, my son first carefully made a pencil mark where he wanted to drill. Next he made an indentation with a center punch, and then he pre-drilled with a 1/8" bit before finally drilling with the 1/4" bit (as shown in photos).
2. Hot glue the legs to the base in the position shown in Fig 7.
3. The next step is to additionally secure the legs to the base with drywall screws. First drill 1/8" pilot holes. Then screw 2" drywall screws into the pilot holes. You will be drilling near the ends of the dowels, so you may split the wood. We split one end (see photos). To fix it, we inserted glue and clamped it while the glue cured. We have had no problems with it.

1. Drill a 1/4" hole 4" from the axle hole, on the finger side of the arm, as shown in Fig. 8. Our hole was about 3/4" from the edge of the arm.
2. Drill a ¼” hole near the end of a popsicle stick. To reduce the chances of the popsicle stick splitting, hold or clamp the popsicle stick firmly against a piece of waste wood. Drill through the stick and into the wood. You may want to drill a small pilot hole first, to make sure that your hole is in the center of the stick. [We used popsicle sticks, because a lot of people seem to have them. However, popsicle sticks are barely big enough to drill the ¼” hole. Any thin, strong, drillable material would work well here. It just needs to hold up the projectile]
3. Repeat this with one more popsicle stick. While you’re at it, you it would be a good idea to make some extras.

4. Use the 1” hex bolt and the wing nut to attach the popsicle sticks to the arm, as shown in the photo.

We used fabric for our design because it is readily available. For my son's competition trebuchet we used a piece of net , which seems lighter and has less wind resistance.

For steps 1-4, refer to Fig. 9. For steps 5-8, refer to the photos.

1. Cut a 10”x10” (approximately) square of fabric. Nylon works well, because you can melt the holes it in it and seal frayed edges.
2. Draw a diagonal line from one corner to the opposite corner.
3. Find and mark the center of the diagonal line.
4. Draw two pairs of dots. Each pair should be about 1 ½” inches apart, and 1 ½” inches from the diagonal line, as shown.
5. Make holes where you drew the dots. As you can see in the photos, we used nylon fabric. That meant we could melt holes with a nail that we heated with a propane torch. My son held the nail firmly with a pair of vise grips.
6. Grab a corner of the fabric that is at one of the ends of the diagonal line. Tie this in a knot. Make the knot as close to the end of the fabric as you can while still getting it nice and tight. Repeat with the opposite corner.
7. Use cable ties to cinch each pair of holes together. Clip off the cable tie excess.
8. Test your pouch. Place your projectile of choice in the pouch. [We used a “high bounce Pinky” ball for the projectile. A small apple or just about any other roundish object should work.] See if the projectile stays in place when you hold both ends of the pouch. Then let go of one end of the pouch and see if the projectile rolls out. If the projectile either won't stay in or won't come out, cut the cable ties and make new pairs of holes. Spacing the holes more widely (before cinching with cable ties) should hold the projectile more securely in place. More closely spaced holes should help it roll out. Placing the holes closer to the center line may also help the projectile roll out more easily.
9. Trim off excess fabric.

Refer to Fig. 10.

1. Clip off or cut off the head-end of one of the nails, so that the remaining pointy end is about 2” long. This will be the trebuchet finger. [If you don't have nail-clipping capabilities, you could try a finishing nail. The minimal head on a finishing nail could be sanded off, or could even be left as it is, without causing much trouble with the sling release.]
2. Drill a small pilot hole into the finger end of the arm. The pilot hole should be a little narrower than the nail that will become the finger. The purpose of the pilot hole is to prevent splitting as you hammer in the nail. It would be a good idea to try a practice hole on some waste wood first.
3. Nail the pointy end of the clipped nail into the pilot hole, so that ½” sticks out. If the end that is sticking out is sharp, file it down until it is rounded. The reason for filing the finger is to keep it from scratching someone.

1. Tie a long length (maybe 30") of cord next to one of the knots, as shown. The knot in the sling fabric keeps the knot in the cord from slipping off.
2. In the same way, attach another length of cord to the opposite corner.

1. (Fig 11) Drill a hole, barely larger than the cord, through the finger end of the arm. The reason the hole is barely larger than the cord is that you will be inserting the cord through the hole, tying a knot in the cord on the other side, and relying on that knot to keep the cord from pulling back through the hole. The hole should be about 2” below the end of the arm wood. This hole needs to be far enough from the end so that it does not hit the buried end of the nail that forms the finger.

2. Insert one sling cord through the hole that you just drilled, as shown in the first photo. The direction is important. The cord should enter the arm on the flat side and come out on the tapered side.

3. Place your projectile in the sling pouch, and arrange both cords as shown in the first photo. Snug the projectile into it its position on the platform (popsicle sticks) and make the cords taut. The popsicle sticks should be perpendicular to the arm.

4. Mark the point where the sling cord exits the tapered side of the arm, and then tie a knot about ½” beyond that point. The purpose of this knot is to prevent the cord from pulling through the arm. After you make the knot, check to see that the sling cord is the correct length. If it is too long, shorten it by tying one or more additional knots near where you tied the first one.

5. Take the other sling cord and pass it around the finger, as shown in the second photo (and also the first video). In the photo, you can see my son pinching the cord and that it is doubled over. Do this. While keeping the cord doubled, remove it from the finger and tie the doubled end in a knot, so that a loop is created at the end of the sling cord. I am a teacher, and my students have a lot of trouble understanding how to tie this knot, even though it is very simple. The video of my son tying the knot and making loop should clarify the procedure.

6. At this point, you have created a weapon called a "staff sling." The second video shows my son testing his, to make sure that the projectile platform and sling work properly. Don't overdo it on the first try. My daughter tried it, and our pinky ball is now lost in the woods. That is why you will see a tennis ball in some of the other videos.

The counterweights need to have a strong base connecting them to the arm. Create the base shown in Fig. 12.

1. Cut three 3 1/2" lengths of 2x4".
2. Sandwich two of the 2x4 pieces on either side of the ½” board, at the end, and screw them together with 3” drywall screws, as shown. These short 2x4 pieces will split easily. To prevent splitting, drill pilot holes for the screws. Do not put a screw in the center, because it may interfere with the lag screw that holds the weights in place.
3. Attach the third 2x4 to the first two, as shown. Do not put a screw in the center, because that is the location of the lag screw.
4. Drill a hole into the center of the counterweight base, a little bit smaller than the lag screw. I think we used a 5/16" bit for the 1/2" lag screw, but a 3/8" should also work. You may want to drill a test hole in some scrap wood, and try the screw's fit.

1. Stand up the arm on its finger end. Do this on grass or dirt. If you do it on concrete, you may mash the finger in too far. You can also drill a hole in a piece of wood and stand the arm on the wood, with the finger in the hole.
2. Stack the weights with the smallest (2.5 lb) on the bottom. The 5 lb goes next and the largest (10 lb) goes on the top [The reason for this is that you want the largest weight to fall as far as possible. This will give your projectile the most energy.]
3. I cut a section of 3/4" PVC pipe and placed it in the weights' holes (see photo). That helped keep the weights aligned, but it isn't necessary.
4. Place the 1/2" washer on the 6" lag screw. Tighten the screw to clamp the weights in place by screwing it tightly into the counterweight base. The screw should take a 3/4" or 19mm wrench.

1. Drill pilot holes diagonally through the triangle, corner edge to corner edge, as shown. We drilled 1/8" holes. They should be small enough to keep the spikes tightly in place, but big enough to prevent splitting. To make drilling into the edge easier, sand, cut, or file a notch into the triangle base where you want to drill. As you can see, my son also likes to use a center punch to make an indentation before drilling. Make the holes about 6” from the triangle legs (far enough so the legs won't interfere with your drill or hammer).
2. Hammer the remaining two nails through the holes.
3. File the ends of the spikes so that they are not so sharp and scratchy. They just need to be able to stick into grass or dirt.

**We like the spikes, because they keep this trebuchet simple. However, if you don't like the spikes, they are not absolutely necessary. In the first and last steps of this instructable, you can see that my 500 pound trebuchet does not have spikes. It falls onto an anchored board that has a lip that prevents the triangle from sliding forward. We have also prevented the triangle from sliding by running cords from the ends of the triangle arm to the launch platform (see the platform in the last step of this instructable). The launch platform is anchored, so it stops the forward movement of the triangle.

Note that the exact dimensions described in this step are not entirely important. They worked for our trebuchet, but other dimensions would also work. If you understand the purpose of this string, you can probably disregard our numbers.

1. Make a mark 15” from the base corner of each triangle leg.
2. At this mark, on each leg, drill a hole barely larger than the cord.
3. Cut a length of cord at least 24” long.
4. Tie a knot in one end of the cord and insert the other end through one of the leg holes that you just drilled, as shown.
5. Insert the free end through the other leg, as shown
6. Tie a knot in the free end so that it will not pull back through the leg, and so that the length of cord between the two legs is 16”.

Attach the triangle to the arm, as shown. Use a washer on each side of each piece of wood – 4 washers in all. Turn the nut by hand until it begins to tighten. The joint should be loose. The video above shows how much play there is in our trebuchet axle. Don't overtighten it (but you also don't want to lose the nut). The reason the looseness is important is that the triangle doesn't always hit the ground at the angle you expect. The counterweight, on the other hand, has so much inertia that it follows a fairly predictable path. Keeping the axle loose allows the counterweight to pass through the triangle in a wide range of orientations. If the axle were tight, the relative positions and motions of the triangle and counterweight would be restricted, and the counterweight would be more likely to break the triangle.

At this point, the total weight of the trebuchet should be around 20 pounds.

Using a file or a saw, make a series of notches on the back of the arm, as shown. These notches are used to hold the tuning string in place. Position the string so that the triangle makes an approximate 45 degree angle, with the arm, as shown. Make a notch here. Make other notches on other sides of this notch. It's helpful to label the notches so that you can keep track of where you set the tuning string during each launch.

The first video shows how to load and fire the trebuchet. There are a few relatively easy ways to fine tune its performance. Here they are in order of decreasing ease and utility.

1. Adjust the triangle's position before firing by moving to tuning string to different notches. If you have a camera with slow-motion capability, take some videos your launches. As a general rule, the counterweight should be at its lowest point when the projectile releases. The counterweight should also be moving as slowly as possible at this point. If the trebuchet flops over forward after firing, or if the shot has a very low trajectory, the triangle is probably too low. If the projectile is released early, flies too high, or the counterweight hits the ground before the projectile is released, the triangle is probably too high.
2. Adjust the sling length. This is more of a hassle. It not only requires changing the sling, but it also requires moving the projectile platform.
3. Adjusting the finger. I have not needed to adjust the fingers on any recent trebuchets of this type, but bending the finger forward or backward is one way to control when the projectile is released. Bending the finger forward holds the projectile in its sling longer. Bending it backward causes the projectile to be released earlier.

In competition we fire this trebuchet from a platform, and we release it with a simple "trigger" mechanism. Without the platform, the trebuchet doesn't fall from as high as it is legally entitled to fall. The maximum allowed height is 41", but the top of the counterweight is probably at about 39" when we just stand the trebuchet in the grass. The triggering system is in place to prevent us from adding any extra energy by giving the trebuchet a push.

I have also included a video showing how this trebuchet can be scaled up. The trebuchet in the final video is 10' tall and 500 pounds. The 5 pound cantaloupe that it threw was never found. Notice that I had to modify the trebuchet to land on a board. On its debut, two years ago, it was so heavy that it sank into the ground and broke itself -- repeatedly. This last year I set it up so that the triangle landed on a 1 1/2" thick plywood pad and was stopped by a lip made of 2x6s.

**The racquetball that my son was launching in the videos above was partially filled with water to bring it up to a regulation weight of 3 ounces. The water was inserted using a needle and syringe. Water-filled racquetballs make fun projectiles.

I have recently seen a few other Walking Arm Trebuchets cropping, and they reminded me that I should include something about how to scale them up or down. One nice aspect of trebuchets is that they are easy to scale. This is what enables Dave Jordan, the VT Pumpkin Chuckin' Festival organizer, to compare the designs of 20 pound trebuchets and 500 pound trebuchets on even footing.

The Walking Arm Trebuchet in this instructable was designed to have a height of 41 inches, a weight of 20 pounds, and a projectile that weighs about 3 ounces. In order to change the size of this trebuchet and have the best chance of it functioning in basically the same way, the scaling factor for weight should be the cube of the scaling factor for height (or any other linear dimension). For example, if you want to double the height (scaling factor 2), you should be multiplying the weight by a factor of 2x2x2 = 8. So if the height increases from 41 inches to 82 inches, the weight should increase from 20 pounds to 20x8 = 160 pounds. Likewise, the weight of the projectile should go from 3 ounces to 3x8=24 ounces. The distance you can expect the trebuchet to throw your projectile should increase by the same factor as height; if it throws 200 feet, and you double the height, it should throw 400 feet.

Here is a procedure that you can follow to check your heights and weights to make sure that your newly-scaled trebuchet will perform like the original. This will work equally well for scaling up or scaling down...

1. Divide the new height by the old height. This is the linear dimension scaling factor.
2. Cube the linear dimension scaling factor (multiply it by itself three times). This result is the weight scaling factor.
3. To get any new weight, multiply the old weight by the weight scaling factor. This should work with the counterweight, the projectile, or the overall trebuchet weight.
4. To estimate the new throwing distance, multiply the old throwing distance by the linear dimension scaling factor.