This is the world’s newest trebuchet design and it’s already setting new hurling records. Not only is the Walking Arm Trebuchet simple, it’s also extremely efficient. Using this trebuchet, my 8-year-old son set the all-time record for best design at the 2018 Vermont Pumpkin Chuckin’ Festival. His lightweight 20-pound, 41-inch tall trebuchet threw a 3-ounce ball 266 feet — scaled up, that’s equivalent to 780 feet in the heavyweight division. My 500-pound, 10-foot version threw a 5-pound cantaloupe over 700 feet — also a new record — but it went so high and so far that the spotter never saw it pass over him on its way into the woods!
This year my 500-pounder threw a 5-pound rice-filled soccer ball 870 feet (and it rolled to 975) but unfortunately at the 2019 Pumpkin Chuckin’ it threw so hard that all the real fruits were crushed during 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 Vermont festival. As you’ll see, my son did all the work himself. I did help out by holding some things in place and showing him how to complete the steps.
I’ve 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.
A STRIPPED-DOWN HURLER
The Walking Arm Trebuchet has many of the advantages of a floating arm trebuchet, but with fewer parts, less friction, and a unique projectile launch path: as the trebuchet walks forward, the projectile never swings backward beyond its starting point, reducing the danger to bystanders behind the trebuchet.
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.
AT MY SIGNAL, UNLEASH FUN!
LOAD AND FIRE
To load your trebuchet, stand it upside down — with the counterweight on the ground. Put a tennis ball in the sling pouch, position the pouch below the popsicle sticks, and loop the sling over the finger. Adjust the sticks to hold the pouch snugly.
On grass or dirt, turn the trebuchet right-side up (counterweight on top) and hold it there while you set the triangle string into a tuning notch of your choice. Aim your trebuchet downrange and make sure it’s all clear.
To fire, just nudge the trebuchet forward so that it falls squarely onto the triangle … step back, and watch it walk and hurl!
TUNE IT UP
There are a few relatively easy ways to fine-tune the trebuchet’s performance. Here they are in order of decreasing ease and utility.
• Adjust the triangle’s position before firing by moving the tuning string to different notches. If you have a camera with slow-motion capability, take videos of 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
• Adjust the sling length. This is more of a hassle. It requires not only changing the sling, but also moving the projectile platform.
• Adjust the finger. I haven’t 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.
COMPETITION LAUNCH SETUP
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.
The racquetball that my son launched in competition 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.
SCALE IT DOWN
In response to one of my Instructables readers, I made a small 3D-printed version of the Walking Arm Trebuchet. I was going for a desktop version, but really it seems at home on carpet. It’s a rough version 1.0, but I posted the .stl file on Instructables if you’d like to print one.
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. There’s no pouch; the sling is just a loop of fishing line that’s permanently attached to the projectile. They fly off together. The proportions are different from the larger versions, which may be why I needed to bend the finger forward on this one to get a good release angle.
SCALE IT UP!
My heavyweight competition trebuchet is 10′ tall and 500 pounds. The 5-pound cantaloupe that it threw was never found! On its debut two years ago, the big trebuchet was so heavy that it sank into the ground and broke itself — repeatedly. So last year I modified it: I set it up so that the triangle lands on a 1½” thick plywood pad and is stopped by a lip made of 2×6s.
EVOLUTION OF THE WALKING ARM
The Walking Arm Trebuchet’s evolution was driven by the unique rules of the Vermont Pumpkin Chuckin’ Festival. Dave Jordan, founder of the festival, set a limit on overall trebuchet weight, rather than the counterweight. If it hadn’t been trying to keep my design as light as possible, I never would have invented this trebuchet.
I wanted a design that would allow the counterweight to fall as far as possible, to maximize potential energy, with a long throwing arm and minimal friction. I also wanted the trebuchet to “shift gears” on its own.
The floating arm trebuchet (FAT) invented by Ron Toms seemed like a good place to start, because the FAT’s weight falls nearly its entire height and it “shifts gears”: because the throwing arm’s axle floats, rather than having a fixed axis, each small movement of the falling counterweight results in an increasingly greater movement of the projectile. (Adding wheels to a medieval hanging counterweight trebuchet (HCW) has a similar effect.)
My first attempt didn’t walk. It was an arm standing on end, with the weight on top, supported at its center by an axle. The axle was on a simple stand that pivoted on the ground, allowing the axle itself to swing forward or backward. As the counterweight’s inertia carried it downward in a relatively straight path, the axle and arm were forced backward, then pulled forward again. This motion flung the projectile in a path similar to that of a FAT, but without all the extra structural weight.
The main problem with this design was that it required high-tension guy wires to hold everything in the right launch position. And creating a trigger to release all the parts simultaneously was tricky.
I added a joint in the arm, like the King Arthur (KA) trebuchet invented by Chris Gerow. This let me get rid of the guy wires. At rest, the trebuchet had become a tripod, with the throwing arm standing on the ground behind an axle stand that extended forward. When fired, the forward inertia of the arm’s top segment pulled the whole trebuchet with it, causing the arm to walk forward and swing over the axle. I was smitten by the simplicity and the walking nature of this design. My daughter used it to win the lightweight division in 2015, but it just didn’t throw things as far as the first design. It was also harder to tune. So I tried to marry the two.
DESIGN #3: EUREKA!
Eventually I realized that if I lifted the axle stand off the ground, the trebuchet would tip forward onto the axle stand, walk, and throw. It worked, but not until I added spikes to keep the axle stand from slipping on the ground. The axle stand that had been rooted to the ground in Design #1 became the triangle in my fully evolved Walking Arm Trebuchet.
The idea of staging the projectile on a platform attached to the arm came from a whipper trebuchet, a HCW variant invented by the late, great Raymond “Ripcord” Goodsell, that I’d seen — where else? — at the Vermont Pumpkin Chuckin’ Festival.