How to Build a Trebuchet (1 Meter Scale)

1. 1
   Like any project, first do your research. There are many other interesting trebuchet sites on the Internet.^[1]
2. 2
   Make a plan and draw it up. Keep it simple; something buildable and within the guidelines. A low braced triangular design was chosen for this project.
3. 3
   It is important to have a plan. You may not follow it exactly but you will find out that you have left out some important detail from the plan. That may become quite obvious when you start building. Here's an overview of what you will need to do:
   
   
   • Make a rectangular base.
   • Install the up-right supports and angled support braces.
   • Build and install the throwing arm (a lever).
   • Build the shot trough.
   • Add the ballast bucket (a counterweight).
   • Add the trigger and trigger adjustment chain.
   • Add the sling/shot-pocket assembly.
4. 4
   

   Build a base
   Build a base and side supports. If it starts out square, it is more likely to stay square, so pay attention. Measure, cut, and lay out 2x2 base parts. Apply wood glue and screw together.
5. 5
   

   Throwing arm detail
   Attach a chain to the throwing arm. The ballast bucket (kitty litter container with rocks in it) will be attached to the chain (so it can swing freely). The chain is anchored by a screw through a washer into both 2x2's. Route or chisel out a small channel to accommodate the chain and washer before screwing the three 2x2's together.
   
   
   • Note the original wooden dowel "cross bar" which was later replaced with a metal tube (conduit). The cross bar is the pivot point or fulcrum between the counterweight and the far tip of the throwing arm. Reduce the friction where the crossbar goes through the supports by using graphite lubricant or better yet, roller bearings.
   • Also note the original drawn plan the arm is lying on.
   • 

     Add supports & crossbar
     The wooden towel rack dowel first used for the pivot was too weak to handle the load of a half a bucket of swinging rocks. A length of metal conduit was substituted. It has greater strength.
   • There is a screw going through the 2x2 and into a hole drilled in the metal conduit so the arm must move when the pivot rod moves.
   • The throwing arm should be light but sturdy. If the arm is heavy you will use energy moving the arm that could be used in throwing the ball.
   • One string is fixed or permanently attached to the throwing arm tip. The other string has a metal ring attached to it. It slides off a short metal rod driven into the tip of the throwing arm. When the ball is thrown, the ring slides off the metal rod at approximatly 90 degrees to the ground. It is like a rockthrowing sling (think David and Goliath), one end stays in the hand, the other is released to toss the stone.
6. 6
   

   Trebuchet assembled
   Install the shot trough and attach the ballast bucket to the throwing arm chain. When the bucket of rocks drops, the throwing arm rotates and the sling throws the croquet ball.
   • Note the wooden end-caps on the metal conduit outside of the support arms, they add lateral stability.
   • Note in the background the custom "double handled" kitty litter bucket. The handle of the attached bucket is bending, so it was replaced with a bucket that had an extra handle added to it.
   • Note the croquet balls this trebuchet was designed to toss. Several rocks were added to the bucket as ballast.
7. 7
   

   Trigger Detail
   As they did in medieval times, you can use a carefully bent coat hanger for the trigger.
   
   
   • The bend in the base of the hook (where it goes into the trigger adjustment chain) is important. That slight offset allows the trigger to catch and stay on the release eye bolt. When the string is pulled, the trigger rotates around the hook and disengages from the eye bolt.
   • This is a safe, reliable, and easy to fire trigger. It uses a fulcrum and an offset "center of tension" for mechanical advantage. The small trigger string can be pulled from a safe distance away from the trebuchet.
   • There is a length of chain attached to the back of base unit. It was marked in 10 centimeter increments. The trigger hook location on the chain is adjustable. Moving the trigger closer to the catapult causes the arm to cover a greater distance; which in turn throws the ball farther. Moving the trigger further away from the base uses less arm swing, and therefore throws the ball a shorter distance. The calibration chart below shows where the trigger hook should be placed on the trigger adjustment chain in order to throw the ball a known distance.
8. 8
   

   Early try at shot pocket
   Add the sling/shot-pocket assembly. At first, the top cut out of a "Gimme" cap was used for the shot pocket. This did not work well. The design was altered to use a square piece of leather for the shot pocket. Note the four attachment points of the string (like a Y). With just two strings to one cloth or leather patch, it is like balancing a ball on a knife blade... difficult. If you bifurcate (split like a "Y") the line to have two attachment points on each side, it makes a natural pocket in the middle.
   
   
   • The shot trough keeps things from getting tangled up and assures a straighter throw. There are raised wooden strips on each side of the flat shot board to keep the ball centered and in-the-groove.
9. 9
   Test.
   
   
   • Find out how too few ballast rock affects the throw. Sling would not let go of the ball which caused it to hit the dirt right just in front of the catapult. Add more rocks.
   • Find out how too many rocks can damage the ballast bucket handle. Make double handled kitty litter buckets to deal better with the stresses.
   • More rocks for ballast make for further throws, but you can also destroy the machine quicker. Find a sustainable balance; too many rocks in the ballast bucket does not throw well, too few rocks does not throw well, it must be "tuned" for what you are throwing.
   • The same thought applies for the sling ropes, they can't be too long or too short but must be "just right" for it to work. You will need to experiment.
   • Make sure the rope lengths are even and knots are secure. Line length is critical, when the arm is vertical, the shot pocket should be just above the cross bar.
   • Having the release string and the "static" string affixed to the same location (throwing rod in tip of throwing arm) is critical for accurate repeatable throws.

Guidelines of Catapult Contest

1. 1
   This is a high school physics class assignment with a simple set of guidelines...
   • The base can not exceed one meter in length and 0.5 meter in width.
   • The throwing arm can not exceed 1.5 meters in length.
   • Wood, metal, and/or plastic can be as construction material.
   • No electrical devices (AC or DC) are allowed.
   • The catapult must have a locking device. You are not allowed to hold the throwing arm in the cocked position.
2. 2
   Construction of a catapult that meets the above guidelines: 55 points.
3. 3
   Distance contest: Each student will get three launches. Each student will be ranked based on their longest launch.
   • 1st place = 30 pts
   • 2nd place = 26 pts
   • 3rd place = 22 pts
   • 4th place = 15 pts
   • minimum distance 6 meters = 10 pts
4. 4
   Accuracy Contest: There will be three targets placed at three different distances. Each student will be required to hit each target with a beanbag launched by their catapult. Their catapult can not be moved more than 3 decimeters in any direction. Each student will be ranked based on the number of launches that is takes to hit all three targets.
   • 1st place = 30 pts
   • 2nd place = 26 pts
   • 3rd place = 22 pts
   • 4th place = 18 pts
   • 5th place = 14 pts
   • Hit all three targets using less than 50 shots = 10 points