Homemade Trebuchet (September 2006)

In 2006, Hurricane Ernesto blew down a 110' Pine tree in my yard. I cut off the branches and put them in a pile to figure out something to do with them. My dad and I cut the trunk into 8 to 10 foot sections and borrowed a Woodmizer to turn it into lumber. My mom told me to dump the branches in the woods, but I had a better idea.

A trebuchet is a catapult that is powered by gravity. A heavy counterweight is attached to one end of a leaver, and a sling to the other end. You can launch projectiles quite a ways by adjusting the mass of the counterweight, how much of your lever sticks out on either side of the fulcrum, and how long your sling is.

Downed Tree and Branches
A big stack of potential destruction. :D

5:1 Throwing Arm to Counterweight Arm Ratio
The arm on a trebuchet is kind of like a teeter toter. You can have a light kid sit on three quarters of the board, but a fat kid sitting on the remaining quarter could still balance it. The two sides of the fulcrum on a trebuchet are known as the "throwing arm" and the "counterweight arm" according to what's attached to them. It's a good idea to start off with 80% of the total arm devoted to the throwing side.

100:1 Counterweight to Projectile Mass Ratio
The mass of the counterweight is what makes things happen. In general, a 100:1 counterweight to projectile ratio is adequate. At the peak of my trebuchet's existence I was using a counterweight of around 150 pounds, and the golf ball I was launching weighs 1.6 ounces. That means I had a ratio of 1,500:1. After a point, adding more mass doesn't make a noticeable difference in throwing distance, but I was trying to see just how much my trebuchet could stand. It was a little scary seeing everything flex as I pulled the rod out that held the arm down.

150 lbs Counterweight
150 Pound Counterweight
     Loaded Trebuchet
Locked and loaded.

1:1 Sling Length to Throwing Arm Ratio
Trebuchets use a sling to magnify the movement of the throwing arm. When the counterweight drops, it twists the throwing arm in an arc. One cord of the sling is permanently attached to the throwing arm, but the other end slips onto a nail that is bent to make the sling release at the correct angle. The trick is to get the sling to release the projectile around 45° to produce the farthest distance. A 1:1 sling length to throwing arm ratio builds up the best amount of speed for the projectile.

I picked the longest and strongest looking branches out of the pile to use for the two A frames that would support the axle and lever. I used ropes to secure everything except the counterweight. I built the A frames fairly quickly, then grabbed some branches for the cross beams. I used some 1.5" galvanized pipe for the axle, and a hose clamp to hold the counterweight on. The trebuchet had a problem of sagging to one side, so I made some guy wires to hold everything relatively straight. I made a "trigger" for the catapult out of two big shackles and a rod that would fit over the throwing arm, holding it down until I pulled the rod out. It took me around three hours to build from start to finish.

Me standing on my trebuchet

First, the throwing arm is pushed down and secured by the trigger. Then the loose end of the sling is attached to a bent nail and a golf ball is put in the pouch. Once you pull out the rod, gravity takes over and hopefully the trebuchet doesn't shake itself to pieces.

I first started off with a single cinderblock for the counter weight, but then decided to see what 150 pounds would do. At one point the trebuchet was throwing golf balls around 75 yards, but I added too much weight and it broke. But no matter how much time I wasted, I still had to put the branches in the woods.

Destroyed Trebuchet
RIP :'(