The Most important part of the Trebuchet
The first thing you need for this project is the axle or fulcrum that the swing arm will pivot on. This is the most important part of the project because a smooth ball-bearing axle like that found in a bicycle will give you an excellent transfer of energy with very little waste.
I used the axle from an old BMX bike. To get the axle you should be able to easily disassemble the wheel. Most bicycles have spokes that are removable with a screwdriver like shown in the picture here. I just removed all the spokes. It was easy enough and I ended up with the axle like this
Assembly of the Trebuchet Swing Arm
I used two pieces of wood (shown in this picture) The wood is 3/4 inch thick and 1 1/2 inch wide. The pieces are 48 inches long and 10 inches long.
Make a mark in the center of the ten inch piece. Then make a mark twelve inches from one end of the long piece. Align these two pieces up and this is where the axle will be placed.
Now screw these two pieces together, keeping the lines aligned. Do not use nails! Nails are not strong enough and you need to take this assembly apart then re-make it. And put the screws so they are at least one inch away from the line you drew. It needs room for the axle.
Measure the diameter of your bicycle wheel axle and drill a hole in the sandwich assembly of the trebuchet arm. This hole needs to be slightly smaller than the axle diameter. My axle was 1 inch thick so I drilled a 15/16 hole and it was perfect. This way you can put the axle right inside and tighten it right up so it doesn't move.
Open up your sandwich assembly by undoing the screws then place the axle inside and redo the assembly by re-inserting the screws. It should hold the axle nice and tight. If your axle slips it will not work well but it is easy to fix. You just open it back up and wrap some tape around the axle to make its diameter bigger.
Now Let's Finish the Trebuchet Swing Arm
Now insert an eyehook in the short end of the arm. It should go close to the end and on the same side as the smaller board.
Think of this side as the underside of the swing arm.
On the long end of the swing arm insert an eyehook on the underside. This eye hook protrudes from the same side as the eyehook on the other end. (we call this the bottom side)
Now, and this is important: insert a headless nail into the end of the swing arm. It should point forward at an angle. And this angle is important because it will determind the arc which your projectile flies. Changing this angle will change the distance and height that you hurl things at. It needs to be a headless nail because you will put a ring on it that will slip off when the arm swings.
Ok, the swing arm is complete now let's continue on with the building of the base of the trebuchet.
Continue with the Bicycle Wheel Trebuchet tutorial
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