After finals this past semester my roommate and I decided to build a "big" trebuchet with K'NEX, so that unorganized mess of plastic in my closet could finally be put to good use.
I decided to submit this instructable after the trebuchet was already built, mostly because I wasn't sure how it would turn out. But I took some pictures and have a few videos so you can get an idea of how it works and how it was put together.
One additional comment before we get started. I'm really not sure what the range of the trebuchet is, considering I cannot test it outdoors because of the weather. However, adjusting the range is pretty easy though: shorter sling yields earlier release; shorter boom tip yields earlier release.
Also, there are plenty of cool ways to rig up a trebuchet, this is just the way my roommate and I did it. Have fun and be sure to shoot as many projectiles indoors as you can (I recommend practice golf balls).
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Step 1: The Five Major Components
Step 2: The Boom & Counterweight
The boom is the first major component we decided to put together. The size of the boom more or less determined the size of everything else. The other stuff in the pictures, including the counterweight was considered after the boom was assembled.
In order to keep the boom from shaking apart from the counterweight we tried to add supports where we could. Ultimately seeking to eliminate twisting of the boom and side to side swaying of the counterweight. See the pictures for notes on the supports.
The distance the axle must span was also an important consideration: if too wide, supporting the boom may become difficult, if too narrow, the counterweight may not have enough room to shake side to side. Ideally the shaking of the counterweight would never be a problem, but in this world, ideal is simply fiction.
Step 3: The Frame
Making the frame is pretty straight forward, but also time consuming. There are a few frame options: red columns with grey ties, yellow columns with red ties, or blue columns with yellow ties. Off the bat I knew I didn't want to use the first method because it would have been too bulky and not rigid enough, and I wouldn't have been able to use the last one cause I am sure I don't have enough pieces.
Step 4: The Trigger
To Tell ya' the truth, I think this little deally is the coolest part.