My friends Ron & Eleanor was coming to visit us on the 4th of July, and I thought that building a trebuchet would be a fun activity for Jenna & their two boys.
There are lots of resources on the web for trebuchet designs, but I wanted something that we could quickly whip up in a few hours, but that would be more than a toy model. After a critical video iChat with my dad, we had a list of materials and were off to the hardware/lumber store.
I used the MacTreb program (downloaded from http://www.algobeautytreb.com/index.html, but seems to be off-line right now) to simulate the design. Though in the end, I think the light weight of the tennis balls we were using meant that air resistance played a major role in distance, thus the range calculated (370ft) was much greater than what we achieved (about 250ft).
Here’s the end result:
Ron & Finished Trebuchet
We used a 20lb dumbbell as the counter weight. The throwing arm (8ft x 1″ x 4″ pine) has 2″x4″ blocks attached on either side of the pivot point, then we screwed 3/8″ lag bolts in from either side. These rotate in similar sized holes drilled in the vertical support boards (5ft x 1″ x 6″ pine).
The sling was made out of an old plastic shower curtain, using a pattern I found on-line…but I can’t locate it right now.
Finally, there’s the always-fun release mechanism – both for the throwing arm, and the sling. I put two hook screws in, and that worked well enough as a way of keeping the arm held down while I fiddled with the sling.
The sling itself was permanently attached on one end, and had a link from a chain that slid off an angled metal rod. Adjusting the angle of the rod let me tune when the ball would actually come free from the sling, and thus the angle of release.
Throwing arm & sling release mechanisms
In the end it all worked, though my wife was less than impressed – she had visions of us launching bowling balls over the neighbor’s roof. Things to change for version two:
- Throw a denser object – a lacrosse ball feels about right. The tennis ball just slowed down too fast once it came out of the sling.
- Make the sling pouch size a bit bigger. We sometimes had launch failures when the tennis ball would pop out just as the sling started moving.
- Use thinner & lighter cord for the sling. The rope we used seemed a bit out-of-scale with the weight of the object we were throwing.
- Angle the support boards in from a wider base. The dumbbell almost didn’t fit, and sometimes hit the boards during a launch. And we wanted to add more weight, but there wasn’t a good way to tie on the second hand weight without adding any width.
- Add bracing from the back of the main base board to the upright boards. The trebuchet would rock back and forth without this bracing.
But all in all it was a fun activity for a 4th of July weekend – highly recommended!
Ken's Techno Tidbits 
As an example of a big trebuchet, see http://www.youtube.com/watch?v=UEho24rgryE for car throwing. Now that definitely would have impressed my wife.
Ha. Don’t aim across Deer Creek or you might hit my brother.
Loved the post re: MIT class of ’83. Your hair was the same in high school IIRC.
Well, that was my freshman year photo.
Where does your brother live?
What did you use as the axle at the top of the throwing arm, and how does your release mechanism work?
Thanks.
@Jonathan – if you’re talking about the axle that the throwing arm rotates around, then I screwed two 2″x4″ blocks on either side of the arm, and put two 3/8″ lag bolts into the blocks from either side.
The release mechanism is just a single chain link tied to the end of the sling rope, which slides on a metal pin (about 1/4″ thick) that I bent and attached to the end of the throwing arm.
If anybody is having trouble with the projectile, try Golf Balls. Golf balls work well because they are lightweight, have little air resistance, and are designed to fly far.