Picture of Floating Arm Trebuchet
This is a general description of how to create a floating arm (f2k) trebuchet.
A f2k trebuchet has an arm that rides on wheels, which roll forward on rails, while the weight bar running through the arm falls down in a vertical groove. (Watch plenty of YouTube videos to understand the motion of this design) The wheels start off above the rails, and the arm is initially supported by a second set of wheels (or one wide wheel) on a stationary axle fixed between the two halves of the frame near the rear end of the rails. The wheels on the arm hit the rails and immediately begin rolling forward, and the arm tip moves in an elliptical path.This is supposed to be a very smooth, quiet transition and the actual impact is very low.

Here's a YouTube video of our trebuchet:

This design was for a school ASME (American Society of Mechanical Engineers) Club competition. The parameters for the contest were as follows: Design a trebuchet that launches pumpkins (about basketball size). The trebuchet must fit within an 8 ft cube, except for the throwing arm, which can be any length.

Our design was very stout and held around 720lbs in counterweight, not including the weight bar and arm itself.
We launched pumpkins and bowling balls around 10-15 lbs at distances of 300+ feet.

Caution: This particular design is a very time intensive project (expect 60+ man hours) and requires some metal fabrication, such as cutting, welding, and drilling.
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_ duke made it!2 months ago

Made this a couple months ago. Got materials from the tip and some plywood from bunnings. Total cost was around $70aud.

Ill try to get a video i can upload but we threw a 3kg rock about 90meters with 80kg counterweights, we didnt have concrete so we had to use 4 x 20L water jerrys. The jerrys take up alot of space to we could only get 80kg worth on.

Over all pretty good thanks for the plans. Im hoping to re make the arm if i can get a drill press, not having a straight axel for the wheels meant that it would sometimes derail.

hammertong3 months ago
This is way too cool. We will be building this for our college pumpkin launch later this summer. I'll post pictures as we get underway. Thanks for writing this 'ible.
urbanmx8 months ago
Oh man that is awesome. I've always wanted one did you ever build a smaller one to start with? Does it take a small army to transport it?

We didn't build any smaller ones, we just went straight for the prize. The machine comes apart in 6 pieces; the base, four upright sections, and the throwing arm. They are bolted together with 1/2 inch hex bolts. We've contemplated building a pair of smaller ones after the fact. Something to launch tennis balls at each other.

randomray urbanmx8 months ago
You can check out YouTube for more versions and there's a really nice one that a high school student built.
snoopindaweb8 months ago

rizdek8 months ago
Neat. I like to play around building twisted rope driven catapults/onagers and also trebuchets. I went for tiny and built a ~2" trebuchet that launched kernels of corn about...6-7 feet. I couldn't get a small enough pouch that was flexible, so I glued 4 lb test fish line into a tiny hole drilled into the corn kernel. I had tied a tiny loop in the fish line...the overall length of the fish line, loop and all was a little less than 2 inches. The loop then fit over a tiny wire hook at the end of the arm. The trebuchet used a 1 oz fishing sinker for weight. The biggest problem was losing the corn kernels{:
bricobart8 months ago
A few times in the year there's an Instructable that stands out above the rest. This is the first of the year. Awesome project, great craftmanship, well explained, crazy as hell! Add some wooden wheels - or mount it on the chassis of a small car - and you're done! This is gonna be a great summer project, thanx for sharing guys!
scull_mech (author) 8 months ago
A good way to estimate the range is to use conservation of energy. I'd maybe assume 50% efficiency as a conservative estimate for the f2k (it's probably higher), so the initial kinetic energy of the projectile is equal to the gravitational potential energy of the falling counterweight times the efficiency. (KE = .5*GPE, so .5mv^2=.5mgh) Solve for the velocity. I used a projectile motion program I wrote using Matlab last year, and assumed a release angle of 45° and initial height of 15 ft. There's tons of assumptions involved there (what's the drag coefficient of a pumpkin?), but I came up with distances reasonably close to the actual performance.
scull_mech (author) 8 months ago
In response to urbanmx, this was our very first trebuchet. We actually did not have motion software, we just looked at lots of YouTube videos. For a traditional trebuchet, the optimum weight ratio is 133:1. I have no clue what it is for f2k, but it is a more efficient trebuchet. I'd guess that for throwing oranges, you'd want to be around 5-6 ft high and an 8ft arm, and maybe 50-100 lbs of weight? You may even be able to get away with less. I'd look around on YouTube and compare different sizes of trebuchets. I've definitely seen some trebuchets in that size range throwing tennis balls, 6" steel shot, etc very far.
scull_mech (author) 8 months ago
We did win, but barely. We threw exactly 300ft, and another team using a traditional trebuchet, a longer arm, and 1200 lbs of counterweight threw 294 ft. I was a bit disappointed in the organization of the competition, and most of the pumpkins were much larger than we had been told they would be. I think we only won because we picked a small pumpkin out.
urbanmx8 months ago
One more question if you did build a smaller one to throw oranges would it throw them a distance much further than throwing them with your arm? I really want to try this at a much smaller scale but it would suck to go through all the trouble and have my buddies throw one farther by hand.
urbanmix, you can use workingmodel software to understand the motion and know how much force is needed to throw your oranges. I think this is the way they did, as there is a working model picture at the beginning. Just don't forget to include aerodynamic drag in your calculations, or you'll end up with a overestimated value.

Great project, if I had the time and resources I really would build it!
Kiteman8 months ago
Awesome job.

Important question: Did you win?