Thread: Trebuchet

Any really big treb functions with a weight ratio of 200 : 1 up to 300 : 1. So the counterweight of 5,000lbs or more makes the thing stationary, as they were in the 12th century. Parking the beast behind a locked gate at a private gun club sort of restricts the opportunities for wholsome havoc & mayhem.

I wanted a treb which was right at the limit of portability, both for size and for all-up weight. As it is, I carry this one in pieces and assemble on site. A little more muscle here. While the little one is good for 15-20m, this one will throw a grapefruit more than 100m at 100kph. Like over the parapet above the 3-storey roof of a gymnasium. 500g ice cubes are nice (the evidence disappears), cordless telephones have no redeeming aerodynamic properties at all.

Today is very cold, it snowed early in the morning. Been snowing HARD up top all dang day. Time to put the treb to bed for the winter so it is just partially assembled. I keep the throwing arm and all the steel in the house. Feet & braces go into the shed, the frame gets a roped-down tarp over it.
1. Pretty much what it looks like. The slide is 2 x 12 x 64. The tower is 2 x 6 x 48", bracing is cut-to-fit 2 x 4. The feet are 2 x 4 x 44, each is bolted under the slide with a pair of 3/8" carriage bolts. The throwing arm is oak 2 x 4 x 70", short side is 14", long is 53.5". The steel axle is 3/4" x 11.5" Some threaded rods and glue. All the bearing blocks are 2 x 4 mahogany.
2. The counterweight axle 3/4" x 6" steel. The counterweight frame is 1 x 2 x 37" steel with a 2 x 2 x 12" crosspiece welded in the bottom.
For portability, I have a stack of 2 x 2 x 16" steel bars and some railroad track. All up is less that 150lbs, I really want 200+lbs. Of course, I could get a lot more steel welded on but that destroys the portability for me. The shape and length need to be designed so that the counterweight won't crash into the slide.
3. I used a combination sling finger and trigger finger this time. Just a gate latch mechanism set below the slide in a notch. Pull rope to release it.
4. C/U of the latch. There's a safety cable but left it in the house. This treb can rearrange the anatomy of your body parts in an instant.
5. Detail of the throwing arm axle arrangement = it simply drops into a couple of 3/4" slots cut in the mahogany.

A way to increase the throwing power is to mount the whole shebang on wheels. The idea is to get the counterweight to fall as close to vertical as possible; the way you've mounted it via a hinged joint helps. If you fit wheels the whole machine will roll backwards and forwards slightly but the counterweight can drop almost vertically. Try popping the banquet buster on a few dowels to test the theory.

Many years ago I watched a British TV show where some mad aristocrat was determined to build a trebuchet that would destroy walls. He'd already designed and built machines that would throw a piano, he just wanted to make it bigger. The throwing arm had to come from Canada as he couldn't find a big enough pine tree in Europe.

The British designer is the wealthy engineer, Hugh Kennedy. History records that the "War Wolf" was a trebuchet large enough to throw a dead horse more than 300 yards into a castle courtyard. 300 yards being about the limit of the defending castle archers. HK was determined to reconstruct such a siege engine. He did so, estimating the horse to be 1,500lbs. It took 40,000lbs in the counterweight. He refuses to show how that treb is readied for firing and what his trigger consists of. Actually, the flaming toilets and the Hillman cars are much more entertaining to watch.

Suppose a castle wall is rammed earth, faced on both sides with cut stone, likely 10-12' thick. When that is hit with a 600lb rock, very little happens on the outside. The shock wave penetrates the wall and the inner surface explodes into the courtyard, killing or injuring anyone in the face of the blast.

I considered a rolling frame and rejected the idea. The post-release oscillations are bad enough as it is. Further, the notion of a fixed counterweight is sheer lunacy (but it was done). The cost of the metal was prohibitive in the 12th century. Nevertheless, I'll try it with the banquet-buster. In fact, the counterweight does fall vertically until the moment of sling release. After that, nobody cares. In situ, in a castle siege scenario, wheels would simply sink into the earth.

I have an 11 year old son who is currently very into Crush the Castle game, and will not be allowed to view this thread in case he gets some ideas. His dad on the other hand is definitely getting some ideas.

There ya go Peter! Wholesome violence that the whole family can participate in.
The two of you need to build a "banquet-buster." Just the right size. Better yet, build 3 and sell two of them $50 each.
The lead ball was $8.95 and there can't be $5 worth of survey-stake quality wood. Bunch of #6 Robertson screws.
Getting the sling pouch the right shape, getting the sling ropes equal(so both pull evenly) and getting the finger bent just right for payload release are the fussy parts. The finger is just a finishing nail with the head cut off.
Gum nuts, peach pits, cherries, what else have you got?

Rich English Guy and his Trebuchet - YouTube

The Medieval Trebuchet - YouTube

Thanks for those links, Woodwould. Nobody ever reveals all the tinkering and fine tuning. The apparent speed increases as I made them smaller. The big ones are positively graceful, the power in the low speed is deceptive.

There are a heap of trebuchet related sites, as well as ones covering torsion powered engines (ballistae). West Point even has an on-line trabuchet calculator, which you can enter your arm dimensions, counter weight & projectile details into & it will calculate the flight path & range for you.

I'm not going to post links as the 'Department of No Fun' seems to be getting stroppy about anything connected to devices that they don't licence. One of the best ballista sites is now only available on the 'way back machine' site.

Using the axle location in the throwing arm as zero, range and trajectory cannot be calculated until you know the ratios for lengths and weights, first. Then tower height so the counterweight doesn't crash into the slide. Do you know the correct length and angle for the sling pin
to get the correct length of the sling ropes to open at the correct moment in the arc of travel? Is the sling basket the correct shape? Quite a few inter-related puzzles to solve.
What solution did I find? The correct one. If you want to change one thing, everything else has to be adjusted to stay in proportion. When you've got the design optimized then you can scale the trebs up and down in size. That's the real beauty of the beast: From the payload, all the rest of the design opens out. That implies that a cocktail olive is exactly the same as a 5kg sack of flour.

Anyway, collect the materials and build a few. By #4 you should have it worked out. #10 - #14 were just issues of assembly.

Readers that get this far may have noticed that I gave measured dimensions for many parts.
Why? From those, you can derive the ratios of lengths to build a treb which takes very little tinkering.
Consider the sling ropes as just one example.
Of course, they must lie flat and untwisted for proper release function.
They must offer equal tension, equal resistance/pull in the acceleration.
Just as 24" won't work well enough to be useful in the least, neither will 88".
Sling length is predicated on tower height and throwing arm proportions.
Have fun.