Introduction: The Skein of Pain - AKA Mangonel - Torsion Catapult

Since I was young I have had an infatuation with all things siege engine. My first love will always be the twelve-foot-tall trebuchet I made in high school shop class 25yrs ago. The sound that a grapefruit makes moving a hundred feet per second is not something I will ever forget.

The Mangonel, a torsion catapult, however, has always called to me and finally I have succumbed to the siren’s song.


This is a siege engine. They were designed by men who destroyed things, for destroying things. The power a torsion catapult can generate is more than you would expect given the materials and size. You could easily give yourself a gap toothed grin by having your face in the wrong place when cocking or triggering the weapon. Unless your idea of fun is spending the weekend in the ER or you love dental work, be careful.

Now… that aside… hold my beer….


I had planned on hewing this Mangonel from oak logs for a more “authentic look” (and cause they’re free and I’m cheap) but to facilitate the building process for others and the fact that my wife wants me to do other projects, I used dimensional lumber from Lowes.

I wish I had gone with my gut. This project had some serious scope creep…

You will need:

2x4’s. I used 8’ studs. 5 of them. You could probably get by with less but they’re $2.25.

Wood glue.

2.5’’-3’’ Nails

String – nylon works and its inexpensive. My 50’ roll was $8.

Something to use as a throwing arm. I had thought that a 2x4 would be a.) too slow and b.) too weak so I used a maple sapling I downed this winter while making trails. Additional bonus, its already tapered.

2 – 6’’ sections of steel pipe

2 metal U bolts

1 eye bolt

So I mentioned scope creep… My first version ended in failure and I had to redesign on the fly. I ended up using odds and ends from my endless supply of garage crap.

Head from a lacrosse stick

some simpson truss nail plates

some metal banding

5’’x1/4’’ bolts

16 gauge nail stop plates

and ratchet straps to name a few items….


I made a quick design on a napkin and built it in my shop from oak and Kreg screws. Worked like a charm, never has a sloth sailed so far through a kitchen. I figure, sure tiny one works, lets just scale it up. Think of how far we can throw sloths with a big one!

Well… scaling doesn’t always…scale well.


When done correctly there will be a lot of load transmitted to the wooden frame from the wound string. This requires a structure built strong enough to support it, or… implosion.

We don’t want implosion.

Version 1.0’s design was not good when scaled up from the model and it failed. I’ll just say its first throw was less than inspiring…and then it imploded.

It relied too much on fasteners and glue doing a good job, and they didn’t.

Don’t be version 1.0.


Having seen a lot of people drill holes through dimensional lumber for the skein and had their frames crack I decided to go another route. Instead of drilling holes I would laminate 2x4’s together and just leave a hole big enough for the rope. This accomplished two things.

1.) it was cheaper and I didn’t have to hog out a 2’’ hole for the rope and

2.) it is stronger as the 2x4’s are now stressed in the 4’’ dimension vs the 2’’.

Plus its laminated so the grains are not all in the same directions and its (presumably) less likely to split. This didn’t necessarily end up being true… more on that later.

Version 2 was built using mostly a laminated mortise and tenon frame. I still used the laminated 2x4 construction but this time used the horizontal pieces more effectively to keep the frame from imploding. V2 also had the pivot further up and I decided to angle the torsion frame forward to get more pull on the arm when at full draw. I also added a metal plate behind the pipe since tensioning V1 the pipe ate into the wood significantly… and just for grins I wrapped some of the parts subject to the most force in steel banding and through bolted all of them.

I had spent too much time on this and didn’t want my family laughing at how bad it was. Again.

Once the torsion box frame was completed I through bolted it together at the corners and strapped it to a quickly built stand with cam buckle straps. I didn’t want to waste too much time on a fancy stand if the darn thing was going to crush itself again.


Round off any areas where the string goes through or could possibly touch. Sharp edges + tensioned string = instant failure.


Up until now you have built the worlds most uncomfortable lawn chair. Its time to lace up the beast.

Take your rope and run it through the skein hole on one side of the Mangonel. Run it through the middle to the other side. Now leave a bit of a loop outside the frame and run the rope back through the hole. Do the same on the other side.

Look at the pics, its easy to do but sounds goofy when I write it down.

I suggest using 50’ or less. Surprisingly this step is quite tedious and you really can’t fit more rope through the 1.5’’-2’’ hole you have anyhow.

Once you either a.) run out of rope or b.) run out of space in the hole then insert your pipes in both of the loops (clamping to the frame keeps them from falling off) tighten everything up and tie the rope off to itself with a knot that can take a lot of load. I used a double fishermans knot.


This is the finger pinching, teeth cracking part. We are going to make the string spring from the skein of rope we just wound. It’s the make or break point for your design.

As mentioned, the torsion skein puts a real hurting on wood and if the structure isn’t strong enough its likely to implode. (see V1) Wear safety glasses.

I started off using a crowbar to get more leverage. Once that no longer worked I moved up to a 5’ long breaker bar and cranked maybe 10 full revolutions before I started hearing cracking noises. Probably best to stop then.


You need a basket, cup, giant wooden spoon… something at the end of the arm to hold you’re ammunition. I used a lacrosse stick head. If you have one laying around then shave down the arm until the head fits. If you use something else, just bolt it to the arm.

Adjusting the head will affect the release angle of the projectile. You’re looking for about 45 degrees off horizontal. I had to adjust the pouch of the lacrosse stick to make it deeper, the first shot line drived it into my yard about 10 yrds out and about took out a flock of chickens.


The wonderful thing about triggers is triggers are wonderful things. They’re three little hoops of metal, that release the power of springs.

It’s simple and it works.

Three hoops. Two hoops on the bottom, third one gets attached to the arm.

Trigger release bolt goes through all three.

Tie a rope to the bolt.

Pull hard, siege begins.


Use a long string on the trigger. And find something to shield yourself for the first shot. Definitely do not let anyone stand directly behind it until you know how its going to release. I had one that fired straight backwards, one that shot straight up, and one that drove it straight into the ground right in front of the Mangonel.

The head will likely need adjustment to get to the 45 degree release angle, just keep adjusting and firing until you get it.


Once you get it dialed in, anything is fair game. After the adjustments we were able to fling rocks, chunks of brick, concrete skulls, sections of logs, and lacrosse balls…

we tried pretty much anything we could fit in the Mangonel’s pouch. Oddly enough lacrosse balls worked the best. Our first trial shots had us flinging them about 180ft. I put the gorilla hand to the torsion box again and tightened the skein string spring two more rotations.

TWAAANG! almost 300 ft… but man… it got so hard to pull back I had to put all my weight on the arm to get it down to the trigger. My wife asked me how on earth I was gonna fire it if I tightened it more. WINCH.

No, not her, I was gonna use a winch. Honestly if I thought the torsion frame could take it I would force a couple more turns on the skein, but alas its already showing signs of frame failure, the ¼’’ through bolts are bending and the sides are starting to cave in. Cest la vie. I should have gone with my gut and made it from whole trees.

Anyhow I built this for the Rope and String Challenge to illustrate the power of a torsion bundle made of ¼’’ rope. It amazes me how much torque you can generate with such small components. I destroyed two Mangonels in as many days by twisting string. The power generated is astonishing.

But remember, with great power comes great responsibility. Don’t aim it at your neighbors house.

Unless you’re planning to invade it.

Rope & String Speed Challenge

Second Prize in the
Rope & String Speed Challenge