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Baby Onager was constructed in preparation for building a mid-sized onager. The basic torsion engine design was taken and improved from my Baby Ballista model. I'm going to recycle metal parts from Ballista Jr to build a mid-sized onager based on this design. If that works well, I may build a second for use in winter snow ball fights.

Baby Onager is two feet long, 15 inches tall, with a 17 inch throwing arm. Base construction is pine 2x4s, with 2x2s holding the upright up. Ferring strips were used for the angle bits.


The sling's length is still a part of the experiment, but is currently about 8 inches long to the center of the sling. The sling is canvas from an ancient WW2 army tent, and sewn into a nice shape by my wife. She recommends a keyed button hole style.

Also part of the experiment is where to brake the arm. The 2x4 arm brake is easily removable and replaceable using wing-nuts to aid in experimentation.


The arm is made from cherry, scavenged from a wood shop's trash bin. Where the arm hits the brake, it is wrapped in 1/8th inch fiber cord. This pads the arm, and also protects against fracturing (which is what happened to the previous arm.)

Note the pouch is tied on a short distance from the release hook. This is supposed to add top-spin to the projectile to aid in distance throwing. The technique worked well on our large trebuchet Juggernaut and Juggernaut2 but I suspect it doesn't do to much at this scale.


Unlike Baby Ballista, this onager has metal face plate between the modiolus, and the frame. It is bolted on with #6 machine bolts. The modiolus is 1/2 inch plywood. The epizygis is a 1/2 inch bolt. The hole is 1 1/8th inch wide. The rope is plain nylon clothes line, and has already added a bend to the bolts. A nail prevents the torsion bundle from unwinding.


The triggering mechanism is a bit of bent steel. I don't know what the kind of steel it is since it was scavenged from a bed spring frame. I do know that it is about 1/4 inch thick, and really hard to bend. The shape is commonly seen in The Book of the Crossbow.


Here is Baby Onager cocked, and ready to fire the dreaded mini-koosh. When I added extra twists to the rope, the trigger started to unbend. I had to add an extra bend to it to prevent a miss fire.


Here is a close up of the trigger in action. Having the projectile lie on the floor in this manner usually results in a high throw. Propping the machine first lends a lower trajectory. This machine has thrown the koosh about 25 feet with a high trajectory without much tuning.

Most machines have the trigger run through a loop beneath the arm. I ran the trigger over the arm due to the simplicity of this design. When the large machine is built, it will probably not use this technique.


The Plans

These are the first set of PDF plans I've made for any of the machines. I get a lot of questions about this little onager, and now you can build one exactly like mine. My recommendation is that you can use these for ideas, but really, there are a few problems with this design you may be best off to design your own.

Unlike the pictures above, I have since replaced the arm break bar with something thinner than the 2x4 mentioned. It allows the arm to swing farther forward.

You should be able to cut the whole thing out of a single 8' 2x4, or even one of those cheap 80" framing beams. You'll need a saw, vice, and drill, with 1 1/8" bit, and something a bit smaller for the bolt holding on the stop bar. You will also need a chisel and hammer to cut out the mortises.

There are no details on the modiolus setup here. See The Torsion Page for ideas on that part.

Baby Onager Frame Layout
Size : 17522


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