Making an Onager

This summer, I built my second Onager, a type of catapult. My original catapult design came from "The Art of the Catapult: Build Greek Ballistae, Roman Onagers, English Trebuchets, and More Ancient Artillery", by William Gurstelle. This is based loosely on that design, but I didn't open the book during the project and made a few modifications to his design.

The assembled onager is about 10 in. x 5½ in. and stands about 8-10" tall.

The Frame. The long pieces of the frame came from four 10" x 3/8" x 1½" sections. I glued two pieces of these together to double the thickness for each of the two main frame pieces. The frame cross pieces are cut from 1" boards to form ¾"x1"x4" boards.Two holes are drilled across from each other in the long boards. The larger is 3/8" and the smaller is 3/16". It's best to clamp all four frame boards together and drill the holes at the same time so everything lines up. Then I glued two holes in the long boards and inserted 3/16" x ½" dowels into the holes to strengthen the uprights. As I completed each step, I gave everything a rough-grain sanding to take off splinters and sharp corners/edges.

The Uprights. Then I cut two uprights from the same board and at the same length as the cross-members in the frame. I cut a 3/16" hole into the bottom of each and glued them to the frame, leaving them clamped overnight. While that was drying, I cut a cross-bar as well as two supports from the same lumber as the uprights. The supports are cut with 45° angles at the ends. Finally, I cut a 5/8" dowel to about 8¼" long for the torsion rod. I cut a notch out of the torsion rod about 1¼" long at one end to sit the cup into.

The Wheels. The glue dried overnight, then I attached the crossbar. As that glue was setting, I used a circle-cutter to cut out wheels. I used the same 3/16" dowel rod to make axles for the wheels. I cut four wheels and four axles. The axles are about 1½" long. I used some nylon string to cap the axles, holding the string with clamps to give the wood-glue time to dry. Once the string had dried at one end, I slipped the axles through the wheels and into holes in 1½" blocks glued to the frame bottom. I then tied more string to the axles and secured them with wood glue. Later, I cut off a bit of the excess axle length with wire cutters. The wheels are thicker than I would prefer, but that's the wood I had on hand and they are stronger that way. If I make any more of these, I'll probably go with a thinner wood; perhaps the 3/8" wood from the frame...

Torsion Spring. Next up was the torsion spring. In the Onager's case, this is provided by ¼" nylon rope. I cut off a piece about 3 inches more than twice the onager's width. I passed a loop through the larger hole on one side of the frame, then stuck a short peg through the loop to brace it. I passed the two ends through the larger hole on the other side of the frame. I used a lighter to carefully melt the ends together, making a single loop out of the rope. (Tying would work, too, but this looks neater to me than a big knot.) I slipped another peg into the smaller hole on one side of the frame to act as a brake. Then I slipped the torsion rod between the two ropes, centering the rod between the frame ends and slid the rop up to the middle of the rod. I slid a third peg into the loop on the open side, and a fourth to act as a brake on that end as well. Then I began twisting the rod away from the direction of the upright braces. When it got tight but not too tight, I slid the rod up enough to stop against the cross-bar. Careful: if you let go of the rod or if you overtighten enough to break one of the pegs, the spring action can hurt!

The Bucket. The bucket was made using a small scrap of 3/8" thick lumber for the bottom with sides of ¼" lumber. The cup is 1¾" x 2 3/8", assembled. I glued the five pieces of the bucket together and let them dry overnight. I then clamped them to the Torsion arm, nestled firmly into the cup. While clamped, I drilled a 3/16" hole through the bucket and the arm. I then glued the arm and bucket together with a short piece of 3/16" dowel to peg the two parts together.

Finished. I let everything sit overnight, then took it out to the driveway and launched some wood scraps. I was able to toss roughly 1" diameter blocks about 15 feet or so. With limestone pebbles, I could get about 20 - 30 feet of launch distance. Not too bad from something this small!