Model Ballista (Crossbow)

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Project by bch posted 02-13-2011 10:38 PM 6509 views 7 times favorited 2 comments Add to Favorites Watch

This roman ballista is my second model siege weapon. See my projects for the first (a roman onager i.e. a catapult). It throws a 1/4” dowel about 10 feet. It will knock down blocks if you are lucky. I find it the most impressive design of my three siege models, but the least functional as far as damage to toy block castles. A 1/4” dowel just doesn’t have much mass compared to a boy block. To be fair to the original Roman designers, it was designed to throw small bolts (arrows) at people, not buildings, and so I am not using it as intended. If you put something heavier in, even a 3/8” dowel, it hardly throws even less distance. It knocks them down if they were wobbly to begin with. Still, it’s one of the most fun to hook and wind up, then release, so please don’t let my comments discourage you. I suppose I could try and rig up a toothpick bolt with pointy tip and fletchings, but that would violate it’s (mostly) kid-friendliness, and perhaps threaten the family dog.

After building it I decided the front legs were not tall enough. Without any upward slope the projectile falls and hits the ground within a foot or two of firing. Were I to redesign, I think an adjustable tilting front would be a fabulous addition to this basic design.

This is not my plan. I got this from a book at the library ( I made some commentary on this book in my first project post.

This is a roman ballista. You might think of it as a crossbow, but it is not, given that there is no bow that bends to provide the resistance. Rather, it is twisted rope, or torsion, which provides the resistance and thrust.

P1: The Ballista
P2: This is the final “pouch” design I came up with. The plan in the book must never have been built, because it was insufficient. It just called for a rectangular block over the groove. My block slid right over the top of the dowel without ever engaging it. So first I added the V-block to the underside. This design failed because as it flung forward, it rolled—right over the dowel. Last, I added the front block to prevent the roll. I’m sure there are better designs, but for the moment, this works. Your ideas as most welcome.
P3: The third photo shows my windlass, or winch. You hook the windlass onto the “pouch”, wind it back, hold it with one hand so it doesn’t fire, then use the other hand to pinch off the hook to fire the model. I think this design could be greatly improved upon with a latch, similar to my onager (catapult). I’ve had some ideas but never implemented them. The simplest would be to have a dowel slide out of the side of the windlass engaging the handles on one side. Then you could tie a string on the hook just like the onager.

-- --bch

2 comments so far

View spunwood's profile


1202 posts in 2862 days

#1 posted 02-13-2011 10:49 PM

Alright! This is a fine woodworking project. Have you waged any battles yet.

-- I came, I was conquered, I was born again. ἵνα ὦσιν ἓν

View bch's profile


296 posts in 2715 days

#2 posted 02-14-2011 01:02 AM

Many a brave, toy soldier has bet his safety and reputation on a toy block castle, and learned his lesson at the able hands of my son. Or as I am fond of orating during battle, and which my son repeats, “Let us in you French dogs!” (nothing against the French, but it is a French/English trebuchet and there is some history there which I reference.)

-- --bch

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