I’ve been recently reading about ballistæ (and their many relatives!), and I’ve come across the assertion that early such weapons were tension-based (like any ordinary bow) while more advanced ones would work akin to a crossbow (torsion-based).
My question is: how the heck do the two differ after all? I can see that the energy-storage mechanism is fundamentally the same (pulled elastic string held back), and I thought it was about the different ways you could pull the string itself, but then I had a look at the Wiki page of the oxybeles (supposedly tension engine), which looks EXACTLY like a torsion ballista. Besides – don’t some crossbows get their string pulled by hand? Are those torsion weapons too?
Bonus: [these](https://www.stormthecastle.com/catapult/images/oxybeles-drawing1.jpg) two [images](https://www.stormthecastle.com/catapult/images/ballista-drawing1.jpg) attempt to explain the concept; I still don’t get it. Can anyone please explain it to me?
Thank you!
In: 8
In your images the “Oxybeles” is powered by the flexing of its bows and the “Ballista” is powered by the TWISTING of the upright members colored gray.
I’d try to find animations or videos, not pictures.
The energy storage is not the same. The storage mechanism is not the string held back. In a felxion based system, it’s the limbs. In a torsional based one, it’s ropes at the base of the limb.
In a flexion based system, the energy is stored by the flexing of the limbs. So when you pull on a bow or crossbow string, the limbs bend. The base of the limb stays perpendicular to the base and the arm deflects so it is no longer straight.
In the torsional system like the second picture, the limbs don’t bend. You’ll notice that the limbs are embedded in rope. When you pull back, the limb stays straight, and the rope twists. And the base of the limb rotates while the limb stays straight.
The pulled string *isn’t* elastic. It transfers the force to (and from, when released) the bow or the twisted (torsioned) ropes.
Torsion is twisting, Make a rope and put it in a loop around two stick like [https://youtu.be/muFVYDyt_MY?t=543](https://youtu.be/muFVYDyt_MY?t=543) When turn the stick i around you will notice that it get harder and harder to do. Try that yourself at home with for example the legs of a chair. Be a bit careful because there is a force that pill the legs together too and you can break the legs of a chair. The stick you use to twist the top will spin around i you release is. Test it to get the principle do not twise it to hard.
That is how the one that is called a ballista in your images works, the amps that hold the sting do not bend and they use many loops of rope that are tensioned. So when you try to twist it a lot of energy can be stored with less than a quarter of a revolution.
You can see the replica in https://www.youtube.com/watch?v=RUBJY-oxeGk notice it is called catapulta. In Roman times a balista threw rock and could look like https://en.wikipedia.org/wiki/File:Ballista_(PSF)_vector.svg A catapult projectile that looks like arrows but larger, a spear is likely a better description.
The name gets changed during the medieval time and a Roman Catapulta became a medieval ballista
The one in your image that is called an Oxybeles has arms that bend, just a bow or a crossbow. It is fundamental a very large crossbow
The difference is in what changes shape to store energy.
What you’re picturing would be a tension system, like a rubber band stretching to launch a projectile. The rubber band gets longer – this is tension.
A bow is a flexion system. The thing that changes shape bends. It curves more or less as it stores energy. A crossbow is the same. The string does not stretch – it does not change shape to store energy – and so this is not a tension system.
A torsion system does not see the member curve. The member stays more or less straight, but it *twists* into a spiral shape.
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