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B Why are Trebuchets better than Torsion Catapults?

  1. Oct 25, 2016 #1
    I would like to know what makes a trebuchet better than a torsion catapult. I know that a trebuchet is more accurate and energy efficient but I am confused on why this is. I would be very interested to know all the details involved in both medieval devices.
  2. jcsd
  3. Oct 25, 2016 #2
    I would start by looking at the component most stressed.
    In the case of a torsion catapult, it is the torsion element and where the arm meets that torsion element.
    In the case of a trebuchet, you have that same place where the arm meets the axle, but the axle itself moves to assist the throw.

    If breaking the throwing arm is not a limit, the torsion catapult can store additional energy in the torsion element. But that is likely more limited than simply adding stones to a basket or allowing the basket to fall further.

    The other issue was scaleability. The trebuchet could be build from available material at the siege site. A catapult with similar range and payload would require the onsite manufacture of a large torsion element.

    As far as accuracy is concerned, I have built and fired a trebuchet. I think the key issue is that there is very little wear on the trebuchet as it is used. So its operation is very repeatable. In contrast, the torsion element will not always wind exactly the same way and the torsion material can slowly loose its flexibility.
  4. Oct 26, 2016 #3
    I think a treb was easier to build. With modern materials I would take a catapult, higher rate of fire, more mobile.....a tactical weapon. Trebs are only good for sieges.
  5. Oct 26, 2016 #4
    I see. Does anyone know exactly what forces occur in the two that allow them to work? I also am more interested in not necessarily the structure of them, but why a trebuchet is mechanically more energy efficient? Like how does its structure cause it to send a huge payload so far vs. the less powerful and accurate torsion catapult?
  6. Oct 26, 2016 #5
    A treb the same size as a catapult would be murdered by the catapult. Treb relies on size to have a weapon effect.
  7. Oct 26, 2016 #6


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    The trebuchet has mainly frictional loses in the bearing and the slide, which can be lubricated to minimize them. A torsion catapult uses deformation of a material, which inevitably dissipates energy as heat.
  8. Oct 27, 2016 #7
    Oh that makes sense. Wouldn't the swing arm's characteristic of being longer on the one side be a reason for more energy efficiency? And also wouldn't the torsion catapult lose more energy from being stopped later in the swing (where the beam [not sure what is called] catches the arm)?
  9. Oct 27, 2016 #8


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    That's just leverage, as the projectile is lighter than the counterweight.

    The trebuchet ends up swinging and also dissipates that energy, just slower.

    Note that energy efficiency wasn't really that relevant for siege machines, but rather range, accuracy, the ability to destroy a wall and ease to build.
  10. Oct 27, 2016 #9
    Okay good to know. You have lead me to another question. Why was the trebuchet better at launching projectiles farther and what made it more accurate? That to me is confusing since I feel the sling would be unpredictable than the bucket of a torsion catapult. Also do you know what forces are at play for both in terms of a free body diagram (for the system)? I know there is the obvious force of gravity downward on the counterweight for a trebuchet and tension force for a catapult, but I can't visualize the others. If you could explain this in some detail i would appreciate it! I am supposed to explain this to my physics class in a week and have not been able to find any videos or sites that can explain this well enough.
    Last edited: Oct 27, 2016
  11. Oct 27, 2016 #10

    launches further because it is bigger.

    cite source that says it is more accurate?

    it is not that hard to hit the side of a large castle, or anywhere inside a castle compound.
  12. Oct 28, 2016 #11


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    It has a longer acceleration path for the same rigid frame size, so it can do more work on the projectile given the same force limits (structural stability). The launch point is also higher for the same frame size.

    It's well reproducible if you lay out the lines cleanly, and you can make small adjustments with the launch finger.
    Last edited: Oct 28, 2016
  13. Oct 28, 2016 #12
    Oh alright, what do you mean by well reproducible? And also what l lines were you referring to? I'm sorry if this is a dumb question. I'm just a bit confused...
  14. Oct 28, 2016 #13


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    Sorry I meant the ropes.
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