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Replacing a bow and arrow.

  1. Aug 18, 2009 #1
    Hi there I'm Matt and I am a geek.

    I love the RPG scene and intend on going to an event held in Derby but, for reasons of personal torment, I want to make the relevant equipment myself. In this case 1 sword two . . . well . . . let's say "ranged" weapons for now.
    The foam sword needed wasn't hard LD45 plastazote foam + 1m carbon fiber tube + dyed latex = done.

    Then I got a bit cocky and out of my depth with springs. Basically I know in events people use a bow (30lbs draw at 28") and arrow (with the head being a big lump of foam for obvious safety reasons) but that got me to wondering why not make something new? I'd already done so with the sword being a flammard to the traditional straight longsword affair. I looked at repeating crossbows and so on but the problem in this case was more with the arrows as, to make them safe, they had lost all of their aerodynamic capabilities. So I came up with the idea to have discs of foam fired by a launcher where the wooden leaf spring is replaced with a smaller metal one.
    This is where my ignorance starts to jump up and down waving flags as it goes. I tried to work out the amount of energy stored in the wooden bow by taking the draw length of 0.7112m (converted into m from 28") the draw weight of 13.6077711kg (as the force of 30lbs converted into kg) and by using the equation E=half the spring constant (which I got as 19.13 N/m) times the change of distance squared I only got 4.84 Joules? Is that right?
    If so can I just work out from there any spring (or combination of) that holds the same amount of energy and use that instead of a bow? Not to even get involved with the fun of seeing how the weight and shape difference of the projectile will affect it's flight. Or a firing mechanism. Or my ultimate hope that I could preload 6 such springs so I could fire off several shots before reloading.

    But first things first.
    Is my idea of using a metal spring which can store the same energy as a wooden spring viable?
    Is that amount of energy that I worked out correct? If not please baby step me through all which is wrong. It's the only way I'll learn.
    To finish will the use of a metal spring really reduce the size of the wooden bow down as much as I think? I mean I'm looking for a reduction of something almost the height of a person down to a mechanism that could fit on a forearm. Am I asking too much?

    Any input greatly appreciated.
  2. jcsd
  3. Aug 18, 2009 #2


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    Wouldn't it be easier to start with determining how fast you want your projectile to travel and how much it weighs? You can then plug those values into the kinetic energy equation (Ek = 1/2mv2)and thereby arrive at a figure which is suitable for your specific applicatiion.
  4. Aug 18, 2009 #3
    The problem with a disc is that its inherently unstable unless its spinning. The faster it spins the more stable and level flight.

    This is your key problem to overcome with any launch mechanism.

    The only thing that comes to mind that would still be accurate is a lever arm.
  5. Aug 18, 2009 #4
    You see the major point is, from the point of view of the safety check performed at these events, I need to prove that the spring (or springs) I use can be proven to be a mechanical equivalent of a wooden bow which draws 30lbs at 28". It must conform to that primarily. The issues of any projectile I use and it's relative shape or weight are irrelevant at this time. otherwise some gentle person with an experience of just working off of no mechanical guideline apart from "30 lbs at 28 inches" will just refuse any device built. I appreciate info on the projectiles but, right now, it is not the concern. It is the spring I, mentally speaking, need to get to grips with.
  6. Aug 21, 2009 #5


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    I might suggest Hooke's law:

  7. Sep 2, 2009 #6
    mini frisbee's ?????
    soft throwing/ninja stars ??
    something like oddjob's hat in the bond films ??
  8. Sep 3, 2009 #7
    That spring constant is wrong. Your force is already 13*g = 130N at .7 m so your spring constant is around 190N/m and the stored Energy at 48 Joules. You can reduce the size with springs, but if you reduce the "pulling distance" the the needed spring gets very stiff very fast. Additionally you run into problems with soft projectiles, because the stronger the spring the more they squeeze instead of accelerating.
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