Can I Use a Metal Spring to Replace a Wooden Bow for My DIY RPG Weapon?

  • Thread starter Matt_741
  • Start date
In summary, Matt is a geek who loves the RPG scene and is planning on attending an event in Derby. He wants to make his own equipment, including a foam sword and ranged weapons. He ran into some trouble with creating a bow and arrow, and is now considering using metal springs instead. He is seeking advice on how to determine the amount of energy stored in a wooden bow and whether a metal spring can be a viable replacement. He also wants to know if he can reduce the size of the bow mechanism and if his idea of using a metal spring is feasible. He has received suggestions such as using Hooke's law and considering other projectile options.
  • #1
Matt_741
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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.
Matt.
 
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  • #2
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.
 
  • #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.
 
  • #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.
 
  • #5
Matt_741 said:
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.

I might suggest Hooke's law:

http://en.wikipedia.org/wiki/Hooke's_law
 
  • #6
mini frisbee's ?
soft throwing/ninja stars ??
something like oddjob's hat in the bond films ??
 
  • #7
Matt_741 said:
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?
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.
 

1. How often should I replace my bow and arrow?

The frequency of replacing a bow and arrow depends on several factors, such as usage, maintenance, and type of materials used. Generally, it is recommended to replace your bow and arrow every 3-5 years or when there is noticeable wear and tear on the equipment.

2. What are the signs that indicate it's time to replace my bow and arrow?

The most common signs that indicate it's time to replace your bow and arrow include visible cracks or damage on the limbs, loose or frayed strings, and reduced accuracy and power when shooting. It is important to regularly inspect your equipment for any signs of wear and tear.

3. Can I replace just one part of my bow and arrow, or should I replace the whole set?

It is possible to replace just one part of your bow and arrow, such as the string or limbs. However, it is important to ensure that the new part is compatible with the rest of your equipment. If the damage is significant or affects the overall performance of the equipment, it is recommended to replace the whole set.

4. What is the best material for a bow and arrow?

The best material for a bow and arrow depends on personal preference and the type of archery you will be doing. Traditional bows are often made of wood, while modern bows use materials such as fiberglass, carbon fiber, or aluminum. It is important to choose a material that is durable and suits your needs and skill level.

5. How can I properly dispose of my old bow and arrow?

Properly disposing of your old bow and arrow is important for both safety and environmental reasons. You can donate it to a local archery club or program, sell it to a secondhand store, or recycle it. If none of these options are available, you can safely break down the materials and dispose of them in the appropriate recycling or waste bins.

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