Advice needed on new DIY Bow and arrow design

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Not to scale in size or proportion. Design is a work in progress, proof of concept in manufacture, no prototype yet made.



What you see above is my design for a small DIY Bow which I have named a rebound bow. The idea behind the bow is the create a small but powerful, easy to make bow that can shoot at 130-140 fps. The problem with small homemade bows is that most materials that are flexible enough are also weak and unable to stand the tension of a single-string bow design. Also, string tense enough are not elastic enough to give much power, hence most home pen-made/wooden bows give around 25 - 60 fps, which is not fun since it gives a range of around 10-15 feet. These designs also ignore how much of the force comes not from the string but from the body of the bow, something pretty obvious in modern compund bows.

Hence, I decided to mount [through holes in the centre of the] 3 poles on a steel barrel [which would help with accuracy]. The main bow is made of bamboo and is highly elasitic. The support pole and the anchor are semi-elastic thin wooden poles and they help to store energy and provide more speed to the release without the required tension by dividing the force applied and energy stored. The main bow bends back and the string expands when the bolt is pulled back. The support pole stores additional energy with the rebound string which would transfer it back to the main bow causing a faster rebound. The straightner limits the pull length and ensures that maximum displacement of the main bow and support string is achieved while also working as a secondary support string. The anchor is short and due to its testile strength, has very little 'give'. It keeps the support pole taut and allows it to bend just enough to maximize the rebound of the main bow but not so much so that the support string loses its optimum output of power. The relative tensions of the strings will have to be calibrated at manufacture, and some advice on doing that is highly appreciated.

What I wish to know from you guys is where the mechanics of the system are optimised and can be done so further and what flaws this design might have; what changes are to be made to the positions and locations of the various components and what changes are to be made to the design itself is also something I wish to know. Also tell me if you like my design, if its original or not and what similiar or other designs you recommend.

Thank you.

 

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This is a crossbow isnt it? I dont understand the design but the best DIY material to use for a small crossbow would be an old leaf spring from a car, cut to size with a hacksaw. You can pick one up for a few bucks at a junk yard.

Far more elastic than ordinary low carbon steel...using an alloy containing tiny qualities of silicon.

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Do you have a closeup of how you are attaching the main bow, support pole, and anchor to the barrel?

 

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Think Carcano´s post 2 suggestion is more what you should be thinking about because:

1) Your main energy storage, I think is in the rubber. Rubber can store a lot of energy per pound, if released slowing - for example when powering a simple model airplane. As you plan a steel tube and lots of other structure, you don´t seem to be concerned with the energy per pound aspect.

2) Stretched rubber when quickly released chills.* (You can demonstrate this easily by holding a heavy rubber band, fully stretched, for a few minutes and then releasing it and immediately touching it to your tongue, which is the most sensitive heat detector of the body.) There may be some rebound with with your design that causes the cold rubber to snap or rubber may have other problems with the limited speed of contraction (compared to rapidly released stretched steel).

3) Bamboo is very inferior to some woods for energy stored when flexed, but an artificial wood composite of bamboo fibers in some binding matrix may be as good as the best woods but at > ten times the cost.


* reason is that in relaxed state the long rubber molecules are very randomly bent in a 3D volume, but more linear when stretched. There are many more ways to be bent than straight so upon release it becomes bent again, but that takes energy, which causes the cooling.

 

I don't think it is a very good design for it seems more complex than what is already available on the market and looks like it would cost allot more to make than what's already being sold. To many things that look like they could snap, pop or brake.

I like the tried and true crossbow myself for its elegant design and ease of use plus can be precise shooting at over 100 yards.

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To Buddha12

Wow - that is impressive. Hope the bolts have some ID hard to remove* so cops will know who silently killed with it from blocks away.
BTW what does DIY stand for?

* Rings along bolt length making a "bar code" perhaps made by neutron activation of long lived very low level radiation isotope. - Cops wrap recovered bold in photographic film for a month or two to expose / read the bar code, but if killer knows they exist he will just toss the bolts in the trash and get similar short steel rods. - Not easy to make hard to remove IDed bolts.

Perhaps very slight (not easily detected without precise instruments) sinasoidal variation in the bolt OD for bar codes is cheaper and better.

Most breast implants now have unique IDs. Who a badly mutilated woman in Brazil was was learned from her implants.

 

DIY = Do It Yourself

 

Yes, gmilam/Billy T, do it yourself. Or maybe: Dart Impeller, Young [as in Young's modulus].

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By manufacture, you mean making them for other people to use? I don't think you can expect it to stay tuned. You'll need tensioners for the archer since strings stretch, and wood sags over time. If you look at how guitar strings are tuned, it's a worm gear which locks without slip. Pianos use a pin block which eliminates the gear, but it may be hard to implement for a string that is subjected to shock. Another possibility is an inline tensioner (turnbuckle) which may be fairly easy to make.

At some point you will want to estimate the tension needed in your cords to develop the desired power. It will increase as the angle to the arrow shaft approaches 90°. In other words, greatest efficiency would be from a cord that's arranged like a common slingshot. You will also have to figure out how much the cord will have to stretch versus the power power required and the length (of the arrow shaft) over which the power is developed. Once this is known, it's possible to estimate the amount of force placed on the structure, and from there you will want to know the specs on your materials, particularly the maximum safe limits in compression and shear. Design to 50% of maximum as a rule of thumb.

Safety: whatever you end up with, make sure it has a foolproof way to lock out against misfires. Also, relook at it as if it were going to break at point A, B C, etc. as far as the consequences. Besides misfires, you don't want to get whipped in the eye, or have a piece go airborne. Consider using a safety, a slack line that will take up the load if the cord snaps, becoming taut and holding the pieces together.

Avoid sharp bends in cords. Every place you put a kink on a cord will be a weak spot. Look at how instrument strings are guarded against kinks. Consider running the cord through little guide wheels where possible. Try to use one continuous cord, or maybe two, and the thing may self adjust. Also this way you need only one turnbuckle. At some point you may be able to devise a variable click adjustment for dialing in the pounds of tension.

As for the struts, you might ask yourself if they need to bend at all. Bamboo seems to me to be risky although it has the advantage of being relatively strong (for grass that is!) due to its tubular geometry. The simplest reinforcement for a strut is a crossbar or triangle. Look at how the arm on a crane is designed. You've seen this before, and also in overhead struts and beams in buildings and so forth.

I don't know about drilling bamboo. I would be inclined to lash onto it, maybe with many wraps of string topped off with glue. The guides on fishing poles used to be lashed on.

As for materials, my first impression, before I got to Billy T's post #6, was also the same concern about bamboo and rubber. Are you specifically trying to stay with "natural" materials? I understand you want light weight, and bamboo offers that. The bungee cord is a great invention because it allows the cord degrade "gracefully". It gives you time to realize it needs to be replaced. Obviously you can develop as much power as a leaf spring using bungees, I'm just not sure how many it will take!

The general rule for struts and beams is that they tend to be stronger in compression or tension than in shear. Bamboo is tricky, because it's not a controlled uniform material. That's not necessarily a problem, just something to think about. So for best use of your material, which is fairly light for its strength, you want to try to keep it out of shear (sideways strain). You can do this by trying to keep the poles parallel to the string tension. Or, if that doesn't suit you, and you think the bamboo is under too much shear, you can try to reinforce it with cross struts (triangles) like you've seen in architecture and machine design. For example, look at how a crane arm is designed. Imagine the shear on that arm.

Another thing about shear is that it decreases in distance from the fulcrum, so you can economize by letting the beam gradually taper off. The leaf spring does this, and typical bows do, too.

The simplest geometry would be an isosceles (or equilateral) triangle, with the base oriented parallel to the string of a conventional bow, and the apex oriented where the grip would be. This can be tested, and, if necessary internal struts can be added (e.g., another triangle, half size, inverted and attached in the center). One you have a sound solid frame that's lightweight cheap and easy to build you can solve the problem of creating tension using the frame for support.

If weight is no concern, then you may be able to take a leaf spring as suggested and adapt it. This would probably require a tripod, as it's quite heavy. You could use a small steel cable with a winch to retract the spring. That sounds pretty dangerous to me, but with adequate precautions it could develop a lot of power. Other lighter spring materials are available, but I have no idea about the cost and specs for them.

The last issue, which I will leave open for now, has to do with figuring out the amount of power needed to launch, and how to develop it using a spring or elastic cord.

Be sure you use safety goggles whenever you're working with springs and things that want to fly apart.

I would say it's novel for using bamboo. I happen to like bamboo, but once it cracks, it's gone. On the other hand, it's expendable - it makes mulch. You might want to make it removable in case it needs to be replaced. It all depends on what you like, what your goals are. I would say go ahead and try it and don't stop there. Try a lot of ideas. If you can get a old leaf spring you can make something really dangerous. Just be careful!

You are getting some additional power by stacking the elastic cords. That's good. Keep in mind you can get the same effect by using multiple wraps of the elastic cord, attaching at the longest point from the fulcrum.

One last thing: you might want to consider investing in a gauge, or making a test jig of your own, that can measure tension.

 

No.

The problem with leaf spring are that they are high tension, for a bow only 1 foot high, they would have a pull weight of some 20 or so pounds, unfeasible for such a small design. Which is why this design is based on dividing the force so that low tension materials at a pull of just 5-6 pounds could provide a high powered release.

 

All three will have a hole drilled in the middle in the barrel will be threaded through then fixed with superglue.

 

How so?

But it is never going to be fully pulled for a long time since there is no holding or release mechanism [like a crossbow].

I want to keep this low cost, but I will look into it.

Wow, nice to know. I would have never thought that some materials would cool upon use, not heat up.

 

This is a more like a toy/science project. Its supposted to be safe and non lethal. But damn, that crossbow looks smashing.

 

Oops. Poor choice of words on my part. What I meant was
Manufacture [v] - Put together out of artificial or natural components or parts
NOT
Manufacture [n] - The organized action of making goods and services for*sale

What are you refering to by the arrow shaft? The bolt? The main bow?

Where will the slack lines be attached? I imagine they have to be just a little less tense than the maximum flex [at full pull] between the points where its attached?

How will the two poles [excluding the anchor] work properly with one cord? wouldnt it slide? Besides, this is a low value project with one primary goal in mind - a low tension, divided force system with fairly good power.

As Billy pointed out, the rubber by itself propably wouldnt be strong enough to provide all the force desired. The strut [support pole] would have some give so that it can contribute to the rebound too. Such division of force would be inefficient at bigger scales, but this bow will be only 1 foot high, so this is the only way to keep it low tension and powerful, no?

Hmm. Could be easier. Is there any problem with drilling bamboo? The barrel would have to be threaded through a central hole which has to be drilled, but do the smaller string holes, placed together, have some effects on splintering or something?

I decided on bamboo and rubber be they are low tension 'springy' materials rather than high tension 'snappy' ones used in real archery weapons. This would obviously reduce the power output, but that is exactly why I added the support pole and rebound strings, so that together it can be more snappy without being high in tension and pull weight. It is, after all, a science project at best. Think of it as being oriented towards understanding and designing, developing and making a toy or a non lethal weapon. Its going to be use primarily for target practice and the focus of the project is the development rather than the use of the product. Also, wouldnt bungee cords to too inelastic and thick for such small lengths?

I dont get what exactly do you mean by keeping it out of shear [in context of the pictured schematic]. Bamboo is grained vertically, so do you mean the actual bending of the bow itself? The strut [support pole] will not be bamboo, but wood so that it is more rigid.

I cant picture what you mean here. Can you pls describe it a bit simply or in reference to a picture?

Will this yield some mechanical advantage as well?

Wouldnt such a configration drastically reduce the amount of deformation the main bow can achieve? You idea would be good for a realistic high tension weapon but I want to make a 'toyey' low tension system.

Yes indeed. Too dangerous and too high tension.

Ok.

Bamboo is good enough since the idea is the make a small bow lasting for about 5-8 months and costing just a few dozen dollars, easy to make and quickly replaceable. I dont know if making it modular would go with this.

The tension! Bamboo would snap if a single bow would have multiple wraps of cord, no? The entire design developed from making small 'plastic spoon' bow and trying to figure out how weak but elastic materials can give high power at low tension to the components.

 

To develop power I think the string needs to be VERY tight at rest...when the bow is strung but not drawn. Rubber isnt going to provide that.

You dont want the arrow slowing down slightly...just before leaving the string on its flight path.