stonebow prod help

Right, well I have most of the kinks worked out for this project except for the prod. I'd like 11-12 inch draw and under 100lbs. I was going to make steel prods, but after some chats on this forum and some additional research I've elected not to make them. The Alchem prods cost too much, and are too powerful. I was looking at the 120 lb fiberglass prods, but it looks like the draw is too little. No real big deal there, as I can make the stocks shorter. They are, however too powerful. These are being built by 12 year olds, and they need to be able to hand draw them.

I was wondering about using fiberglass strip like some of you have done, but I don't know the specs. I have in the past used the fiberglass rods that serve as driveway markers, but keeping the ends on always was troublesome. 

Any ideas?
Cheers in advance
chriso

You can use bamboo slat like I did with the "Chinese pellet crossbow". It is cheap, available everywhere, and you can chose the power you need puting the number of slats you prefer.

When thinking about your question I want to point out one thing:

People always demand prods for certain measurements which is very contraproductive when building crossbows of any kind.

There was always one rule to follow: first the prod, then the tiller!

For example, it is relatively easy to build a bow with under 120lbs from metal and then build a tiller fitting to its draw length.

So, if you get rid of those measurements in your mind, you could easily forge/bend a working prod for your systems.
(for (metric) example: center 4x20mm, limb end 4x10mm, limb length 200mm (working 180mm) should give you around 100lbs at brace height of 80mm and total draw of 183mm; make the limb 250mm long with 230 working you should be at around 80lbs at 240mm)


Consider: even when doing work with kids, a crossbow stays a some kind of weapon that strongly depends on storable energy. If going to low with the draw weight the system will eventually not work the way it is intended to do.

Funniest thing is: non guided bullet crossbows are the last thing I would consider building with kids because of their really dangerous behaviour when shot.
Dangerous end is not always the front side, ask Geezer, I am sure he can tell you a story.
A tiller ricochet can easily cost the shooter an eye!

Tuning the system to a safe working state can take a lot more time than of a regular crossbow with a bolt.

But I dont want to scare you from the project, I am curious to see some progress and wish all the best.

Greetings Rizzar

I was thinking along the same lines. It isn't too late to change things. I'll talk to the kids next monday. As for the tillers, we haven't built them yet for exactly the reason you suggest: no prods yet. I could design around the little fiberglass prods I bought and go from there. The result would be a tiny bow.
I'll be using all this discussion as part of the lesson. The idea is to not only learn the history of the bow, but the process of building them. Everything will be written up. There is talk of e publishing it as well.
cheers!
chriso

I would recommend you go ahead with the larger fiberglass prods and shorten your power-stroke to something acceptable.  That will give you an affordable, workable project and not leave everybody hanging.  Geezer.

I was thinking the same thing as Rizzar Chriso.From a psychological viewpoint,when a kid makes something,makes it right,and it works as it should,he or she is extremely proud of it,thinks"I can do this",and sometimes,it can lead to a life long interest.Comparing a crossbow to a stonebow,I would think that they are much more likely to achieve success with a crossbow.Then there is the safety aspect,which is even more important.Others much more experienced than I on this forum have confirmed the safety concerns that I,and no doubt others could foresee.Personally,a stonebow is something that I might tackle after I had gained some experience building a crossbow or two.
                                             As a teacher,I'm sure you are aware that kids have parents,especially Mothers!.Imagine this scenario.Kid comes home from school and says to Mum,"hey Mum,I'm building a crossbow/stonebow in shop...........Mum says "Oh,what's that?".Kid enthusiastically explains.Mum has a meltdown,and the brown stuff hit's the fan.Scenario 2.School phones Mum,and tells her "Wayne has been taken to hospital because the stonebow he's been making in Shop blew up".Mum grabs Dad's 12 guage,and goes teacher hunting.
                                   I am sure you are a good and responsible teacher,and have your kid's best interests allways in mind,but getting kids to build weapons I would think is likely to be a hard sell,especially to Mothers,so building something that could be dangerous to the user,during construction,and use,where kids are involved,should give pause for thought.
                                                       Personally,I think what you are doing is great,and the skills and attitudes that the kids will get from building a crossbow or stonebow will be invaluable in later life,but going for the safer option(in my view the crossbow)makes sense. Best of luck,and success with your project,I look forward to seeing your progress postings.
                                                            Hermit.

Geezer here:  I have tried to stay out of the safety aspects of stonebows, having already voiced my opinion numerous times.  But since others have brought it up, in short, I have built several stonebows in my time.  The last one misfired- the doubled-string touched the top of the stock just at the end of its run, which turned the pocket around 180 degrees.  When the string stopped, the pocket rebounded and fired the shot (a marble in this case) back into my eye at about half power.  That was sufficient to smash the lens of my spectacles and permanently injure my right eye.  I didn't lose it, but it is scarred and the iris-muscles are damaged, so that eye doesn't dialate and contract normally.  In bright sunlight, I tend to keep the eye closed.  At night, it's actually better for seeing in limited light.
So yes, a stonebow can be dangerous to the shooter.  However, if you build a payne-gallwey barreled bullet-bow or a slurbow, I can't see how you could shoot backward even if you tried.  You might bounce the ball off a hard target, but I don't think you could replicate my mishap.  So by all means, build stonebows with your shop class, but I would recommend shooting down a slotted tube, or fitting a slur-plate to the top of the track.  That will let you opt for the simpler single string rather than a double, and you can even shoot bolts instead if you'd rather. 
Be safe, have fun!  Geezer

Geezer:
Thanks. That is enough for me to approach the kids with. We'll look into making a different style than the early one described in Gallway's book.