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    How to span a heavy prod on a budget

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    gcostello65
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    How to span a heavy prod on a budget

    Post by gcostello65 on Thu Aug 15, 2013 8:53 pm

    Hello all,
    I just recently completed my first crossbow build. But I cannot span the bow. I used an old chevy leaf spring and ground it down as much as I possibly could. I do not know the exact draw weight, but all I know is I cannot even begin to span it by hand. I need a spanning mechanism that would be cheap and easy to make without the aid of a forge. I do not have the money to spend on a windlass or cranquin. Please help with any info!

    PierreC
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    Re: How to span a heavy prod on a budget

    Post by PierreC on Thu Aug 15, 2013 9:50 pm

    In your car, you will most likely find a jack, near the spare tire....

    If that can't span your bow, then you have a seriously powerful bow.

    If you like to do some redneck engineering, you could set up a decent spanning tool using one of these: http://www.princessauto.com/pal/en/Straps-And-Cambuckles/2-in-x-16-ft-Gunwale-Tie-Down/8435455.p

    They say 
    Working Load833 lb
    Breaking Strength2,500 lb

    That'll git-r-done.  I've used straps like that to un-warp 4x6 lumber so I could bolt it down.

    Rizzar
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    Re: How to span a heavy prod on a budget

    Post by Rizzar on Fri Aug 16, 2013 3:33 am

    If your bow is too powerful, you will be running into several problems:

    Spanning and stringing are the first ones

    Building a sufficient string is due to modern material no problem, but you need a hint to your draw weight.

    You need to know your maximum range of spring, which I guess you do not know.

    Your tiller must be strong enough to hold against your draw weight

    Your mechanism must be strong enough to hold against your draw weight


    Considering you are at the point to have a bow constructed, are you sure you can handle all the other points without endangering someone nearby or yourself?

    The more powerful a steel bow is, the higher the danger when it comes to part failure (which is generally with used car springs not low).


    At the point, when you say you can not even begin to span a bow without mechanical aid, I tend to say you will need more efford to span it than just some rope action.

    Spanning high power bows ist expensive. Always was!
    A cranequin for example was as valuable as a good horse.
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    hoopslaboratories
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    Re: How to span a heavy prod on a budget

    Post by hoopslaboratories on Fri Aug 16, 2013 12:43 pm

    What about a fence stretcher? Replace the wire grabbers with the same hooks that cranequins have. I'm sure someone has already thought of this and I apologize if it is already covered somewhere else on the forum, but the first time I saw a cranequin it's rack reminded me of the thousands of hours I spent as a youth fixing cattle fences with one of those old yellow fence stretchers.

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    Re: How to span a heavy prod on a budget

    Post by PierreC on Fri Aug 16, 2013 12:58 pm

    A fence stretcher is interesting.  One of my long-term goals is to build a small siege-engine with a hi-lift jack for a tiller.  Unbolt the foot, bolt in a prod, cut off the foot from the moving part, and attach a suitably strong latch to that part.

    The nice thing about those type of jacks is that, unloaded, the moving part slides freely, which lets you rapidly slide it forward to pick up the string.  Then they have a pretty massive compound lever to walk the moving part back 2 inches at a time.

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    Re: How to span a heavy prod on a budget

    Post by Stonedog on Sat Aug 17, 2013 6:53 am

    I use a ratchet strap....brace the prod so it can bend...put the (padded) hook of the ratchet strap in the center....slowly crank it down until you can just slip on the string....carefully remove from the braces....and voila!  A strung prod!
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    Gnome
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    Re: How to span a heavy prod on a budget

    Post by Gnome on Sat Aug 17, 2013 2:39 pm

    Gcostello,
    Can I ask what method of hand-spanning you tried? I'm wondering if it was the foot in the stirrup, one tip braced against the ground, and pulling up on the other tip with both hands while some brave, dear soul slipped that end of the string on for you. For a weapon that can be spanned without mechanical aid it is the simplest and cheapest, since it requires no additional hardware. It does take a knack, you have to be very thoughtful about applying force to the prod only and not the tiller, or you can easily loosen or break your attachment points.
    If you are seriously on a budget and would like a mechanical aid to string your weapon, I'd suggest a bastard string solution, for a few reasons. I have to make a few assumptions since I don't know the details of your build (some photos and details of your builld would be very helpful for hopefully helpful folks to help!) For instance, I assume that you have aquired or made an appropriate string. If you've done that, you can get/make another one to use as a bastard. depending on the shape of your prod, the bastard can be attached with hooks or looped around the prod itself and clamped near the nock tips so that it is secure. I use small C clamps for that, cheap but not too cheap or they'll fail and fly across the yard and kill the neighbor's cat.
    My reasons for reccomending this method is that it is cheap, and also because you will use the same method to string your bow as you do when you span it for firing, so you'll be testing out your lock mechanism under full load at the same time. For actually drawing the string back to lock, I'd reccomend a rope cocking aid, cheap to buy and cheaper to make. I wouldn't test a new weapon by attemting to draw it back by hand, even if the prod is weak enough to allow for that, if your lock mech isn't quite perfect and slips it hurts. A lot. Speaking from experience there. So a hands off approach is safest, I think, and the rope-cocker will allow you to release the bastard string without "Dry firing" it, just hold it back while your friendly assistant pulls the trigger for you, or if you have a tickler you can release it yourself with your knee.
    Anyhow, if your prod is too stiff to draw back and lock a bastard string using a simple force multiplier like a cocking rope or goats foot or other type of lever, you may want to think seriously about how you're going to ever fire the weapon anyway, if you don't want to pony up for a cranquelin or windlass!
    I was stumped getting the string on my first build, too. I ended up with the crossbow held in a vice, clamping the scissor jack from my car to the shoulder stock, hooking a chain to the prod ends and wrapping it around the jack, then cranking the jack slowly while I cowered behind a big piece of plywood!
    So yeah, there's lots of solutions to the problem, but they aren't all good ones. How about some pictures of your work so far?
    Gnome

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