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    string making and bolts

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    panne
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    string making and bolts

    Post by panne on Sat Nov 10, 2012 10:44 am

    i have some mil spec dacron upholstery thread (VT. 00285A) that i couldn't find the old specs for, but came up with an average breaking strength of 14 pounds testing it myself. i've used it with great success on my pvc horse and long bows in continuous loop and flemish twist styles on bows under 45 pounds. it seems to outperform my #6 waxed linen strings by a mile, but is a pain to flemish twist.

    after polishing the nocks and trigger, i made a string for my jayhawk prod using 30 strands (15 loops)which comes to a 420 pound string on a 75 pound prod. i served it with the same thread. brace height was 3 1/4" and the power stroke is 9". on my second shot the string hit the bolt a little high flipping the forward end skyward resulting in a misfire and broken string. on the first shot the bolt hit something hard embedded in my styrofoam target snapping it in half. 5/16" x 16" port orford cedar with a 100 grain, 2 blade glue on point. bolt too light?

    also wondering if bamboo would work ok for bolt shafts. the species of bamboo i have is small diameter, has long internodes and is extremely thick walled. it makes great arrow and atlatl dart shafts, so i'm wondering if it will work for bolts?

    on my bows i usually make the string between 4 and 5 times the draw weight. though they are never dry fired and my arrows tend to be a bit heavy for the draw weight. the string i broke on my crossbow was 5.6 times the draw weight. how much heavier should i make them to make them a bit more forgiving of misfires?

    i'm also wondering what my minimum assembled bolt weight should be for my crossbow?

    thanks
    panne
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    Geezer
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    Re: string making and bolts

    Post by Geezer on Sat Nov 10, 2012 10:56 am

    remember that a 30 strand Dacron string that's a continuous loop is only half-thickness at the ends. So you don't have a 400 lb. string, you have a 200 lb. string where the stress is greatest. I read somewhere that a complete misfire momentarily loads the ends of the string with 4 or 5 times the 'draw' weight. In your case, if the draw weight is 70 lb, the maximum stress on missfire is close to 300, which is far in excess of the approximate fail-point for 15 strands over the ends. So a string of this weight should handle ordinary shooting with ease... in fact, you could make thinner ones, but on misfire, it's goin' West in a hurry. For perfect safety, look at making doubled-strings, which are full thickness over the ends. You can either use Payne-Gallwey's 'auxilary loop' string, or follow the instructions found in Harmuth's "Die Armbrust" or Paterson's "Guide to the Crossbow." Harmuth's book is very rare and expensive, but you should be able to get Paterson's wonderful little monograph from Manchester University Museum's Simon Archery Collection. Highly recommended, lots of good stuff. Geezer

    panne
    Fresh Blood

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    I'm new to crossbows


    Fresh Blood Doesn't meanI'm new to crossbows

    Posts : 46
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    Re: string making and bolts

    Post by panne on Sat Nov 10, 2012 12:59 pm

    i hadn't thought of the half thickness ends on a continuous loop. thanks geezer for the smack to the forehead. definitely a selling point for full thickness flemish twist loops. unfortunately my arthritic fingers aren't very fond of flemish twisting and they also don't like pushing on the thin 30 strand continuous loop string to span it, so at least double the fibers in the string sound like a plus to me. the 30 strand string was also hitting the bolts a bit low.
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    Geezer
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    Re: string making and bolts

    Post by Geezer on Sat Nov 10, 2012 3:34 pm

    Yes, doubling the number of fibers will give you stronger ends, and the string will push closer to the middle of the bolt, but of course the string will be heavier as well. That will slightly decrease your efficiency and make the bolts slower.
    Using a flemish or doubled central-european string will give you full thickness ends without increasing the thickness of the center. If you want the string to strike bolt-butts higher, you'll have to make the bolt-groove deeper. Ya see, everything involves compromise
    Geezer.

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    Re: string making and bolts

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