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    brace height and draw length for fibreglass bow

    backgardenbowyer
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    Post by backgardenbowyer on Sat Feb 09, 2013 6:18 am

    I'm just assembling the materials and tools for my second build - a modified medieval style using a fibre glass prod. Got a nice piece of oak, some 36mm delrin (matching forstner bit), 10mm square for the tickler is on the way. The fiber glass prod I have is marked 150lbs wit hthe trade name "Armex" and is about 27" long. I've braced it with a tillering string to about 3.25" (from the belly) and this looks about right to my longbowyer's eye. I have, however, no indication of the draw length or power stroke - any suggestions?

    My bow scales only go up to 90lbs and as far as I can tell the bow is reaching 100lbs at about 10" draw (from belly), or a power stroke of 6.75". A weight of about 125lbs will be enough for my purposes. I was thinking of going back to 12" draw and a power stroke of 8.75". Does that sound about right? Obviously once I drill the nut hole I am committed so thought it best to get some advice first!

    Thanks so much for your help to a beginner.
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    Post by stoneagebowyer on Sat Feb 09, 2013 8:02 am

    I cant off the top of my head think of the formula for calculating bow weight at a given draw, though on Primitive Archer website, you may find that answer. If you have a tillering stick and a bathroom scale, you can at least get a rough idea of weight at a given draw length.
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    brace height and draw length for fibreglass bow Empty Re: brace height and draw length for fibreglass bow

    Post by Geezer on Sat Feb 09, 2013 8:37 am

    For Backgardenbowyer, re this:
    "The fiber glass prod I have is marked 150lbs wit hthe trade name
    "Armex" and is about 27" long. I've braced it with a tillering string
    to about 3.25""
    Your glass prod sounds suspiciously like one of the many knockoffs from the old Barnett Panther. Their glass prods are approx 1 inch wide and 27-28 inches long. They're designed to work from a brace @ 3 inches and a draw of perhaps 8.5 from there. Just off the top of my bald-head, I'd say you should be reasonably safe putting the lock 12 inches behind the belly (inside surface) of the prod. Just my two-bits. Geezer.
    Todd the archer
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    Post by Todd the archer on Sat Feb 09, 2013 8:56 am

    Made one with a similar prod it. It is set up with a total draw of 13 3/8" from inside prod to the nut, and 10 1/2" from brace to nut (powerstroke). Have shot a couple dozen times so far no problems. Guessing it is close to 150 pounds.

    I am thinking your set up will be fine.

    brace height and draw length for fibreglass bow 100_9591_zps15ff04a6

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    Post by backgardenbowyer on Sat Feb 09, 2013 10:14 am

    Thank you guys. Done a bit more research and I think this might be the same prod as found on the model sold here as an Armex "Jaguar" here which seems to have a power stroke of 10" (though one site said 16" which is clearly the bolt length.)I'll go for a power stroke of about 9", maybe a tad more.

    Todd - I like you bow: interesting way the prod is attached to the tiller: are the some wedges in the slot for the prod? It looks like less hassle than a bridle.

    I'll let you know how I get on.
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    Post by Todd the archer on Sat Feb 09, 2013 10:35 am

    Yes there are wedges in front of the prod which are held in place by the sides of the stirrup. I have come to prefer this method. Simple and secure and easy to change out.

    Todd
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    Post by Taxus on Mon Feb 11, 2013 5:45 am

    backgardenbowyer: I have an Armex crossbow with the same 150lb fiberglass bow you've bought. You might find the dimensions of this bow useful for you own.
    Brace height is 3 inches
    Draw length/powerstroke is 10 1/2 inches (exactly 27cm)


    Last edited by Taxus on Mon Feb 11, 2013 5:45 am; edited 1 time in total (Reason for editing : correction)
    backgardenbowyer
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    Post by backgardenbowyer on Tue Feb 12, 2013 12:25 pm

    Thanks Taxus, I roughed out the tiller on Sunday, and have drilled the nut socket and rough cut the bow slot. There is a little over 12" from the belly to the centre of the nut - so a power stroke of 9"with a 3" brace. I estimate weight to be around 120lbs at this point and the bow feels to be stacking here. I've only drawn the bow by putting my foot on it and and 12" total draw is as much as I can manage. Maybe it will feel easier on the stock with a stirrup. Hopefully it will be enough for my purposes.

    10mm square bar for the tickler arrived today, with a small vice and a light bench it's going to be a bit of a challenge to bend cold, but facing new problems are the fun part of a new build!
    Taxus
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    Post by Taxus on Tue Feb 12, 2013 3:17 pm

    Sounds like your making good progress.

    A technique I use to make cold bending easier (especially with a small vice) is to use a hollow, round or square metal rod, the longer the easier it will be.
    You slide this over the trigger which is held in the vice and use the rod as a lever to bend the 10mm bar.
    Its is possible to put twists in this way but they can be undone by bending in the opposite direction.

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