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Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

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4 posters

    Power Tuff prod tip question

    Zardoz
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    Post by Zardoz Thu Dec 06, 2012 6:50 am

    I will be making my first prod out of Power Tuff (GC-67-UB). Being that the glass is unidirectional, should I reinforce the limb tips to prevent the string from possibly splitting the prod lengthwise? Also, is it safe to use Fast Flight string on it? Kiwijim did a fantastic job with that stuff and it seems WAY easier to work than steel and will perform better. I read his post about it, but I forgot to ask about the limb tips.
    Todd the archer
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    Post by Todd the archer Thu Dec 06, 2012 3:54 pm

    I would just safer and also allows you to narrow the tips more if you choose. Just do not make them too massive as that would hurt performance.



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    Post by Zardoz Thu Dec 06, 2012 5:12 pm

    Todd, it may be a stupid question, but do you think I should reinforce the belly side of the tips as well as the back side of the tips? I was thinking of maybe a thin piece of Bow-Tuff on the belly side with the grain at 90 degrees to the prod's grain.
    Todd the archer
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    Post by Todd the archer Thu Dec 06, 2012 5:58 pm

    No I would not, I believe it is most effective on the back. No need to add any more mass than needed.



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    Post by kiwijim Fri Dec 07, 2012 3:02 pm

    Hi guys, I have found no reinforcement is needed on the belly. For the nocks I uses several thin layers of phenolic epoxied together. I roughly shaped phenolic blocks and then expoxied then onto the prods. I cut in two sets of nocks into each one, one set for the bastartd string, the other for the string. I cut the nocks mostly into the phenolic; with minimal violation of the sides of the powertuff prod. Finally, for some extra insurance, I drilled a 4mm hole into the very tip of the nock and epoxied in a 4mm fibreglass rod. A couple of thousand shots later I have had no problems; even with such a heavy prod and fast flight strings.

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    Post by Hotspur Sun Dec 09, 2012 4:24 pm

    Zardoz wrote:I will be making my first prod out of Power Tuff (GC-67-UB)

    Hi Zardoz & all, I'm thinking of making a prod from this stuff as well. What is the design logic in deciding brace hight and power stroke? To date I just purchased alchem prods with these measurements dictated.
    Zardoz
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    Post by Zardoz Sun Dec 09, 2012 6:04 pm

    I really don't know for sure, but I read somewhere that for a longbow, brace height should be about 1/8 of the nock to nock lenth of the bow. I looked at the brace heights for longbows and recurves on the Martin archery site and that formula seems kind of right. As for draw length, I do know if it is really thick, you have to make it longer. Fiberglass can have a longer draw than wood. I plan on using 1/2" power tuff and making the nock to nock length 28" with a total draw length of 12" from about a 3 1/2" brace height for an 8 1/2" power stroke. All width tapering on the bottom edge with the rest of the tillering on the belly. Very much like Kiwijim did. I am ordering three 60" pieces so I can get 6 prods out of it because I expect to mess up at least four of them. I am getting the 2 inch wide stuff so it will let me experiment with the width. I plan on trying 1 1/2 to 1 3/4 inches wide.
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    Post by Todd the archer Sun Dec 09, 2012 6:57 pm

    I think Zardoz is on the right track. I would say that draw length (from prod to string catch) should be roughly equal to one limbs working length.



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    Post by kiwijim Sun Dec 09, 2012 8:16 pm

    Hi Guys,

    Zardof is spot on. For a prod that bends over it's entire length (ie comes compass) a good length of the prod, nock to nock, is twice the total draw length. My prod is 34" nock to nock. It has a 13" power stroke and is braced about 4".

    Therefore: Brace height (4") + powerstroke (13") = total draw length(17")

    17" inches x 2 = 34"

    Brace height is a bit more inexact. You want the brace height to be high. This is to reduce downward pressure from the prod onto the string and stock. I originally braced this prod at 3", but at this brace height I was going through too many strings. After increasing the brace height to 4" the problem was solved . You also don't want to brace the prod too high, or you will lose power and it will be difficult to string. I think between 4 and 5" of brace height is a good compromise. If you are using B50 for the string, I would brace at 5" to allow for stretch.

    Regards

    James

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