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    Prod Shape

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    directdrive8
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    Prod Shape

    Post by directdrive8 on Wed Mar 30, 2011 9:36 am

    Hi, All:

    In a previous post, I stated that I plan to build a crossbow using a laminated wood/fiberglass prod. I had intended to make the prod straight with recurved ends. However, I just purchased Ralph Payne-Gallwey's "The Crossbow" and it seems most, if not all the prods of crossbows "back-when" were made with a purposeful set in them. Having made a number of longbows, I know that set in the limbs is generally something to be avoided as it robs the bow of power. That being said, should I make my laminated prod with the limbs set as the old laminated and steel prods were made or go ahead and make it straight as originally planned? I understand why they made the prods the way they did, based on what I've read. However, it seems to me that I'd have a much more powerful prod were it straight rather than set. All suggestions and opinions are appreciated.
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    mac
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    Re: Prod Shape

    Post by mac on Wed Mar 30, 2011 10:10 am

    Direct,

    Steel bows need "deflex" or "set" to keep within the elastic range of the material. Likewise, a self-wood crossbow lathe must have deflex. Without it, the bow would blow up. It's all about keeping within permissible values of elongation and compression on the back and belly of the bow.

    Have you got the Traditional Boyer's Bible" series? http://www.amazon.com/Traditional-Bowyers-Bible-1/dp/1585740853
    It is an essential read. They don't deal with crossbows at all, so you have to read between the lines.

    Mac
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    Re: Prod Shape

    Post by directdrive8 on Wed Mar 30, 2011 12:12 pm

    Hi, Mac: Yes. I have the TBB. I suppose you've answered my question for me regarding set in the limbs. I didn't realize the metal prods had the limitations you mention. I suppose that's why the term "metal fatigue" was coined. Thanks
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    Re: Prod Shape

    Post by Todd the archer on Wed Mar 30, 2011 12:57 pm

    Without getting too technical, much has to do with what style and use you have in mind. If building a medieval replica with it's short power stroke you could use a straight or even a reflexed prod. Something more modern with a longer power stroke might need a deflexed prod or a longer prod. Like Mac said it's all about working within stress limits.
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    Re: Prod Shape

    Post by Basilisk120 on Wed Mar 30, 2011 10:59 pm

    I always thought that some of those old bows had just taken a set simple from time. Likely from spending too much time strung. And I'll have to look for the picture again but more than one medieval crossbow really looks like it was restored with the prod on backwards so it looked like it had a nasty set when it was really reflexed.



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    Re: Prod Shape

    Post by mac on Thu Mar 31, 2011 6:23 am

    Basilisk,

    You are probably thinking of the crossbow at Köln (Cologne). Here is a link to a picture of it with the bow correctly mounted. http://www.flickr.com/photos/8435962@N06/2579981662

    Mac

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