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    affects of brace height

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    panne
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    affects of brace height

    Post by panne on Wed Dec 12, 2012 9:46 am

    looking for some insight on how different brace heights affect performance on a crossbow, without changing the prod mounting and trigger locations, using the same weight bolt.

    thanks,
    panne
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    Todd the archer
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    Re: affects of brace height

    Post by Todd the archer on Wed Dec 12, 2012 2:26 pm

    Lower brace height gives a longer power stroke and thus faster speed. However most crossbows we build using a one piece prod have the prod canted. Braced too low and you increase string drag and the string might jump off the nocks, too high and the string might travel across the deck too high and could cause a miss fire.



    Todd

    panne
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    Re: affects of brace height

    Post by panne on Thu Dec 13, 2012 7:24 am

    so if the string pressure and brace height is correct the string should have enough pressure in the unspanned position to keep the string in contact with the rail through the whole draw length, since the string pressure will be decreased in the spanned position, directly proportional to the degree of prod angle? so if it misfires, lowering the brace height in small increments to increase the string pressure should help correct misfires, but will also slow the FPS due to the increase in drag as it moves forward?
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    mac
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    Re: affects of brace height

    Post by mac on Thu Dec 13, 2012 9:52 am

    Panne,

    There are many different ways that a bolt can "misfire". So long as the center of the string travels in a way that will keep it pressed against the center of the end of the bolt, it does not much matter what the brace height is.

    The danger which Todd mentioned is that a low brace height can sometimes cause the string loops to get over the upper shoulders of the nocks. This will cause the string to leave the bow, and the prod limbs to continue traveling forward unchecked. The resulting stress reversal is decidedly prejudicial to the life of the prod. Indeed, an organic prod might fail right then and there. Therefore, this particular mode of "misfire" should be guarded against.

    In general, I think you will find that if you lower the brace height a bit, the increase in friction of the string on the top of the tiller is more than compensated for by the longer stroke. That is, so long as the string still has tension in its resting position.

    Mac

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