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Thread: Does a higher brace height make for higher draw weight at a given draw length?

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    In the Blue jonUK76's Avatar
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    Does a higher brace height make for higher draw weight at a given draw length?

    Just wondering, at the risk of sounding like a noob... All things being equal, at a given draw length, does increasing the brace height (i.e. shortening the string) result in a higher draw weight? I've heard people say it doesn't make a difference to draw weight - just the power stroke (higher brace height = shorter power stroke). I understand that the power stroke is shorter. But at the same time, the limbs are bent back further at the braced position, and so I think I'm right in saying that the limbs also will be bent further back at full draw? This should actually increase the draw weight, should it not?

    Any thoughts?





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    I think that you are correct. However, it isn't quite as simple as it first seems. As the string is drawn back, the angle the string makes with the limb itself, changes . The angles are bigger for the shorter string from start to finish of the draw. The string is pulling on the limb and bends it, but bending a leaf spring is easier when the bending force is applied at right angles to the spring. As we draw a bow the initial angles are small so a relatively large force has to be used, compared to a force applied at right angles. The shorter string gives slightly larger angles(closer to the right angle) all through the draw which compensates a little for the extra bend on the limb.
    This explains why it is so difficult to string a longbow by trying to slide the loop up to the nock in the top limb. The string angle with the limb is very small, so we are doing more of a compressing of the bow rather than applying a force at right angles.

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    jonUK76 (05-07-18)

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    It's an X I've taken part in an AIUK Ironman Shoot.The Fonz Award.AIUK subscriber. Timid Toad's Avatar
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    What you are inerested in is how far they are bent back at full draw, which will be the same.
    So with a high bh you have some energy in the bow (at brace) and you put some in yourself (through the draw).
    With a low bh there is less energy already in the bow (at brace) but you put more in yourself (through the draw, to anchor, which is the same in both examples)
    *however*
    When you loose the energy is rapidly imparted to the arrow. This increases until the string is straight and the arrow is flicked off. So a low bh will impart more energy to the arrow. In addition the residual energy has to go somewhere, so with a high bh your bow has more to absorb, which is mostly taken by the limbs. So higher bh can damage limbs, and it is extremely important to follow manufacturer's recommendations. The safest bh for limbs is no tension in the string at all, but that comes with it's own set of problems!
    "Time is an illusion. Lunchtime doubly so." Douglas Adams

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    Some real world figures I've just measured agree with JonUK and contradict Timid Toad:

    58" bow nominally rated at 55# with a 7 5/8 BH pulls 59# at my 30"ish according to my scale. Swapping the string for one from a 60" bow gives 6 5/8" BH and measures 56# at full draw

    Shortening the string to increase brace height also increases the amount of bend on the limb at full draw, and therefore increases the draw weight

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    jonUK76 (05-07-18)

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    Shortening the string does *not* increase the bend significantly, and 55 to 56lbs, is well within the error of most bowscales, even electronic ones.
    Cut a draw stick to 30". Pop it on the bow, hang on the wall and draw round. Change strings, repeat. It's an interesting exercise.
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    Sorry think you have misread the figures in my previous post. The bow is rated 55# @ 28" and one inch extra brace height takes the actual weight from 56# to 59# at my draw length.

    As the rough rule of thumb states 3# per inch of extra draw length, it suggests to me the extra inch of brace height roughly equates to an inch of extra draw length.

    So if we take the "interesting exercise" a step further, I reckon if you lengthen the draw stick by an inch, the bend will be exactly the same.

    Anybody else care to measure and report your findings?

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