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Re: [SCA-Archery] poundage question

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  • eulenhorst@juno.com
    No. The pre-stress is higher but the total stress is identical. A spring tensioned to the same point will only show a certain poundage regardless of the
    Message 1 of 24 , Apr 2, 2004
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      No. The pre-stress is higher but the total stress is identical. A spring tensioned to the same point will only show a certain poundage regardless of the prestress. The archery mechanics site posted earlier has a good discussion of spring loading and prestress under plunger buttons. My results are empirical based on actual tests with a bow scale, not theoretical.

      In Service to the dream
      Carolus von Eulenhorst
      eulenhorst@...
      BTW, that mechanics site has some references on the home page and seems well grounded.
      CvE

      -- Siegfried Sebastian Faust <crossbow@...> wrote:
      Ok Carolus, I see a slight error here, and I want to try to explain this,
      if the explantion doesn't work, I'll draw a graphic ... just let me know.

      I find the
      >poundage the same at the same draw length regardless of where I have the
      >brace height..The bow is flexed the same amount at the same draw length
      >though it is, indeed, flexed more at rest with a higher brace height.
      [snip]
      >The effect you see is not the result of poundage change but rather
      >distance change. For example, a crossbow braced at 4 inches and drawn 10
      >inches with a draw weight of 62.5 pounds will yield 625 inch pounds of
      >pull. If the brace height is raised to 4.4 inches it reduces the draw
      >length to 9.6 inches. When multiplied against the same 62.5 pound pull
      >the result is 600 inch pounds.
      [snip]
      >If you do actually see the poundage change I would like more information
      >on the poundages at each brace height, draw lengths, and construction of
      >the bows.

      Ok Carolus, simple physics ... and I'm going to use crossbows as an
      example, just because I find it a better mental model.

      Ok, Let's say you have a 28" wide crossbow prod, and a 26" long
      string. That gives you some amount of brace height.

      Ok, now imagine the crossbow drawn back and cocked. Assume now, that
      there is a 1" wide nut/shelf/etc it is in.

      If you measure the length now, of the string on either side of the nut to
      the tip. You will find it is 12.5" ... half the width of the string, after
      the 1" of nut is taken into consideration. So the angle the prod is being
      held at, is to allow for a 12.5" length of string on either side of it to
      exist. Measure the poundage here.

      Ok, now, take the string off and twist it tighter until it is only 25" long.

      Ok, now put it back on the crossbow, and you will have a higher brace
      height. Agreed?

      So now, pull back/cock the crossbow. Do the same measurements. Now, there
      is only 12" on either side of string. By simple physics, the prod has been
      pulled back farther. There is no other way around it. Now, unless this
      prod 'somehow' has magical properties that give it a smooth unchanging
      poundage (I know of none), the fact that it is being stressed farther,
      means it took more force to get it there, and therefore, if you put a
      bowscale on it now, the poundage will be higher.

      Now, as stated, the power stroke is now shorter, as the string stops
      sooner, and this counteracts 'somewhat' the difference. However, each bow
      is different, and some stringheight/poundage is the optimum for that
      bow/prod, others are less efficient, therefore, depending upon the bow, you
      can adjust it's poundage, and it's power, by playing with that, because in
      my experience, it isn't a smooth curve.

      Siegfried
      (Who will also sit down and do lots of string pulling on one of his
      crossbows for ya if he has to *grin*)



      ___________________________________________________________________________
      THL Siegfried Sebastian Faust http://crossbows.biz/
      Barony of Highland Foorde Baronial Web Minister & Archery Marshal
      Kingdom of Atlantia Deputy Kingdom Earl Marshal for Target Archery
      http://highland-foorde.atlantia.sca.org/ http://archery.atlantia.sca.org/



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    • Scott Jaqua
      Actually Carolus, as a logic thought problem, I come up with more total stress too. Lower brace height equals a shorter string. A sorter string at the same
      Message 2 of 24 , Apr 2, 2004
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        Actually Carolus, as a logic thought problem, I come up with more total
        stress too. Lower brace height equals a shorter string. A sorter string at
        the same draw length means the tip of the limb had to flex further. And thus
        more total stress. But I can't tell you if this is one of the cases where
        the real world observations don't match theory.

        Njall
        (jr rocket scientist, for a real rocket scientist, talk to my dad)

        Scott B. Jaqua
        ----------------------------------------------------------------------------
        -----------------------------
        The most import right of all, is that of free speech. Without that, all your
        other rights will soon be taken away. So, I may disagree with what you say,
        but I will defend until death, your right to say it!
        ----- Original Message -----
        From: <eulenhorst@...>
        To: <SCA-Archery@yahoogroups.com>
        Sent: Friday, April 02, 2004 2:16 PM
        Subject: Re: [SCA-Archery] poundage question


        >
        > No. The pre-stress is higher but the total stress is identical. A spring
        tensioned to the same point will only show a certain poundage regardless of
        the prestress. The archery mechanics site posted earlier has a good
        discussion of spring loading and prestress under plunger buttons. My
        results are empirical based on actual tests with a bow scale, not
        theoretical.
        >
        > In Service to the dream
        > Carolus von Eulenhorst
        > eulenhorst@...
        > BTW, that mechanics site has some references on the home page and seems
        well grounded.
        > CvE
        >
        > -- Siegfried Sebastian Faust <crossbow@...> wrote:
        > Ok Carolus, I see a slight error here, and I want to try to explain this,
        > if the explantion doesn't work, I'll draw a graphic ... just let me know.
        >
        > I find the
        > >poundage the same at the same draw length regardless of where I have the
        > >brace height..The bow is flexed the same amount at the same draw length
        > >though it is, indeed, flexed more at rest with a higher brace height.
        > [snip]
        > >The effect you see is not the result of poundage change but rather
        > >distance change. For example, a crossbow braced at 4 inches and drawn 10
        > >inches with a draw weight of 62.5 pounds will yield 625 inch pounds of
        > >pull. If the brace height is raised to 4.4 inches it reduces the draw
        > >length to 9.6 inches. When multiplied against the same 62.5 pound pull
        > >the result is 600 inch pounds.
        > [snip]
        > >If you do actually see the poundage change I would like more information
        > >on the poundages at each brace height, draw lengths, and construction of
        > >the bows.
        >
        > Ok Carolus, simple physics ... and I'm going to use crossbows as an
        > example, just because I find it a better mental model.
        >
        > Ok, Let's say you have a 28" wide crossbow prod, and a 26" long
        > string. That gives you some amount of brace height.
        >
        > Ok, now imagine the crossbow drawn back and cocked. Assume now, that
        > there is a 1" wide nut/shelf/etc it is in.
        >
        > If you measure the length now, of the string on either side of the nut to
        > the tip. You will find it is 12.5" ... half the width of the string,
        after
        > the 1" of nut is taken into consideration. So the angle the prod is being
        > held at, is to allow for a 12.5" length of string on either side of it to
        > exist. Measure the poundage here.
        >
        > Ok, now, take the string off and twist it tighter until it is only 25"
        long.
        >
        > Ok, now put it back on the crossbow, and you will have a higher brace
        > height. Agreed?
        >
        > So now, pull back/cock the crossbow. Do the same measurements. Now,
        there
        > is only 12" on either side of string. By simple physics, the prod has
        been
        > pulled back farther. There is no other way around it. Now, unless this
        > prod 'somehow' has magical properties that give it a smooth unchanging
        > poundage (I know of none), the fact that it is being stressed farther,
        > means it took more force to get it there, and therefore, if you put a
        > bowscale on it now, the poundage will be higher.
        >
        > Now, as stated, the power stroke is now shorter, as the string stops
        > sooner, and this counteracts 'somewhat' the difference. However, each
        bow
        > is different, and some stringheight/poundage is the optimum for that
        > bow/prod, others are less efficient, therefore, depending upon the bow,
        you
        > can adjust it's poundage, and it's power, by playing with that, because in
        > my experience, it isn't a smooth curve.
        >
        > Siegfried
        > (Who will also sit down and do lots of string pulling on one of his
        > crossbows for ya if he has to *grin*)
        >
        >
        >
        >
        ___________________________________________________________________________
        > THL Siegfried Sebastian Faust
        http://crossbows.biz/
        > Barony of Highland Foorde Baronial Web Minister & Archery
        Marshal
        > Kingdom of Atlantia Deputy Kingdom Earl Marshal for Target
        Archery
        > http://highland-foorde.atlantia.sca.org/
        http://archery.atlantia.sca.org/
        >
        >
        >
        > ---8<---------------------------------------------
        > Brought to you YahooGroups Ad Free in 2003 by Medieval Mart
        > Get Medieval at Mad Macsen's http://www.medievalmart.com/
        >
        > [Email to SCA-Archery-unsubscribe@yahoogroups.com to leave this list]
        >
        > Yahoo! Groups Links
        >
        >
        >
        >
        >
        >
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      • eulenhorst@juno.com
        This only works if the draw distance remains constant thus resulting in more draw with the higher brace height. If the bow is drawn to 28 , for example, it
        Message 3 of 24 , Apr 2, 2004
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          This only works if the draw distance remains constant thus resulting in more draw with the higher brace height. If the bow is drawn to 28", for example, it doesn't matter if it started with a brace of 7" or 8". The draw length in the first case is 21" and in the second 20". If, however we use a draw distance of 20" we get a draw length of 27" with a 7" brace and 28" with an 8" brace. In this case the draw weight does change.


          In Service to the dream
          Carolus von Eulenhorst
          eulenhorst@...

          -- "Scott Jaqua" <jaqua@...> wrote:
          Actually Carolus, as a logic thought problem, I come up with more total
          stress too. Lower brace height equals a shorter string. A sorter string at
          the same draw length means the tip of the limb had to flex further. And thus
          more total stress. But I can't tell you if this is one of the cases where
          the real world observations don't match theory.

          Njall
          (jr rocket scientist, for a real rocket scientist, talk to my dad)

          Scott B. Jaqua
          ----------------------------------------------------------------------------
          -----------------------------
          The most import right of all, is that of free speech. Without that, all your
          other rights will soon be taken away. So, I may disagree with what you say,
          but I will defend until death, your right to say it!
          ----- Original Message -----
          From: <eulenhorst@...>
          To: <SCA-Archery@yahoogroups.com>
          Sent: Friday, April 02, 2004 2:16 PM
          Subject: Re: [SCA-Archery] poundage question


          >
          > No. The pre-stress is higher but the total stress is identical. A spring
          tensioned to the same point will only show a certain poundage regardless of
          the prestress. The archery mechanics site posted earlier has a good
          discussion of spring loading and prestress under plunger buttons. My
          results are empirical based on actual tests with a bow scale, not
          theoretical.
          >
          > In Service to the dream
          > Carolus von Eulenhorst
          > eulenhorst@...
          > BTW, that mechanics site has some references on the home page and seems
          well grounded.
          > CvE
          >
          > -- Siegfried Sebastian Faust <crossbow@...> wrote:
          > Ok Carolus, I see a slight error here, and I want to try to explain this,
          > if the explantion doesn't work, I'll draw a graphic ... just let me know.
          >
          > I find the
          > >poundage the same at the same draw length regardless of where I have the
          > >brace height..The bow is flexed the same amount at the same draw length
          > >though it is, indeed, flexed more at rest with a higher brace height.
          > [snip]
          > >The effect you see is not the result of poundage change but rather
          > >distance change. For example, a crossbow braced at 4 inches and drawn 10
          > >inches with a draw weight of 62.5 pounds will yield 625 inch pounds of
          > >pull. If the brace height is raised to 4.4 inches it reduces the draw
          > >length to 9.6 inches. When multiplied against the same 62.5 pound pull
          > >the result is 600 inch pounds.
          > [snip]
          > >If you do actually see the poundage change I would like more information
          > >on the poundages at each brace height, draw lengths, and construction of
          > >the bows.
          >
          > Ok Carolus, simple physics ... and I'm going to use crossbows as an
          > example, just because I find it a better mental model.
          >
          > Ok, Let's say you have a 28" wide crossbow prod, and a 26" long
          > string. That gives you some amount of brace height.
          >
          > Ok, now imagine the crossbow drawn back and cocked. Assume now, that
          > there is a 1" wide nut/shelf/etc it is in.
          >
          > If you measure the length now, of the string on either side of the nut to
          > the tip. You will find it is 12.5" ... half the width of the string,
          after
          > the 1" of nut is taken into consideration. So the angle the prod is being
          > held at, is to allow for a 12.5" length of string on either side of it to
          > exist. Measure the poundage here.
          >
          > Ok, now, take the string off and twist it tighter until it is only 25"
          long.
          >
          > Ok, now put it back on the crossbow, and you will have a higher brace
          > height. Agreed?
          >
          > So now, pull back/cock the crossbow. Do the same measurements. Now,
          there
          > is only 12" on either side of string. By simple physics, the prod has
          been
          > pulled back farther. There is no other way around it. Now, unless this
          > prod 'somehow' has magical properties that give it a smooth unchanging
          > poundage (I know of none), the fact that it is being stressed farther,
          > means it took more force to get it there, and therefore, if you put a
          > bowscale on it now, the poundage will be higher.
          >
          > Now, as stated, the power stroke is now shorter, as the string stops
          > sooner, and this counteracts 'somewhat' the difference. However, each
          bow
          > is different, and some stringheight/poundage is the optimum for that
          > bow/prod, others are less efficient, therefore, depending upon the bow,
          you
          > can adjust it's poundage, and it's power, by playing with that, because in
          > my experience, it isn't a smooth curve.
          >
          > Siegfried
          > (Who will also sit down and do lots of string pulling on one of his
          > crossbows for ya if he has to *grin*)
          >
          >
          >
          >
          ___________________________________________________________________________
          > THL Siegfried Sebastian Faust
          http://crossbows.biz/
          > Barony of Highland Foorde Baronial Web Minister & Archery
          Marshal
          > Kingdom of Atlantia Deputy Kingdom Earl Marshal for Target
          Archery
          > http://highland-foorde.atlantia.sca.org/
          http://archery.atlantia.sca.org/
          >
          >
          >
          > ---8<---------------------------------------------
          > Brought to you YahooGroups Ad Free in 2003 by Medieval Mart
          > Get Medieval at Mad Macsen's http://www.medievalmart.com/
          >
          > [Email to SCA-Archery-unsubscribe@yahoogroups.com to leave this list]
          >
          > Yahoo! Groups Links
          >
          >
          >
          >
          >
          >
          > ________________________________________________________________
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          > Surf the Web up to FIVE TIMES FASTER!
          > Only $14.95/ month - visit www.juno.com to sign up today!
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          >
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          >



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        • Siegfried Sebastian Faust
          Hey Carolus ... Sorry for the delay in my reply, been sick. ... The point is, that if you take a bow/prod ... and put a shorter string on it, then pull it back
          Message 4 of 24 , Apr 8, 2004
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            Hey Carolus ... Sorry for the delay in my reply, been sick.

            Anyway ... I think you are missing something:

            >No. The pre-stress is higher but the total stress is identical. A spring
            >tensioned to the same point will only show a certain poundage regardless
            >of the prestress. The archery mechanics site posted earlier has a good
            >discussion of spring loading and prestress under plunger buttons.

            The point is, that if you take a bow/prod ... and put a shorter string on
            it, then pull it back to the same draw length (not power stroke, but draw
            length) ...

            Then because of the shorter string, the 'spring' is NOT tensioned to the
            same point. To get the shorter string to come back to the same spot, it
            has to pull the 'spring' farther into deflection. Thereby, stressing the
            material more, thereby, raising the 'poundage' (While very possibly not
            raising the power because of the shorter power stroke).

            Again, clear your mind for a second and imagine a crossbow prod with a 3"
            brace height, and a 28" string on it. Draw it back to a pin, and you will
            measure 14" of string on either side of the pin. Now take it off and put
            a 26" string on it. You have your higher brace height.

            Now, put it back on the same 'jig', and pull it back to the same pin. The
            prod is now deflected MORE than it was before, because there is only 13" of
            string on either side of the pin, forcing the prod to bend more to allow
            for this.

            Again, if you need it, I'll draw a picture or take a few photos of this in
            action.

            > My results are empirical based on actual tests with a bow scale, not
            > theoretical.

            As are mine. I can easily put a shorter string on the same prod and have
            my bowscale read a higher poundage at the same draw.

            Siegfried



            ___________________________________________________________________________
            THL Siegfried Sebastian Faust http://crossbows.biz/
            Barony of Highland Foorde Baronial Web Minister & Archery Marshal
            Kingdom of Atlantia Deputy Kingdom Earl Marshal for Target Archery
            http://highland-foorde.atlantia.sca.org/ http://archery.atlantia.sca.org/
          • Siegfried Sebastian Faust
            ... Exactly ___________________________________________________________________________ THL Siegfried Sebastian Faust
            Message 5 of 24 , Apr 8, 2004
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              At 06:45 PM 4/2/2004, Scott Jaqua wrote:
              >Actually Carolus, as a logic thought problem, I come up with more total
              >stress too. Lower brace height equals a shorter string. A sorter string at
              >the same draw length means the tip of the limb had to flex further. And thus
              >more total stress.

              Exactly


              ___________________________________________________________________________
              THL Siegfried Sebastian Faust http://crossbows.biz/
              Barony of Highland Foorde Baronial Web Minister & Archery Marshal
              Kingdom of Atlantia Deputy Kingdom Earl Marshal for Target Archery
              http://highland-foorde.atlantia.sca.org/ http://archery.atlantia.sca.org/
            • Siegfried Sebastian Faust
              ... No carolus, it does make a difference, the shorter string causes a higher angle of deflection to get to 28 Siegfried
              Message 6 of 24 , Apr 8, 2004
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                >This only works if the draw distance remains constant thus resulting in
                >more draw with the higher brace height. If the bow is drawn to 28", for
                >example, it doesn't matter if it started with a brace of 7" or 8". The
                >draw length in the first case is 21" and in the second 20". If, however
                >we use a draw distance of 20" we get a draw length of 27" with a 7" brace
                >and 28" with an 8" brace. In this case the draw weight does change.

                No carolus, it does make a difference, the shorter string causes a higher
                angle of deflection to get to 28"

                Siegfried



                ___________________________________________________________________________
                THL Siegfried Sebastian Faust http://crossbows.biz/
                Barony of Highland Foorde Baronial Web Minister & Archery Marshal
                Kingdom of Atlantia Deputy Kingdom Earl Marshal for Target Archery
                http://highland-foorde.atlantia.sca.org/ http://archery.atlantia.sca.org/
              • Carolus Eulenhorst
                Angles might be slightly different. but poundages have measured the same. There may be some small difference but not enough to read on a scale with 1 pound
                Message 7 of 24 , Apr 9, 2004
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                  Angles might be slightly different. but poundages have measured the same.
                  There may be some small difference but not enough to read on a scale
                  with 1 pound resolution. I would still like to see someone else do some
                  empirical tests on this. Thinking on it while writing this, I might be
                  able to see some difference on a longbow (all my work was on recurves)
                  were the rotation of the tips was not a factor.
                  Carolus

                  On Thu, 08 Apr 2004 23:25:14 -0400 Siegfried Sebastian Faust
                  <crossbow@...> writes:
                  >
                  > >This only works if the draw distance remains constant thus
                  > resulting in
                  > >more draw with the higher brace height. If the bow is drawn to
                  > 28", for
                  > >example, it doesn't matter if it started with a brace of 7" or 8".
                  > The
                  > >draw length in the first case is 21" and in the second 20". If,
                  > however
                  > >we use a draw distance of 20" we get a draw length of 27" with a 7"
                  > brace
                  > >and 28" with an 8" brace. In this case the draw weight does
                  > change.
                  >
                  > No carolus, it does make a difference, the shorter string causes a
                  > higher
                  > angle of deflection to get to 28"
                  >
                  > Siegfried

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                • Carolus Eulenhorst
                  What numbers have you recorded? Carolus On Thu, 08 Apr 2004 23:22:25 -0400 Siegfried Sebastian Faust ...
                  Message 8 of 24 , Apr 9, 2004
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                    What numbers have you recorded?
                    Carolus

                    On Thu, 08 Apr 2004 23:22:25 -0400 Siegfried Sebastian Faust
                    <crossbow@...> writes:
                    > Hey Carolus ... Sorry for the delay in my reply, been sick.
                    >
                    > Anyway ... I think you are missing something:
                    >
                    > >No. The pre-stress is higher but the total stress is identical. A
                    > spring
                    > >tensioned to the same point will only show a certain poundage
                    > regardless
                    > >of the prestress. The archery mechanics site posted earlier has a
                    > good
                    > >discussion of spring loading and prestress under plunger buttons.
                    >
                    > The point is, that if you take a bow/prod ... and put a shorter
                    > string on
                    > it, then pull it back to the same draw length (not power stroke, but
                    > draw
                    > length) ...
                    >
                    > Then because of the shorter string, the 'spring' is NOT tensioned to
                    > the
                    > same point. To get the shorter string to come back to the same
                    > spot, it
                    > has to pull the 'spring' farther into deflection. Thereby,
                    > stressing the
                    > material more, thereby, raising the 'poundage' (While very possibly
                    > not
                    > raising the power because of the shorter power stroke).
                    >
                    > Again, clear your mind for a second and imagine a crossbow prod with
                    > a 3"
                    > brace height, and a 28" string on it. Draw it back to a pin, and
                    > you will
                    > measure 14" of string on either side of the pin. Now take it off
                    > and put
                    > a 26" string on it. You have your higher brace height.
                    >
                    > Now, put it back on the same 'jig', and pull it back to the same
                    > pin. The
                    > prod is now deflected MORE than it was before, because there is only
                    > 13" of
                    > string on either side of the pin, forcing the prod to bend more to
                    > allow
                    > for this.
                    >
                    > Again, if you need it, I'll draw a picture or take a few photos of
                    > this in
                    > action.
                    >
                    > > My results are empirical based on actual tests with a bow scale,
                    > not
                    > > theoretical.
                    >
                    > As are mine. I can easily put a shorter string on the same prod and
                    > have
                    > my bowscale read a higher poundage at the same draw.
                    >
                    > Siegfried

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                  • Carolus Eulenhorst
                    I just ran this through the calculator with an assumption of 3 being normal brace height and 10 of draw. This changes the angel of deflection by 12 degrees
                    Message 9 of 24 , Apr 9, 2004
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                      I just ran this through the calculator with an assumption of 3" being
                      normal brace height and 10" of draw. This changes the angel of
                      deflection by 12 degrees or about 15.5% while shortening the power stroke
                      30% and raising the brace 100% to 6 inches (not taking any rotation of
                      the tips through prod recurve into account. Realizing that the flexion
                      of a spring in not a linear curve and the fact that I would be highly
                      suspect of a bow over braced by this amount, yes, in this case you might
                      see such a change in poundage. However, in practice I don't find it so.
                      Of course, I have only been working with brace height changes in the +-
                      1" range, not the 3" in this case. Extremes do make a difference as
                      shown in some of the "scientific" studies used to prove political points.

                      Carolus

                      On Thu, 08 Apr 2004 23:22:25 -0400 Siegfried Sebastian Faust
                      <crossbow@...> writes:
                      > Hey Carolus ... Sorry for the delay in my reply, been sick.
                      >
                      > Anyway ... I think you are missing something:
                      >
                      > >No. The pre-stress is higher but the total stress is identical. A
                      > spring
                      > >tensioned to the same point will only show a certain poundage
                      > regardless
                      > >of the prestress. The archery mechanics site posted earlier has a
                      > good
                      > >discussion of spring loading and prestress under plunger buttons.
                      >
                      > The point is, that if you take a bow/prod ... and put a shorter
                      > string on
                      > it, then pull it back to the same draw length (not power stroke, but
                      > draw
                      > length) ...
                      >
                      > Then because of the shorter string, the 'spring' is NOT tensioned to
                      > the
                      > same point. To get the shorter string to come back to the same
                      > spot, it
                      > has to pull the 'spring' farther into deflection. Thereby,
                      > stressing the
                      > material more, thereby, raising the 'poundage' (While very possibly
                      > not
                      > raising the power because of the shorter power stroke).
                      >
                      > Again, clear your mind for a second and imagine a crossbow prod with
                      > a 3"
                      > brace height, and a 28" string on it. Draw it back to a pin, and
                      > you will
                      > measure 14" of string on either side of the pin. Now take it off
                      > and put
                      > a 26" string on it. You have your higher brace height.
                      >
                      > Now, put it back on the same 'jig', and pull it back to the same
                      > pin. The
                      > prod is now deflected MORE than it was before, because there is only
                      > 13" of
                      > string on either side of the pin, forcing the prod to bend more to
                      > allow
                      > for this.
                      >
                      > Again, if you need it, I'll draw a picture or take a few photos of
                      > this in
                      > action.
                      >
                      > > My results are empirical based on actual tests with a bow scale,
                      > not
                      > > theoretical.
                      >
                      > As are mine. I can easily put a shorter string on the same prod and
                      > have
                      > my bowscale read a higher poundage at the same draw.
                      >
                      > Siegfried

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                    • Bruce R. Gordon
                      Greetings I approach this discussion with a certain amount of trepidation, since I am definitely not well-versed in the physics of archery or math analysis,
                      Message 10 of 24 , Apr 9, 2004
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                        Greetings
                        I approach this discussion with a certain amount of trepidation,
                        since I am definitely not well-versed in the physics of archery or math
                        analysis, but...
                        On a standard bow, say, a #35 recurve, it is often said; 35 pounds
                        AT 28 INCHES (emphasis mine). I have always taken this to mean that if
                        I underdraw the bow, I get less than 35 lbs, and if I overdraw it more
                        than 28 in., I get a higher poundage. Is this the case, and does it
                        therefore have relevence to the discussion at hand? Or am I missing
                        some factor here that would make this not to the point?

                        Nigel FitzMaurice

                        > I just ran this through the calculator with an assumption of 3" being
                        > normal brace height and 10" of draw. This changes the angel of
                        > deflection by 12 degrees or about 15.5% while shortening the power
                        stroke
                        > 30% and raising the brace 100% to 6 inches (not taking any rotation of
                        > the tips through prod recurve into account. Realizing that the
                        flexion
                        > of a spring in not a linear curve and the fact that I would be highly
                        > suspect of a bow over braced by this amount, yes, in this case you
                        might
                        > see such a change in poundage. However, in practice I don't find it
                        so.
                        > Of course, I have only been working with brace height changes in the
                        +-
                        > 1" range, not the 3" in this case. Extremes do make a difference as
                        > shown in some of the "scientific" studies used to prove political
                        points.
                        >
                        > Carolus
                        >
                        > On Thu, 08 Apr 2004 23:22:25 -0400 Siegfried Sebastian Faust
                        > <crossbow@...> writes:
                        > > Hey Carolus ... Sorry for the delay in my reply, been sick.
                        > >
                        > > Anyway ... I think you are missing something:
                        > >
                        > > >No. The pre-stress is higher but the total stress is identical.
                        A
                        > > spring
                        > > >tensioned to the same point will only show a certain poundage
                        > > regardless
                        > > >of the prestress. The archery mechanics site posted earlier has a
                        > > good
                        > > >discussion of spring loading and prestress under plunger buttons.
                        > >
                        > > The point is, that if you take a bow/prod ... and put a shorter
                        > > string on
                        > > it, then pull it back to the same draw length (not power stroke,
                        but
                        > > draw
                        > > length) ...
                        > >
                        > > Then because of the shorter string, the 'spring' is NOT tensioned
                        to
                        > > the
                        > > same point. To get the shorter string to come back to the same
                        > > spot, it
                        > > has to pull the 'spring' farther into deflection. Thereby,
                        > > stressing the
                        > > material more, thereby, raising the 'poundage' (While very
                        possibly
                        > > not
                        > > raising the power because of the shorter power stroke).
                        > >
                        > > Again, clear your mind for a second and imagine a crossbow prod
                        with
                        > > a 3"
                        > > brace height, and a 28" string on it. Draw it back to a pin, and
                        > > you will
                        > > measure 14" of string on either side of the pin. Now take it off
                        > > and put
                        > > a 26" string on it. You have your higher brace height.
                        > >
                        > > Now, put it back on the same 'jig', and pull it back to the same
                        > > pin. The
                        > > prod is now deflected MORE than it was before, because there is
                        only
                        > > 13" of
                        > > string on either side of the pin, forcing the prod to bend more to
                        > > allow
                        > > for this.
                        > >
                        > > Again, if you need it, I'll draw a picture or take a few photos of
                        > > this in
                        > > action.
                        > >
                        > > > My results are empirical based on actual tests with a bow
                        scale,
                        > > not
                        > > > theoretical.
                        > >
                        > > As are mine. I can easily put a shorter string on the same prod
                        and
                        > > have
                        > > my bowscale read a higher poundage at the same draw.
                        > >
                        > > Siegfried
                        >
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                      • Siegfried Sebastian Faust
                        ... Tonight, and/or this weekend, I will take a few of my crossbows and do the measurements on them and post it here ... Siegfried
                        Message 11 of 24 , Apr 9, 2004
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                          >What numbers have you recorded?

                          Tonight, and/or this weekend, I will take a few of my crossbows and do the
                          measurements on them and post it here ...

                          Siegfried



                          ___________________________________________________________________________
                          THL Siegfried Sebastian Faust http://crossbows.biz/
                          Barony of Highland Foorde Baronial Web Minister & Archery Marshal
                          Kingdom of Atlantia Deputy Kingdom Earl Marshal for Target Archery
                          http://highland-foorde.atlantia.sca.org/ http://archery.atlantia.sca.org/
                        • Siegfried Sebastian Faust
                          ... Ok, at least we are riding the same thought train now :) Now that we agree that it can change the poundage ... I agree that all springs are not linear
                          Message 12 of 24 , Apr 9, 2004
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                            >I just ran this through the calculator with an assumption of 3" being
                            >normal brace height and 10" of draw. This changes the angel of
                            >deflection by 12 degrees or about 15.5% while shortening the power stroke
                            >30% and raising the brace 100% to 6 inches (not taking any rotation of
                            >the tips through prod recurve into account. Realizing that the flexion
                            >of a spring in not a linear curve and the fact that I would be highly
                            >suspect of a bow over braced by this amount, yes, in this case you might
                            >see such a change in poundage. However, in practice I don't find it so.
                            >Of course, I have only been working with brace height changes in the +-
                            >1" range, not the 3" in this case. Extremes do make a difference as
                            >shown in some of the "scientific" studies used to prove political points.

                            Ok, at least we are riding the same thought train now :)

                            Now that we agree that it 'can' change the poundage ... I agree that all
                            springs are not linear curves (not even close), and that the difference may
                            in fact, depending on the prod be unmeasureable, or visible.

                            In my experience, on at least some of my crossbows, it has been
                            measureable, as I have measured differences.

                            Again, this weekend I will grab a couple of crossbows and play.

                            Siegfried


                            ___________________________________________________________________________
                            THL Siegfried Sebastian Faust http://crossbows.biz/
                            Barony of Highland Foorde Baronial Web Minister & Archery Marshal
                            Kingdom of Atlantia Deputy Kingdom Earl Marshal for Target Archery
                            http://highland-foorde.atlantia.sca.org/ http://archery.atlantia.sca.org/
                          • Carolus Eulenhorst
                            Semantics here. might not can depending on construction and design. I haven t seen it on recurve designs, it is not reported by any manufacturer (their
                            Message 13 of 24 , Apr 9, 2004
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                              Semantics here. "might" not "can" depending on construction and
                              design. I haven't seen it on recurve designs, it is not reported by any
                              manufacturer (their documentation always gives a range of brace heights
                              with no reference to poundage change) and only appears relevant when
                              taken outside the safe range of variation of a bow. Many changes can
                              happen when design specs are exceeded. Please be sure to give all
                              relevant measurements with your stats. My tests were with a crossbow
                              using a Iolo prod, braced from 3-5" pulled to a nut 14" from the prod.
                              Measured a constant 86 pounds.
                              Carolus


                              On Fri, 09 Apr 2004 09:39:54 -0400 Siegfried Sebastian Faust
                              <crossbow@...> writes:
                              >
                              > >I just ran this through the calculator with an assumption of 3"
                              > being
                              > >normal brace height and 10" of draw. This changes the angel of
                              > >deflection by 12 degrees or about 15.5% while shortening the power
                              > stroke
                              > >30% and raising the brace 100% to 6 inches (not taking any rotation
                              > of
                              > >the tips through prod recurve into account. Realizing that the
                              > flexion
                              > >of a spring in not a linear curve and the fact that I would be
                              > highly
                              > >suspect of a bow over braced by this amount, yes, in this case you
                              > might
                              > >see such a change in poundage. However, in practice I don't find
                              > it so.
                              > >Of course, I have only been working with brace height changes in
                              > the +-
                              > >1" range, not the 3" in this case. Extremes do make a difference
                              > as
                              > >shown in some of the "scientific" studies used to prove political
                              > points.
                              >
                              > Ok, at least we are riding the same thought train now :)
                              >
                              > Now that we agree that it 'can' change the poundage ... I agree that
                              > all
                              > springs are not linear curves (not even close), and that the
                              > difference may
                              > in fact, depending on the prod be unmeasureable, or visible.
                              >
                              > In my experience, on at least some of my crossbows, it has been
                              > measureable, as I have measured differences.
                              >
                              > Again, this weekend I will grab a couple of crossbows and play.
                              >
                              > Siegfried

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                            • Siegfried Sebastian Faust
                              ... But of course :) And I will be doing it with a Gladius prod, one of my own custom Power-Tuff Fiberglass ones, and maybe something else. I swear I have
                              Message 14 of 24 , Apr 9, 2004
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                                >Please be sure to give all
                                >relevant measurements with your stats. My tests were with a crossbow
                                >using a Iolo prod, braced from 3-5" pulled to a nut 14" from the prod.
                                >Measured a constant 86 pounds.
                                >Carolus

                                But of course :)

                                And I will be doing it with a Gladius prod, one of my own custom Power-Tuff
                                Fiberglass ones, and maybe something else.

                                I 'swear' I have seen the differences with these :)

                                Siegfried



                                ___________________________________________________________________________
                                THL Siegfried Sebastian Faust http://crossbows.biz/
                                Barony of Highland Foorde Baronial Web Minister & Archery Marshal
                                Kingdom of Atlantia Deputy Kingdom Earl Marshal for Target Archery
                                http://highland-foorde.atlantia.sca.org/ http://archery.atlantia.sca.org/
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