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Re: Spine

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  • Duane Elms
    Hi, Dwane is going to talk about spine. Normally I don t do this, but the way a number of people are discussing this topic may leave some other folks with
    Message 1 of 1 , Nov 30, 2000
      Hi,

      Dwane is going to talk about spine.

      Normally I don't do this, but the way a number of people are discussing
      this topic may leave some other folks with mistaken impressions about
      what spine really is and how to deal with it.

      Spine is a measure of stiffness. In our particular case, it is a measure
      of the ability to resist bending by a wooden rod of nominally circular
      cross section. If you were to create an equation to derive the spine
      (stiffness) of such a rod, the equation would NOT contain a term for the
      length of the rod. For our purposes, the actual stiffness of the rod
      depends only on its diameter and its composition. It does not depend on
      the length.

      Archers have devised a method of measuring that stiffness and assigning
      to it a figure of merit. We suspend the rod between two knife blades 26
      inches apart and hang a 2 pound weight from the center of the span. Then
      we measure the deflection (typically in thousandths of an inch). The
      less deflection, the stiffer the rod. Lots of years of experimentation
      and empirical evidence and jawboning and consensus have resulted in the
      convention that if we divide the number 26 by the
      deflection (in inches), we get a number fairly close to the draw weight
      of a bow for which the shaft under test will make a reasonable 28 inch
      long arrow. For example, if the deflection is .5 inches then 26 divided
      by .5 is 52 and we would say that this shaft is spined 52 pounds and
      would expect a generic 28 inch long arrow made from that shaft to be
      more or less matched to a 50 pound (at 28 inches) draw long bow.

      So far so good. The problem is that not all arrows are 28 inches long,
      not all arrows have 'generic' heads and fletching, and not all bows are
      created equal.

      The flexibility of an arrow is most important when the arrow must bend
      to get around the bow. This requirement is most prominent in ELBs and
      other bows without any shelf or cutout. Here is a simplified description
      of what happens when an archer releases an arrow on a longbow. First
      note that the string will want to travel the shortest distance to its
      rest position, i.e. straight toward the center of the back of the bow.
      Also note that the arrow cannot be pointed straight at the target since
      the bow itself is in the way. When the string is released, it
      immediately tries to accelerate the arrow. This effort is resisted by
      the inertia of the arrow, largely concentrated in the mass of the
      arrowhead. The force of the string pushing on the rear of the arrow, but
      slightly out of alignment with the shaft, and resisted by the inertia of
      the arrow and arrowhead will cause the arrow to start to bend. The
      amount the arrow will bend depends, not surprisingly, on:

      1. The stiffness of the shaft.
      2. The force of the string.
      3. The mass of the shaft and arrowhead.
      4. The alignment of the string with the arrow.
      5. The efficiency of the bow.
      6. The will of Thor.
      7. Any misalignments due to an imperfect release.
      8. A whole bunch of other things outside this discussion.

      If everything is well matched, the arrow will bend just enough to get
      cleanly around the riser and continue straight to the target.

      If the shaft is not stiff enough, the arrow will bend too much and
      either miss the target, or in extreme cases, break.

      If the shaft is too stiff, the arrow will not bend enough and will be
      forced off target by the side of the bow.

      If the archer is too stiff, the arrow will be forced off the target by
      the will of Thor.

      Now as I mentioned above, the stiffness of a shaft is not a function of
      its length. However, it's clear that length must get into the total
      equation somewhere. Here's where that happens.

      When the archer releases the string, for an instant, due to inertia, the
      arrow behaves as if it were placed against a solid object and a force
      was placed on the other end. This basically means there's a lever at
      work and where there's a lever, there's a mechanical advantage related
      to length. I expect that we've all had the experience of being able to
      bend a long metal rod and then after cutting off a shorter piece,
      finding that
      shorter piece much more difficult to bend. Our difficulty did not result
      from some inherent change in the stiffness of the rod, but from a much
      shorter lever. The same process is at work when we make longer or
      shorter arrows.

      Here are some examples.

      1. We have an archer and bow with a 26 inch draw at 35 pounds and we
      wish to make 26 inch arrows that will be well matched to his bow. If we
      use shafts with a stiffness suitable for a 35 pound bow and 28 inch
      arrow, the shaft will be too stiff for our archer since his shaft needs
      to be shorter and therefore the lever arm used to bend the shaft will be
      shorter. To have the arrow bend correctly, it must be made from a more
      flexible shaft.

      2. We have an archer and bow with a 28 inch draw at 50 pounds. We wish
      to make 28 inch arrows that will be well matched to his bow, but he
      wants to use a very heavy broadhead. If we use shafts suitable for a 28
      inch arrow for the 50 pound bow, but put the heavy broadhead on the
      arrow, the shaft will be too flexible since there will be more inertia
      resisting the flight of the arrow and the arrow will bend more before
      leaving the bow. To have the arrow bend correctly, it must be made from
      a stiffer shaft.

      3. We have an archer and bow with a 31 inch draw at 40 pounds. We wish
      to make 31 inch arrows that will be well matched to his bow. If we use
      shafts with a stiffness suitable for 28 inch arrowa and a 40 pound bow
      and make 31 inch
      arrows, the shaft will be too flexible for our archer since his long
      arrows provide a greater lever arm for the string force to act through.
      To have the arrow bend correctly, it must be made from a stiffer shaft.

      A number of contributors to this list have provided various rules of
      thumb to determine how much change in stiffness of the arrow shaft is
      necessary to compensate for the above (and other) situations.

      The point of all the above is that:

      Spine is only a measurement of the stiffness or flexibility of the
      shaft.
      Spine numbers reported in pounds are only a convenient shorthand
      convention
      to help us get in the ballpark.
      There are a lot of things that can affect the stiffness requirement for
      a
      well matched arrow shaft.


      One final note. If you are going to err in selecting the spine for your
      arrows, err on the stiff side.


      For additional information on spine and other archery topics, try my
      pages
      at:

      http://pages.cthome.net/dkelms/sca_arch.htm


      Enough for now.

      Duane
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