## Re: Spine

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• 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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