Spine
Spine is basically a measure of the stiffness of an arrow shaft. More
accurately, it is a measure of the deflection a shaft exhibits when a two pound
weight is suspended from the middle of the shaft when the shaft is supported at
two points 26 inches apart and the shaft is rotated so that the grain of the
wood is vertical. This measurement is generally made with a device called,
surprise, a spine tester. It is important that the grain of the shaft be
oriented properly since a grained material exhibits different stiffness with and
against the grain. Arrow shafts are measured to determine the greatest stiffness
and that measurement is across the grain. Note that this is important because it
tells you how to orient the fletching on the shaft.
There are tables that convert shaft deflection into pounds, a more usual
description of the stiffness. This conversion can also be done using the simple
formula Spine (in pounds) equals 26 divided by the deflection in inches. This
says that if a shaft under test bends half an inch then is it has a 52 pound
spine measurement. I suspect that the formula is simple because of the selection
of the measurement method (the two pound weight seems to be the item most easily
used to tune the conversion). In any case, the above information should be
enough to allow a handy archer to jury rig a spine tester if one was needed.
The spine of an arrow is important because of a phenomenon called Archer's
Paradox. Archer's Paradox is essentially the process by which an arrow shaft
bends around the bow and shoots straight. Note the word bend. The force of the
bow string on the arrow causes the arrow to bend during the process of
accelerating the arrow off the bow. The amount of bend affects the flight of the
arrow and the accuracy and consistency of the shot. This is particularly
important where the arrow rest is significantly offset from the path of the
string such as with longbows shot off the hand. It is less important in center
shot bows where the motion of the arrow is essentially along the path of the bow
string upon release.
For the bow and arrow combination to work well, the arrow must bend just enough
to get around the riser of the bow. If it doesn't bend enough, the back end of
the arrow will impact the riser and the arrow will deflect. If it bends too
much, the back end of the arrow will fly off to the side and the arrow will
deflect. Just right and the arrow goes straight and everyone is happy.
Selecting arrow shafts for your bow is a little more complicated than simply
buying shafts with the same spine rating as your bow weight. There are a couple
more things that affect the effective spine of a completed arrow. The first is
the weight (mass) of the arrow head or point. The heavier the point of an arrow,
the lower the effective spine of the shaft. This is because of the increased
inertia provided by the greater mass. With a heavier point, acceleration of the
arrow will be slower, and more energy will accumulate in the arrow shaft
reflected in greater bending and a lower effective spine. All other things being
equal, an arrow with a 30 grain target point on it will act like a more heavily
spined arrow than the same arrow with a 125 grain field point.
The second thing that affects the effective spine of an arrow is the efficiency
of the bow. All bows of the same draw weight are not created equal. My 35# 70"
Wing recurve will put significantly more of its stored energy into an arrow shot
from it than someone's 35# "D" section hickory selfbow. Once again, more energy
in means more bending during acceleration and a lower effective spine.
One more thing to remember is that spine is measured over 26 inches of the arrow
shaft. If your draw length, and consequently your arrow length, is significantly
different than 28 inches, the effective spine of your arrow shafts will be
different. The rule of thumb here is that you require about three to five more
pounds of spine for every inch increase in arrow length over 28 inches. You
require two to three less pounds of spine for every inch decrease in arrow
length under 28 inches. Stiffness increases faster as a shaft is shortened.
All this said, however, the best way to select shafts with the correct spine for
your bow and shooting style is through bare bow testing. Here you shoot
unfletched arrows of differing spines while observing the flight of the arrows.
The ones that fly straight and true even without fletching are correctly spined
for your shooting. If you must err, err on the side of stiffer arrows. It's
safer and allows easier archer adjustments.
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