Tackle matching (specifically wooden arrows to bows) is a subject that seems to cause
much confusion within the ranks of traditional bowhunters. Everyone has a theory...or
two or three! Much of that is due to the fact that individual shooting form, bow design and
arrow materials all introduce variables that mean 2 honest archers may recommend arrow
spines 20 lbs different for the same poor soul trying to select arrows. I write this in hope
that it will clarify and simplify things rather than deepen the confusion.
What is spine?
Spine is a measure of stiffness. How much backbone does that arrow have? That is
correct-spine as in backbone, not ”splines” which are a series of ridges and grooves that
allow multi-piece drive shafts to fit together to transfer power to the wheels of your pick-
up (Sorry, incorrect terminology is a pet peeve of mine). Long ago W. J. Rheingans and
Forrest Nagler worked out a formula for measuring arrow stiffness. Most arrow makers
use spine charts to sort out their shafts into various categories but the formula used to
make that confusing chart is simple. 26 divided by bow poundage gives you the deflection
in inches (usually a fraction of an inch) of a matched shaft. Deflection? Yes.
The arrow
shaft is supported at two points 26 inches apart and a 2 pound weight is hung from the
center. (A measurement of how much the shaft bends, or deflects, is taken.) Thus a 50 lb spine is 26/50 = 0.520. Or 26/70=0.3714285 (call it .371) You’ll
note that the higher the poundage the less the arrow deflects under its 2 pound load.
Voila! -”Spine” measurements.
What about draw length?
The rests for measuring deflection are 26 inches apart but that doesn’t mean they are for
26 inch arrows. Rheingans’ and Nagler’s formula became the AMO standard and provides
a baseline to work from to choose the arrow shafts for our own set-ups. That baseline
matches bow poundage to arrow stiffness for 28 inch draws.....in theory. At the time the
spine formula was developed nearly all bows in America and England were selfbows.
Bows made from a single piece of wood or two pieces spliced at the handle. Those bows
were almost never centershot though many may have had a small arrow shelf. Let’s wait a
bit to conclude about draw length and discuss...
Bow design?
For those unfamiliar with the term centershot or confused about it Centershot refers to
where the arrow lines up in relationship to the center of the bow. As a practical matter this
really means where the line of the string passes in relation to the arrowplate when the bow
is in shooting position-the limbs can wander all over the place between the nocks and the
nocks may lie on different sides of the bow’s center (on some selfbows they DO). On most
selfbows the arrow must be “shot around the bow” -The side of the bow holds the arrow
tip out so pointing the bow at the target leaves the arrow pointing several degrees to the
side (left for a right handed shooter) The fact that the arrow actually can be shot where the
bow is pointed is the archers paradox-the correct arrow will bend just far enough that it
takes off oscillating back and forth and eventually stabilizes flying in line to the target
where the bow hand was pointing rather than where the arrow shaft seemed to be
pointing. Obviously arrows for selfbows need to be more flexible than an arrow for a
centershot bow that launches the arrow in line with the string, the bow’s force and the
target. Does this mean that the AMO charts are recommending spines too weak for your
bow? Maybe.
My own findings have been as follows:
1. Wide at the arrow pass selfbows usually need arrows 5-10lbs lighter in spine than
the AMO spine charts indicate.
2. Bows with a cut in shelf that approaches centershot but doesn’t go all the way to
center usually match the spine charts.
3. Bows that are cut to or past center need stiffer arrows than the chart indicates.
Sometimes a whale of a lot more. 5 lbs extra may suffice for a bow cut to center or just
beyond but a bow cut deeply past center may need as much as 20 pounds of extra spine.
Back to drawlength.
There is a rule of thumb that says for every inch of arrow shaft above 28 “ one should add
5 lbs of spine and for every inch below 28inches one should subtract 5 pounds. While not
perfect, it is a good working rule. Please understand that this is talking about selecting
stiffer or more flexible shafts based upon your actual drawweight at your drawlength. If
you have a bow marked 50lbs @28” and only draw 26 then your shaft selection should
reflect the actual draw weight (probably 46lbs), the length of shaft you wish to shoot and
the degree of centershot of the bow.
Superbows At this point my advice is going to diverge from what many believe. For many years I
advised archers to select stronger or weaker shafts based upon bow performance. No less
an authority than Easton Aluminum had published charts based upon string material and
bow efficiency. Unfortunately the results were very spotty. Sometimes bow X which was
faster than a politician palming a campaign contribution did indeed need 20 extra pounds
of spine for perfect flight but the next screamer might only need 10lbs extra and a third hot
bow might shoot arrows spined directly corresponding to its drawweight. Over time a
pattern emerged- Spine requirements tended to correlate much more strongly with the degree of centershot than with overall bow performance. This made me stop and
reconsider the reasoning behind our expectation that very efficient/speedy bows need more
spine. We might suppose that a bow that stores more energy or imparts a greater
percentage of stored energy to the arrow would stress the arrow shaft more than a less
efficient design. That seems logical and it is to some degree true. However, the big issue
causing shaft flexion is inertia. The drawn bow suddenly accelerates the stationary arrow
by pushing on the nock end and because objects at rest tend to remain at rest the point
doesn’t move until it can resist no longer. In that tiny interim the nock end is moving while
the point end is resisting and the shaft flexes. Whether your bow is a 50lb selfbow with 3
inches of string follow or a radically reflexed carbon lam recurve drawing 50 lbs, that
initial kick in the pants is still 50lbs. The overall energy storage and delivery are very
different but the force on your fingers at full draw is the same and the initial force at the
arrow nock upon release is the same. Your Brand X bow may indeed be a marvel of
efficiency and may need arrows spined 20lbs over actual draw weight but I’d lay odds that
if it does need such stiff shafts it is cut well past center. Actually I have a rather doggy old
heavily deflexed, slightly recurved bow from the 70’s that no one would call fast that
needs arrows spined 20lbs over actual weight. It is cut far past center.
My experience with the super string materials is limited-(I still rather like Dacron)- but
within that limited experience the rule has remained true that centershot has far more
influence on spine requirements than efficiency.
Point Weight
Because inertia is the principle that causes shafts to flex upon release, point weight plays a
critical role in spine requirements. Heavy points mean greater inertia and more flex -light
ones less. The “standard” point weight is considered to be 125 grains. Considering that
Nagler advocated 90 grain broadheads in some of his writings and many target arrows of
his day were tipped with things such as bullet jackets It is debatable whether he had this
weight in mind but 125s do work for us with our baseline spine measurements. There have
been many rules of thumb proposed for adjusting spine ratings to differing point weights.
The folks advising 5lbs more spine for each additional 15 grains, 20 grains or 30 grains of
point weight are all correct as far as their experience is concerned. If shooter A has arrows
that were adequately spined with 125s and they act weak with 140s then 15 grains did
indeed push things into a 5lb heavier spine category. (We group shafts into categories such
as 50-55 or 50-54 depending upon the supplier). On the other hand shooter B may swap
points for some that are 20 grains heavier and still find his shafts fly perfectly. The
unknown is whether shooter A. had arrows that were barely stiff enough and shooter B
had some that were almost too stiff to begin with. The best advice is shoot and see but if
you are trying to startup from scratch and have decided upon a specific broadhead weight
my own rule of thumb is that each 25- 30 grains departure from the 125grain standard
means 5 lbs of spine in common hunting weights. Heavier points need stiffer shafts (higher
spine) lighter need lower spines.
Another issue occasionally mentioned is overall shaft weight-Some archers report a need
for greater stiffness when shooting Ash (heavy) compared to Port Orford Cedar
(moderately light). There is something to this but it isn’t easy to quantify-some Ash shafts
are fairly light and some cedars are fairly heavy. Even if you know the weight of the shafts
you’ll be getting, weight distributed along the full length of the arrow has a less dramatic
effect than weight at the ends. To complicate it all, differing woods have different average
modulus of elasticity ratings. If I may suggest...just ignore these differences in trying to
make an initial shafting choice. The variables are small and there are already so many other
variables in selecting arrow shafts you need not complicate your life with them. If you
discover later that extra heavy shafts mean buying one spine category stiffer shafts is
necessary you can do so.
The magic of long shafts
I struggled for a while to convince a family member that spine is important. He said that
everything shot pretty well for him. I watched him shoot. He was right. They all shot
about the same if they weighed close to the same. His groups were okay. The flight
seemed fair if not perfect. I also observed that he only drew those 29 and 30 inch shafts
with a 20 lb span in spine about 25 inches. Interestingly, many primitive archers used long
arrows-some of the S. American tribes actually using arrows 5 feet long! Long shafts have
an amazing ability to negate differences in stiffness. We could get into a big discussion
about physics and nodes of oscillation etc. but for now suffice it to say that arrows a few
inches over draw length tend to forgive minor mismatching. This principle is helping a lot
of guys who set up their bows based upon the 29 inch draw they think they ought to
have but are shooting with an actual 25 inch draw. On the other hand, target archers have traditionally selected arrows that were barely long enough, perfectly matched and perfectly
spined. They know what they are doing-uniformity is a key to consistency. For the
hunting archer, starting with arrows a little longer than absolutely needed and
progressively shortening and /or changing point weight will allow one to tune arrows to a
new bow without purchasing dozen after dozen of arrows in search of a perfect match-
up. Remember to allow at least an extra 1/2 inch for broadhead clearance on hunting
arrows.
The archer
The archer’s style and the smoothness of his/her release makes a difference but only
experience is likely to tell you if you consistently need more or less spine than ‘standard”.
A very smooth and clean release tends to allow the shooter to shoot a weaker spine. A bit
of a pluck (pulling away from the face upon release) may mean better flight from slightly
“overspined” shafts.
Too stiff or too weak?
There are many popular tuning methods. This isn’t intended to be a tuning primer but for
quick reference.... How does one tell if one has the right spine for the setup he/she is
shooting? A right-handed archer shooting in the style most of us use grasps the bow in
his left hand and draws the arrow with the right. The arrow passes along the left side of
the bow. If the arrow is too stiff it will fail to flex around the bow sufficiently and may hit
left of the spot it was aimed at. If it is too weak if will flex around the bow too much and
tend to hit right of the target. If one is using paper tuning or tail watching methods an
overly stiff arrow flies nock right. A weak arrow flies nock left.
As you think about the dynamics involved in this bending around the bow it becomes
clear why bows with deeply cut shelves require extra spine. Please note that cutting past
center isn’t a design flaw.-It actually allows the most possible tuning options but it does
change arrow behavior. A centershot bow that isn’t cut deeply past center will keep the
central axis of the arrow slightly to the left of center. That will probably need about 5
pounds of “extra” spine to tune well but will continue to shoot progressively stiffer arrows
quite well.-We say that centershot bows are very ‘forgiving‘ with good reason. If the bow
is cut 3/16ths of an inch or more past center the central axis of 5/16ths and 11/32nds
shafts will actually lie to the right of center and lots of stiffness is needed to cause that
arrow to kick back into line. Otherwise the arrow starts out pointing a bit right and acts as
if it were weak. Of course the archer also has the option of building the arrowplate out
and thus change the alignment, dynamics and tune of the arrow. All of this was written
for right-handers. -The sinisterly inclined suffer through life having to reverse all
instructions.
Summation
Correctly matching arrow stiffness to the bow is essential for best arrow flight and
maximum enjoyment of your bow not to mention optimum penetration on game. Check
length. Decide on desired point weight. Add or subtract poundage to the actual draw
weight based upon degree of centershot, deviation from 28 inch standard length, and
deviation from 125 grain standard points. Consider the inter-relationship of the variables.
If your shafts seem too stiff you can add point weight. If they act as if they are too weak
and your bow is cut past center you can build out the arrow plate with a bit of leather or
moleskin. Starting with your arrows cut long allows you to play with other variables and
still have a fine tuning adjustment available by shortening in small increments. Nothing is
more satisfying than perfect arrow flight to the center of your target. I hope this helps
some to achieve that perfect flight.
Disclaimer
Actual results may vary with humidity and barometric pressure. Dedicated archer on test
course. Your mileage may vary. Never drink and drive. No vegetarians were harmed in the
making of this article. Always wear a helmet. Not valid with any other offer.
Justin Scott
Canon City, Colorado