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| Spine,
Straightness and Weight Information by Rick McKinney" |
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| As
many more companies are attempting to build arrow shafts
it gets more and more confusing as to what makes a good
arrow. Since most companies do not actually build their
own arrows and others who may build arrows are not archers
of any length of time, it becomes more and more critical
to explain the mechanics of an arrow and why it is important
to choose arrows that are truly the quality you should
expect. There
are three specific areas that determine a good arrow
which are spine, straightness and weight. These three
items are discussed by many but it appears that they
are a little misunderstood. Let’s dissect each
one and determine what makes them critical. Obviously,
we could go further in talking about other reasons for
good arrows as to surface finish, durability and ease
of use (bonding adhesion and pulling out of targets),
but we will stick with the areas that may be difficult
to tell without the proper equipment.
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| Weight: |
| Since
weight is the easiest category for most consumers to determine
accuracy of an arrow, companies focus their attention
on it. Let’s face it, anyone can purchase a simple
grain scale and then check out the weight of each arrow.
However, the weight of an arrow shaft only gives you a
hazy picture at best when you have to add glues, vanes,
nocks and points/broadheads. How many people weight their
broadheads? Few at best. When you switch the broadheads,
do you really think they are exactly the same? However,
the weight of the arrow shaft is critical? To a point,
yes, but if you learn to weigh the broadhead or points
and the complete arrow with vanes, nocks and inserts installed,
you can get a dozen arrows close to 2 grains apart with
no trouble at all.
But we are getting ahead of ourselves.
First let’s determine how much does weight deviation
affect the impact of the arrow. About 15 years ago,
I ran several tests to see how much can you be off in
weight and still impact the same at 90 meters (approximately
99 yards). Since 25% of your performance was determined
at 90 meters in the Olympics, it was a critical part
of the game. It was determined that with a recurve bow
and shooting fingers with a speed of 200 feet per second,
1 grain of weight difference gave you an estimated 1”
difference at 90 meters. Now this is with some extremely
accurate people.
Let’s try to put that into today’s
equipment. With average compounds giving you about 250
feet per second or more, the drop is less. Also, using
a release gives you more consistency in speed as well.
Thus, it is estimated that the drop is about .5 inches
at 90 meters! The next step is to determine the average
shot made for hunting. This is a bit under 20 yards
for most! This would mean that a one grain difference
would cause the arrow to deviate at most .1 inches!
This does not include gravitational pull, air resistance
nor aiming capabilities which when added all together
are 10 times greater at 90 meters!
Working with scientists at the Olympic
Training Center in Colorado Springs at this same time
we determined that 1 millimeter of movement of your
sight pin at 90 meters gave you a 1 inch deviation as
well. How many people can hold a bow without moving
it 1 millimeter? Only a machine can do that! And yet,
some people are claiming that a one-grain weight difference
of an arrow can cause many problems. Only psychologically
(can we say P.T. Barnum?)!!!! Jay Barrs captured the
silver medal at the first World Indoor Championships
with total arrow weight deviations of 9 grains! He was
keeping his arrows consistently within a one inch diameter
for over 120 arrows! Let’s see, he was using a
bow that was giving him a speed of less than 200 feet
per second, a recurve, with fingers at near 20 yards
(18 meters). Oh, by the way, the guy who beat him was
the next Olympic Gold Medalist Sebastian Flute.
A
simple test is to take the heaviest and lightest arrow
in a group of 12 arrows. Without knowing which is the
heaviest and which is the lightest, mark them as 1 and
2. Then go to 20 yards or even 50 if you want to and
shoot them. Plot their impacts and do this about 5 or
more times. Then go weigh them to determine which one
is the heaviest and which is the lightest. Usually if
the arrow is 7 or 8 grains difference you will probably
not notice much impact difference at 50 yards, unless
you are one of the top 50 archers in the world. Thus,
the gimmick of weight deviation is just that, a gimmick
to scare you into believing weight has a huge difference
on impact. You can’t even aim good enough to determine
the weight differences!
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| Straightness: |
The
second most highly talked about category that many manufacturers
push is straightness tolerances. It’s funny that
several years ago, when there were no carbon arrows,
arrow straightness was constantly being drilled into
our heads about how important it was to have super straight
arrows. This may be true with aluminum arrows and aluminum
and carbon mixed. But when it comes to all carbon arrows
it is not as factual. In the late 1980’s AFC and
I ran tests to determine how straight does the arrow
need to be in order to carry a 3 inch group at 50 meters
or 55 yards. We used a recurve, fingers with a speed
of near 200 feet per second. We found that .010”
T.I.R. (Total Indicator Reading) was the maximum in
order to keep a 3 inch group at this distance.
Although
when you spin a .010” T.I.R. arrow you would freak
out, it proved to me that the impact is the key, not
just physical observations. Thus, if you shoot an arrow
of .005” plus/minus, you actually have a .010”
T.I.R. and it will group exceptionally well. However,
those 50 top archers of the world will argue on this
point, and rightly so. They are the 8 hours per day
training athletes and demand accuracy of the highest
nature. Over 90% of the population will not even notice
this deviation or I should say that they may use it
as it really is, an excuse and not the real reason for
poor performance!
There are many ways to determine straightness
and since there is no consistency in the industry, it
makes it very difficult to determine what companies
are really saying in their advertising. Most give you
a number but may not state that it is a T.I.R. If you
see a plus or minus type statement, it means that it
is half of the T.I.R. Thus a .003” plus/minus
is actually a .006” T.I.R. Now you have to find
out what distance do they measure this reading. Some
use a 14” reading while others use 28” and
anything in between, thus again you need to find out
just what they are really stating. Straightness has
some effect on shooting performance but not as much
as one would think.
The
question still comes up about why carbon straightness
is not as critical as aluminum straightness. Let’s
look and see why aluminum straightness is so important.
There is a frequency vibration occurring when launching
an arrow. When you have a straight arrow, this frequency
is fairly consistent and the shooting impact is very
good. However, when the arrow becomes bent, the frequency
of the arrow changes, thus causing the oscillation to
change as well.
This
causes bent arrows to not impact in the same place as
straight arrows. The all carbon arrow cannot be bent.
It can be bowed but not bent. A straight all carbon
arrow and a bowed all carbon arrow have the same frequency.
Thus, the frequency harmonics do not change and the
impact of the arrow does not change. Now, can we say
if an aluminum arrow is slightly bowed, would it have
the same frequency as a straight one? Yes. However,
bows very seldom occur in an aluminum arrow. You may
even have heard some people commenting that depending
on where the arrow is bent it still may fly into the
group. Generally this is a “bowed” aluminum
arrow. Does an aluminum/carbon arrow have the same characteristics
as an all carbon or a 100% aluminum? It has more qualities
as an aluminum. Thus, keep an eye on those aluminums.
You can straighten the all aluminum, but it is almost
impossible to straighten a carbon/aluminum by the average
person.
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| Spine: |
Spine
is probably the most important part of the arrow shaft
and the most ignored. I presume the main reason for
this is because it is the hardest for a manufacturer
to get right and keep consistent. Also, it is one that
cannot be measured very easily by the average person.
Let’s determine what spine is and do not confuse
it with spline! Spline is what the fishing industry
uses in order to get sort of the “back bone”
of the fishing rod.
This
is sort of an overlap of material in order to get the
stiffer side. Keeping this stiff side on the upper side
makes it easier to handle when reeling in that big one!
In archery you do not want a spline! You want an even
consistent spine all the way around the shaft (circumferentially).
Spine was established in modern times by Easton who
uses a 29” arrow. You place this arrow on two
posts measured out 28” apart. You then place a
1.94 pound weight in the middle of the shaft and measure
how far the arrow shaft drops down. This gives you a
static (non-moving) spine.
When an arrow is launched from a bow,
the arrow flexes (dynamic spine). This flex needs to
be a specific amount and stay consistent among all the
arrows in order to carry a group. If the arrow flexes
too much it becomes exceptionally critical. The smallest
mistake made by the arrow increases substantially if
the arrow is too weak.
If the arrow is too stiff it is not
as critical, but does not give the best possible grouping.
Thus it is far better for the arrow to be too stiff
than too weak. That is why you may note that some companies
fudge on the size arrow recommended towards the stiff
side. This is far better than on the weak side. Since
the arrow flexes upon being launched, you would want
it to flex the same.
If the arrow is too stiff it will favor
the left side while if the arrow is a bit weak, it favors
the right side. Thus you will get lots of rights and
lefts if you have lots of inconsistent spines in your
arrows and that is exactly what you will get with many
of the arrows on the market today. Since most of the
archers do not know how to measure this spine, they
are unaware of why they are not grouping so well. Also,
you will note that most arrows that are sold in dozen
groups, only 6 to 8 arrows will group and the rest will
not. Again, this is due to the spine more than anything
else. Sometimes they can get a few more arrows to group
by moving the nock around the shaft a little in order
to find a near correct spine.
Many companies do not keep very tight
tolerances on spine consistency. This causes all types
of problems for the archer and the dealer. Of course,
since most archers are not very good or accurate, they
do not realize that the arrow is making them look even
worse than what they really are. According to tests
that I have been involved with, the tighter the spine
tolerances the more accurate the arrows become. Keeping
them .005” plus or minus is what was set years
ago with aluminum arrows and their accuracy has been
proven over the years. Some companies have spine deviations
of over .040” plus or minus! Thus, it would be
like putting spines of a 2113, 2116 and 2119 all in
one group of arrows and expect them to shoot well. It
will not happen!
Part of the reason for having so many
spine inconsistencies is due to the material used. Some
companies look for the cheapest product they can find
in order to keep costs down. This severely causes huge
spine deviations. Also, how the arrow is manufactured
will cause spine inconsistencies. Most companies put
the spine determining material on the outside and then
grind it down to get as close to the weight they can
get. However, this causes spine inconsistencies and
breaks down the fibers that actually determine the spine.
Cutting the materials requires tremendous precision
in order to get the exact spines and many companies
use like a paper cutting device to get their patterns.
This gives a lot of spine inconsistencies as well. It
also gives them a “spline” as talked about
in the above paragraph.
Now
if you consider the inconsistencies of spine, the straightness
factors and weight factors, you can see why there is
so many discrepancies in arrow shafts. The degree of
importance is determined by what material is used. With
aluminum arrows, the degree of importance is straightness,
spine and then weight. With all carbon it is spine,
straightness and then weight. The spine of an aluminum
arrow is normally very good to start with.
However,
this spine breaks down over time. Depending on the wall
thickness spines of an aluminum arrow can break down
as fast as 10 shots! This has been proven time and again
by some of the best archers world wide. Although the
only American manufacturer of aluminum shafts disputes
this, the “proof is in the pudding”! Top
archers will replace these arrows very quickly without
anyone knowing any different.
Most
all carbon arrows start to loose their spine over several
hundred shots due to wear. As the arrow penetrates the
target, the friction microscopically wears down the
outer layer of carbon and since most companies have
their spine determining layer on the outside, the spine
gets weaker and weaker over time. The aluminum arrow
breaks down for different reasons. The flexing of the
shaft upon impact of the target, pulling the arrow out
of the target and the launching of the arrow from the
bow continues to flex the aluminum tube constantly and
we all know what happens to metals when continuously
flexing them back and forth.
Now
you can understand some of the simple physics of what
is happening to an arrow and why it is important to
choose wisely when purchasing arrows.
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