FOC Balance Point

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Arrow balance point
presents us with some tradeoffs. If
you keep the arrow’s nose light
it will remain a little more level
in the air and actually plane or
sail along a flatter trajectory
than it would if the nose were
heavier and it flew pointing more
nose downward. On the other side
of the tradeoff is stability. The
closer the center of gravity gets
to the physical center of the arrow
the more unstable the arrow becomes
while in flight. Take it to the
extreme. If the center of gravity
were behind the physical center
of the arrow it would actually
flip around as soon as it left
the bow and try to fly tail first. The
closer you get to a tail heavy
arrow the more unstable it becomes.

There’s an
archery term that’s used to describe an arrow’s balance point. It’s
called Forward of Center, or FOC. You arrive at FOC by making a few
measurements and then running the numbers
through a simple formula. Here’s the process:




Balance point:

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Install the tip you will be shooting. If you are testing stability for
3-D shooting put your field point or nibb into the arrow. Of course,
for hunting install your broadhead. Find the arrow’s balance point by
sliding it back and forth along a fairly sharp edge. You’ll find the
spot where the arrow just balances. Mark it carefully. Now measure
from the bottom of the nock groove to the balance point and write this
number down for later.

Overall length:

There are different conventions for
measuring overall arrow length depending upon the type of point you are
using.

Arrows that
include inserts
: Measure from the bottom of the nock groove to the
end of the arrow not including the insert.
This is often referred to as the arrow’s cut length.

Shafts with swaged tips: The overall
length is measured from the bottom of the nock groove to the most forward
extension of the full diameter of the shaft, just behind the swage.

Shafts that include outserts: Measure
from the nock groove to a point ¾ inch forward of the rearward end of the
outsert.

Shafts with glue-on heads: Measure from
the nock groove to the most rearward portion of the glue-on point.

Determine FOC:

To find the FOC (which is always expressed as a percentage) divide the
overall length by two. This should produce the physical center of the
shaft. Now subtract this number from the balance point and divide by
the overall length. Multiply by 100 to express the fractional value as
a percentage.

Most expert
archers agree that an FOC value that is between 7 and 10 percent will produce
the best compromise between stability and a flat trajectory. The American Society for Testing and
Materials, in their specification for measuring balance point, state that a
value of 9% is typical. But, they also
state that the range can be as wide as 7% to 18% while still producing good
arrow flight characteristics.

The best way
to achieve your desired FOC is to try several different weight field
points until you hit the right balance. However, if you are sold
on a particular broadhead that’s too heavy to permit the arrow to fall
into the desired FOC range, you can change from feathers to
vanes. You can change from aluminum inserts to lighter composite
inserts or you can even play some games with the weight of the nock
end. For example, you can experiment with adding weight by
placing a narrow strip of lead tape around the shaft just behind the
fletching. Make sure it fully circles the shaft so you don’t
introduce a wobble. You can get lead tape from any full-service
golf supply shop.


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1 Comment

  1. I no way agree that 7-10% foc is ideal. Maybe if your shooting a 300gr total weight arrow. Today’s hunters are going way too light with way to little foc. It seems all is about speed. Well, phoee,phoee,phoee on that. Speed is great if your a 3d shooter/target shooter. If your hunting you need weight, weight produces momentum. Throw a ping-pong ball against a friend at say 350fps, then throw a lacrosse ball at him at 200ftps. Ask him what hurt the most. This is the same with arrows, although it isn’t about hurting, it’s about penetration. Think back 25-30 years ago. We shot at ABO speeds with heavy arrows. 2315 [email protected] 30inches that weighed around 540-600grains. Nothing stoped those arrows. Today I shoot arrows that weigh 650 grains with a 1.7/8ths inch cutting diameter (Silver Flame XXL) with FOC in the 20% range at 217fps. This set up will shoot lengthway through a whitetail, it will enter/exit at any angle and blow through both shoulders and keep on going. Plan for the worst, not the best. And when your do make a perfect double lung they are dead so quick you don’t need a blood trail to follow.

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