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.
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:
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.
There are different conventions for
measuring overall arrow length depending upon the type of point you are
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
Shafts with glue-on heads: Measure from
the nock groove to the most rearward portion of the glue-on point.
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
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.