CALCULATION FOR EXTREME FOC

[Mr Green 740]

What is the formula for calculating foc?

[BobW]

check the "how to" forum

[Dr. Ed Ashby]

Tony, The "Prologue" to the 2007 Study Updates is an article totally about FOC, what it is, what it does, and how it is measured. On the last page you will find the formula(s) too.

You should be aware that there are two commonly used methods used from measuring the relative FOC of an arrow. Neither is 'more correct' than the other; both are a relative value that allows you to comapre one arrow to another. The important thing in making any comparison is to be sure that the FOC formula used is the same for both arrows. I use the AMO Standard method.

Hope that helps,

Ed
TGMM Family of the Bow

[Dr. Ed Ashby]

Tony, here’s a excerpt from a PM reply I sent today. Since I had it written out already, I’ll add it here. Hope it helps you out some.

“lets do the FOC formula first; step by step. This is the AMO Standard method. Some folks use a different method. It really doesn't matter which you use, as long as it's the same one you are trying to compare with. The FOC we use is just a relative measurement. It's an easy way to compare one arrow to another. True FOC is a dynamic characteristic of a body in flight. It reflects the relationship between the body's center of gravity and the CENTER OF PRESSURE exerted by the air moving around that object when it is in flight. It really has nothing to do with the length of the object; merely the weight distribution and the aerodynamic pressure point.

The 'relative FOC' we use does, however, have a relationship with True FOC. As our 'measured' FOC goes up, so does the true FOC.

So, that out of the way, here we go with the AMO measured FOC"

(1) Measure the length of the arrow SHAFT, from the bottom of the nock throat to the end of the shaft. DO NOT include the length of the insert's lip or the length of the point or broadhead. (I find this easiest to do if I measure in millimeters; it avoids changing fractions of an inch into their decimal values.)

(2) With the point installed, find the balance point of the arrow by balancing the arrow on a knife's edge. Mark that point.

(3) Measure the distance from the bottom of the nock's throat to the balance point you marked in step 2.

(4) Divide the distance to the balance point (from step 3) by the overall shaft length (from step 1). This gives you the decimal value of where the balance point falls in relation to the overall shaft length.

(5) From the decimal value you get from step 4, subtract 0.50. That gives you the decimal value of how far the balance point is forward of the center (mid-point) of the arrow shaft.

(6) Now all you have to do is convert the decimal value from step 5 to a percentage; by multiplying it by 100, or simply moving the decimal point two places left.

It's a lot easier to do than to explain.

Example: if your shaft measures 737mm and the balance point is 538mm you would have:

538 divided by 737 = 0.73
0.73 minus 0.50 = 0.23
0.23 x 100 = 23% FOC

I hope that's clear enough. It's really simple, but harder to explain.”

Ed
TGMM Family of the Bow