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FOC is there a difference?

954 Views 8 Replies 5 Participants Last post by  THF
Trying to learn as much as I can and see two ways to figure FOC presented.

In the articles section above back in 2010 Urban Legend has a great presentation using the total length of the arrow to measure this from tip of broad head to end of arrow.

You look on line and see where the measurement is from the nock point to the end of the insert on the arrow is used by Eastman and the length of the broad head is not considered.

Is one more accurate than the other?

It seems there is a percentage change with the over all length vs just arrow length will change the % number.

Just looking at couple of the broadheads and they tend to be about 2" in length from the insert to tip.

Does it really make any difference how you figure this if the % FOC is not too little?

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A small difference. Lots of discussion on which method to use on this forum. Personally I measure the length including the chosen point. As you know some heavier points are shorter than the longer lighter points. May be OCD on my part. IMO, most important is to maintain an FOC of greater than 15%. My setup groups arrows tighter with FOC of ~17%. Either method will provide the information needed for accurate, tight arrow groupings.
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If I figured it correctly then using the Mossy Oak / Decimators at 325gr wt and Blood Sport at 300gr wt
Arrow 20.38" used 100 gr Swhackers at about 2" in length
for the wt forward I had 4.3 inches on the 325 grain and 3.67 on the 300 gr arrow.
I got 21% on the 325 arrows and 18.2% for the 300 gr arrow.

I guess the question is even with the FOC that should not change with the different length of broad heads which are

about 2" long compared to a short fat field point or does it for mechanical broad heads?

I guess I am how does this length of broad factor into the equation with mechanical BH's or does it.

Do not even want to get off on variance with fixed broad heads where the blades come into play which seems would pull many other variables into play depending on the different blades and lengths

I used to do serious rifle reloading and had trouble weighting arrows till I figured on the old balance beam scales you can put a small loop of fishing line off the support barand to be able hang the arrow off the side of the table and get an accurate measurement.
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Since I don't target shoot I quit worrying so much about FOC. My set up is very accurate to 40 yards and I always get pass throughs. If a problem develops I will then have to double check everything including FOC in order to pinpoint the problem.

Many variables, are the vanes and nocks the exact same weight? Are the inserts the exact same weight?

Use the FOC option without the broadheads in place and see the differences.

IMO, your good with either arrow.
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As I understand the determination of FOC, it is intended to focus on the center of mass (balance point of complete arrow ready to hunt) relative to the geometric center of dynamic activity of the shaft (the middle of the dynamically active portion of the shaft). A 20" carbon tube with 1" of lighted nock inside the shaft and 2" of insert inside the shaft would have 17" of dynamically active shaft. The geometric center would 10 1/2 " from the front of the tube. That's how I figure it. Makes sense to me.
Didn't know how to measure FOC until seeing the link above. Using the "recommended" method today, I measured the FOC for the CAMX Accuspine arrows provided with both my A4 and my 330X (different length arrows are supplied for each). Using 100 grain field points and the calculator above, both arrows were almost identical. Right at 19.5% FOC.

Coincidence? I think not. I do know this, neither bow is as accurate using the Nocturnal lighted knocks as they are without them. Is it the difference in FOC causing the variation or the geometry of the lighted knocks? I've done away with the lighted knocks recently. See the thread for below for reasons.
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