Whats my FOC???

[Predator1]

I have AD trad lites cut to 29 1/2" with the std inserts. I am using a 200gr WW. Total arrow length is with tip is 32". The balance point is 11 1/4" back from the point.

[SC Bowhunter]

I may be mistaken but I think that is 29.68 %.

[MikeW]

I am using a 200gr WW

How/Where do you get a 200 grain Wensel Woodsman?

[wihill]

Head to Goldtip's website, they've got a great web app that'll calculate FOC and KE for you under the tools section.

[SC Bowhunter]

My mistake. It is actually 14.84%.

[O.L. Adcock]

You're at 13%....If an arrow balances 3" in front of center and it's 30" long, that's 10%....O.L.

[katman]

MikeW, without the insert the WW is 125gr so put in a 75gr steel adapter instead of the 25 gr Aluminum the screw in comes with at 150 gr.

[LazerRay]

I use a 250 gr WW with 125 gr insert from 3 Rivers

[Daddy Bear]

AMO Standard % FOC as used by Ashby:

(Length of balance point with Broadhead measured from throat of nock) divided by (length of shaft from throat of nock to BOP) equals the decimal%. (Decimal%) subtract (.50) equals the decimal fraction. (Decimal fraction) multiplied by (100) equals %FOC.

You gave the wrong measurement for your balance point, it needs to be from the nock throat to the balance point.

[Daddy Bear]

You have 21.55% AMO FOC.

[Daddy Bear]

Sounds like a good math problem:)

Your arrow length with broadhead is 32" with you shaft length measured 29" from nock to BOP, that means your broadhead is 3" long in front of the shaft. Your balance measurement is backwards being 11.25" from the front. That would be 11.25" subtract the broadhead length of 3" would equal 8.25". Subtract that 8.25" from your 29" shaft and that would indicate that your true balance point is 20.75" measured from the nock. Now we have a measurement to work with:

20.75"/29" equals 0.7155 decimal%

0.7155 minus .50 equals 0.2155 decimal fraction

0.2155 multiplied by 100 equals 21.55%FOC

[fowlarcher]

All the different answers made me curious, so I searched the internet for an AMO standard for measuring FOC %. I did not find one. If anybody knows of a page showing a standard for this measurement please post a link. I don't know why AMO would be concerned with FOC % anyway.

Finding percentages is just simple math, but it seems some people are measuring OAL differently. I have always measured from throat to tip.

My answer to Predator1's question is 14.84%.

[Ghostman]

try this site - he has FOC calculators -  http://home.att.net/~sajackson/arrow_nitro.html

I came up with 17.4% assuming you're using 3 5" feathers

[Daddy Bear]

There are several formulas for %FOC which will give you different answers. Everyone I know speaks of %FOC in the same terms as Dr. Ashby. He uses the Standard AMO %FOC formula which would be 21.55%FOC. You can use whichever formula you desire, but when speaking in terms of extreme %FOC, we should all be on the same page and use the same math.

Dr. Ashby explains the formula in detail at this link:

 http://www.ubbc.ca/resources/documents/archive/understanding_and_applying_foc.pdf

[Widowbender]

Okay, here is an easy way to calculate FOC...all you need is a tape measure, a pencil, a caculator and something to balance the arrow on....
(1) Find the balance point of your arrow with the desired head installed.
(2) Mark this point with a pencil.
(3) measure the arrow from the throat (bottom of nock groove) to the back of the point/insert/taper.For our model lets use 29".
(4) Divide this in half and make another pencil mark on the arrow. This would be at 14.5" in our model. This the center of the arrow hence the term FOC.
(5)Measure the distance between the pencil marks.
Lets use 6.25" in our model.
(6)Divide this measurement by the total lentgh.
6.25" divided by 29" equals .2155 or 21.55 %FOC.

If you use the other method it would be 4.75" divided by 32" equals .1484 0r 14.84 %FOC. The balance point has not changed, just the measurement because of adding in the tip of the head. This causes you to have to shift the center of the arrow forward 1.5" with this arrow

I think this may account for the reason that some folks have a hard time coming up with High or extreme FOC numbers. They are either not using the same measurements as Dr. Ashby (and AMO)or using an online calculator that doesn't use the same measurements.

David