# FOC - Why calculate W/O a head



## BarneySlayer (Feb 28, 2009)

That's a good question.

I have no idea.


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## thunderbolt (Oct 11, 2002)

Not sure where you got your information from, but FOC is calculated with a broadhead or field point on the arrow. It may be measured from the nock throat to the edge of the insert though. Here's how I do it http://home.att.net/~sajackson/archery9.html


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## edthearcher (Nov 2, 2002)

*foc*

so what idiot are you listening to??? front of center FOC what does that mean to you. to the rest of us its with the point on.:mg: the posts get better and better


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## SonnyThomas (Sep 10, 2006)

Ches said:


> OK, I know you are to calculate FOC from the throat of the nock to the end of the arrow (to the insert ring) prior to putting your field point or broad head on. My question is why it is done this way instead of finished arrow (with tip installed)? If I have 7% FOC without a 100 grain field tip, I should have a lot more with it.


I believe you misread the instructions. The balance point is found with the tip in place.

At one time there was in fact two formulas for figuring FOC. One is in General Archery Information, http://www.archerytalk.com/vb/showthread.php?t=453845

The second formula, if I remember correctly, was in the link given in the above link,
http://www.socalarchery.com/Information/front of center.htm

I can not get it to come up. However, years back, I received the same formula in a pamphlet in one my hunting magazines (Bowhunting, perhaps?). The formula was given by, I think, Bill Winke. As such, the shaft is measured from the bottom of the nock groove to the end of the cut shaft, not including the insert. Having built target and hunting arrows using both methods I could not tell the difference in accuracy or performance. 

If you read the General Archery Information post you should note; "There is no perfect F.O.C. for each setup." And then given are starting points.

I'd like to read the technical reports of the findings/differences between the two formulas.


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## Maxtor (Jan 7, 2007)

So does determining the FOC actually do anything or help in any way?

I've never known what mine was until I saw this post and now know that it's 11.2%. Which is chinese to me....lol


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## SonnyThomas (Sep 10, 2006)

Maxtor said:


> So does determining the FOC actually do anything or help in any way?


For us mere mortals, no. Virtually any arrow with a 100 gr tip and of proper spine for the bow will have enough weight forward of center to perform adequately. There are a lot of us that feel Carbon Express went overboard with their "weight forward technology." Basically, they came up using the same arrow of old, only heavier. So we switched to something else. Me, I bought a bunch of the older CX 200s and 300s that were on sale to bring in the newer arrow and have enough to last me a long, long time.

Here; Common F.O.C.s for each style of archery.
FITA 11-16%
3-D Archery 6-12%
Field Archery 10-15%
Hunting 10-15%


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## Spotshooter2 (Oct 23, 2003)

> so what idiot are you listening to??? front of center FOC what does that mean to you. to the rest of us its with the point on. the posts get better and better


 Now here is a truly *WORTHLESS *post. Just trying to get your post count up there edthearcher?


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## Maxtor (Jan 7, 2007)

SonnyThomas said:


> There are a lot of us that feel Carbon Express went overboard with their "weight forward technology." Basically, they came up using the same arrow of old, only heavier.


 Hmmmm, my 11.2% is with a CE Maxima 3D Select


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## BarneySlayer (Feb 28, 2009)

I read the original post thinking he was asking why the FOC was used not including the length of the tip to figure the center of the arrow. Maybe I read that carelessly.

My guess would be, if I read the question correctly, because the lengths of different tips may vary highly, which begs the question, how much does that matter? I.e., does FOC with a short blunt vs. FOC with a long extended broadhead still behave the same way? Then again, considering that the aerodynamics will be fastly different, at that point, does it even matter?

I'm not going to hurt my brain pondering this, but it's an interesting question, as I read it.


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## FIGJAM (Jan 12, 2009)

Mmm Mine is 18% want to know how to get this FOC?


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## dwagoner (Sep 27, 2007)

FIGJAM said:


> Mmm Mine is 18% want to know how to get this FOC?


250g point on your indoor 27s??


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## FIGJAM (Jan 12, 2009)

dwagoner said:


> 250g point on your indoor 27s??


no a 130grn Brass insert I machined for my Axis 300 plus a 125grn BH total on the tip 255grn


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## SonnyThomas (Sep 10, 2006)

BarneySlayer said:


> I read the original post thinking he was asking why the FOC was used not including the length of the tip to figure the center of the arrow. Maybe I read that carelessly.
> *You read it somewhat incorrectly as he incorrectly as he figured FOC. Confused me too.*
> 
> My guess would be, if I read the question correctly, because the lengths of different tips may vary highly, which begs the question, how much does that matter? I.e., does FOC with a short blunt vs. FOC with a long extended broadhead still behave the same way?


You have the same questions I do of length of point. The 2nd method I wrote of does not include the length of point in the measurement, but does for the balance point.

Seems everyone wants to be super perfect at doing things, but if they are, why the h___ don't they include the full length of the nock in the measurement instead just to the bottom of the nock groove? Remember, the full length of the nock is included in finding the balance point.


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## Ches (Aug 3, 2009)

Thanks all, I always thought the calculation should be from nock groove to end of point, however, I have read many places that you only measured shaft length from nock groove and wondered why. EDTHEARCHER thinks I am listening to IDIOTS. That could be, I looked for some examples of these IDIOTS and see that even the Easton Tuning Guide has some IDIOTS working for them. They do use the point weight in their calculation, but also use the shaft length without the point as the basis for center. I am just going to measure total length from nock groove to end of point divided by two to get center, then find ballance point with tip included to calculate FOC. It's not like this has kept me up nights, I just wondered what the reason for calculating without a point or by finding center of a shaft without a point has to do with anything.

Again, thanksl


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## SonnyThomas (Sep 10, 2006)

Well, Ches, idiots run amok I guess. I started looking through my books. Lo and behold, I found Bowhunting Equipment & Skills by The Complete Hunter and Editors of Bowhunter Magzine, copyright 1997. Now this book was fine tuned by none other than 4 of the best; Dwight Schuh, Fred Asbell, Dave Holt, and M.R. James. The passage of FOC reads "measure from the end of the nock."

Like I said, I've assembled arrows from both methods and all my arrows fly great. And as the all wise Oxford had in his Post; "There is no perfect F.O.C. for each setup." 

Sort of gives merit to; Measure with a yard stick, mark wanted length with chalk and cut off with a axe. Mmmm??? I guess you just have to get close like in a game of hand grenades.


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## dwagoner (Sep 27, 2007)

FIGJAM said:


> no a 130grn Brass insert I machined for my Axis 300 plus a 125grn BH total on the tip 255grn


your crazy LOL


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## ceebee (Dec 3, 2002)

Back in the good old days, we shot wood arrows and glue on field points. Along came JDE and aluminium shafts with inserts for changing points. Now we have screwin field points "and" glue in target points. makes a different length of arrow. I have played with my arrows for years and I have settled on about twelve percent foc for target arrows for indoors. For outdoors, depends on the range, open or in the woods, but I would never drop below ten percent foc. 
Charllie


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## konrad (Mar 29, 2009)

The calculations are made with the tip on; however, the measurements are taken to the outer edge of the insert because not all tips are the same length and would slant the calculation improperly. 

There would be no common frame of reference.


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## SonnyThomas (Sep 10, 2006)

konrad said:


> The calculations are made with the tip on; however, the measurements are taken to the outer edge of the insert because not all tips are the same length and would slant the calculation improperly.
> There would be no common frame of reference.


I have read many articles on FOC. Some books and articles are quite well written, BUT nearly all of them vary. Some don't mention the insert as the measurement takes it in. Others have; from the bottom of the nock to the end of the broadhead or tip. One mentions "cut length," bottom of nock to end of shaft as in below and the one I pointed out in my last reply has the measurement using the full length of the nock. Some mention that the insert (lip) is not to be measured.

Taken from Easton's Tuning Guide;
L= Correct Arrow Length-Distance from bottom of nock groove to end of *shaft*. Note that insert is not listed, but listed in Easton's next step.
A=Distance from bottom of nock groove to finished arrow balance position (includes weight of point [+ insert], nock system and fletching)


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## konrad (Mar 29, 2009)

I defer to the good doctor:

How does one “correctly measure” FOC? The AMO Standard uses
shaft-length; ignoring insert/tip/broadhead length. The other
common formula uses overall arrow length; includinginsert/tip/broadhead. Which is “correct”? Neither. True FOC
is based on the center of pressure. We simulate the CP
location in both formulas.
The AMO formula was adopted as “standard” because; of the
two common formulas; it uses a “simulation point” nearer the
actual CP location for most arrows in flight. Note the CP
does not reflect the point of greatest shaft flex, but that
upon which “flexional force” is greatest. Shaft design and
material has a significant effect on both CP location and
where the shaft will flex most.
For practical applications; those for which archers use FOC;
either common formula works equally well. All that matters
is that the method used be stated; so everyone “reads off
the same page” when making comparisons. As with static
spine, in-and-of-themselves the “numbers” mean little. They
allow relative comparison of one arrow to another; nothing
more.
For example, static spine measures relative stiffness of a
shaft; how much it flexes when a weight of specified mass is
suspended mid-way between two shaft-supporting points; which
are located a specified distance apart. Everything about the
measurement is relative, not absolute.
Static spine tells nothing of an arrow’s dynamic spine. From
it one gleans only an indication of relative stiffness. What
it does do is provide a reference point. This helps when one
needs to move to a stiffer or softer spine. It allows
comparison of shafts; relative to each other.
This is why tuning arrow to bow is important. No static
measurement or calculation contends with the myriad variables
encountered when one shoots an arrow from a bow. That’s why,
besides charts, Easton publishes 35 instructional pages on
attaining “the right arrow”. Charts provide no magic number
saying; “Pick me. I’m the right one!”
FOC measurement is the same; relative. Neither common formula
is “correct”, nor is either “wrong”. Each serves its purpose;
providing a reference point. As long as one knows which was
applied, they can duplicate results. If preferred, then remeasure
and state it in another format; that’s perfectly
alright. It still provides “relative reference”.
If FOC indicates fletching’s “lever arm” and resultant
stabilizing effect, what led to its inclusion as a tissue
penetration factor? FOC measurement has been around in
archery a long time, but shaft materials limited the FOC
achievable with good arrow flight. Carbon shafting has
created an abundance of possibilities. They behave
differently. “Old rules” may not be applicable. Carbon
shafts offer great stiffness at low mass, with forgiving
flexional characteristics. I’ve found no shafting, other than
carbon, giving good flight at Extreme FOC. Carbon shafts
allow unprecedented FOC with exceptional flight.


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## SonnyThomas (Sep 10, 2006)

konrad said:


> I defer to the good doctor:
> //////
> Which is “correct”? Neither.
> //////
> ...


No problem. I think I replied as such.


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## Ches (Aug 3, 2009)

I know, I am probably splitting hairs again, but has anyone tried those insert weights made by Gold Tip that can be screwed into the back side of the insert after being installed. I orderd some the other day from Mountain, but when they came, the wrench was bent in shipping. Nice people at Mountain are sending me a replacement, so I have not been able to try them yet.


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