figuring FOC

[Pat B.]

I would like the formula for figuring FOC. Used the search function but came up empty handed.

Can anyone help ?

                     Thanks

[Apex Predator]

Measure the total length of your shaft including point and nock.  Find the measured center and mark it.  Find the balance point using a knife edge fulcrum and mark it.  Measure the distance between the marks and dIvide it by the total length.  That should be the FOC%, unless I've been doing it wrong.

[Pat B.]

Thanks AP..

 What is the desired window here, 15%, 20%,25%?  Or just where ever you find a good set up that tunes and shoots well...?

[Orion]

Divide the distance from the throat of the nock to the balance point by total shaft length (from the throat of the nock to back of point, not including point).  Subtract .5 from that number and multiply by 100.  Works every time.  There is some uncertainty though as to whether the length of the point should be incorporated into arrow length.

In times past, most arrows had an FOC of about 10%, give or take a percent or two because that's what a 125-135 grain head, which was a  pretty common weight, would yield on a wood shaft.  It's difficult to get much more than 15% FOC on wood unless you go to a tapered and footed shaft and a fairly heavy head, i.e., 190 or so grains.  It's much easier to front load carbons to 20-25% FOC.  I believe Ashby's research defines high FOC as 15-19% and extreme as 20% and more.  A well tuned, straight flying arrow is very important to get the full benefit out of a high FOC or any other arrow.

[Blackhawk]

Many "experts" claim that the high FOC (heavy point weight) simply provides better penetration. They claim that if the two arrows weigh the same and one is weighted up front rather than with some sort of weight tube it will outpenetrate the other shaft every time.

I'm no expert,  but if I am getting perfect flight from an arrow without the high FOC, I would not hesitate to use it on just about any North American game.  

It's hard to figure how two arrows of equal weight flying perfectly straight will not penetrate equally.

If going after rhino, buffalo, or large bears, then I may experiment with high FOC to confirm their claim of penetration superiority.  Until then, I'm happy to hunt my deer and elk without it.

[Jerry Jeffer]

check this out
 http://home.att.net/~sajackson/arrow_xx75.html

[p1choco]

Check    this   out.  Just plug in the numbers and the math is done for you.

[TradPaul]

Try this..

FOC via manual calculations  

Use a finger to find the balance point of yer arrow.

BAL = the distance from the groove in the arrow nock to the balance point.

LENGTH = the distance from the groove in the arrow nock to the back of the arrow point.

HALF = LENGTH divided by 2

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here's the FOC formula ...

(BAL minus HALF) times (100 divided by LENGTH)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Example ...

29" = arrow LENGTH

14.5" = arrow length HALF

22" = BALance point

22 - 14.5 = 7.5

100 / 29 = 3.448

7.5 x 3.448 = 25.86

25.86% FOC


P.

[Fletcher]

I'm going to agree with Apex on this one, and I come at this from 35 years as an aircraft mechanic, so I have a pretty fair understanding of things that fly.  

Gremlins added to provide some lighthearted humor to an otherwise dry post.   :D

[Daddy Bear]

There is more than one formula to measure %FOC. Most people I know speak in the same terms of %FOC as Dr. Ashby. If so, he uses the AMO Standard formula for %FOC which is what Orion posted above but Orion has the division reversed. Dr. Ashby goes in a bit more detail in breaking down the formula as such:

Divide the measured length (from nock throat to the balance point with broadhead installed) by the measured length (from nock throat to BOP) which will equal the decimal%. Subtract .50 from the decimal% which equals the decimal fraction. Multiply the decimal fraction by 100 which equals Dr. Ashby's (AMO Standard) %FOC.

Using Trad Paul's arrow from his version (which also works) as an example:

22 / 29 = .759

.759 - .50 = .259

.259 x 100 = 25.86% FOC (ASHBY/AMO)

later,
Daddy Bear

[fyrfyter43]

Originally posted by Blackhawk:
It's hard to figure how two arrows of equal weight flying perfectly straight will not penetrate equally.

Because the higher the FoC, the less the arrow flexes on impact. The more the arrow flexes, the more energy is wasted.

[TradPaul]

It's really the arrow point pulling the arrow, instead of the shaft pushing the point....Did i say that right    i think that is the principle.


P.

[Daddy Bear]

Having the weight concentrated directly behind the broadhead versus distributed further back to the tail end of the shaft helps to keep the energy behind the broadhead so that it tends to travel straight ahead when meeting resistance. As more weight is distributed to the rear of the shaft, the tail end of the shaft will have a tendancy to attempt to pass the front end when the broadhead meets resistance, think of bullet yaw.

Also think of the arrow flight long before the terminal performance of striking game. In flight, having a high %FOC requires less input from the fletching at the tail end to steer the broadhead and arrow straight. This makes for quicker recovery and greater forgiveness when you have a less than ideal release such as you may find when sitting in the cold for hours before loosing an arrow on a game animal. It also allows for smaller fletching which may aid in increased downrange velocity and energy while still making for quick recovery.

There are many benefits.

later,
Daddy Bear

[Orion]

Dah!.  You're right Daddy Bear.  Don't know how I mixed them up, but I did.  Went back and edited my post to fix it.  

[longbowben]

Just go to gold tip web page you fill in the blanks .

[WidowEater]

http://tradgang.com/noncgi/ultimatebb.php?ubb=get_topic;f=2;t=000089

This is one Rob did on Excel works great