FOC or Front of Center

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Carpdaddy
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FOC or Front of Center

#1 Post by Carpdaddy »

Okay fellers; here is an important subject to talk about and learn if you are not already familure with it. This first part I copied from Easton's site, Dr. Ed Ashby gives much more detail, I may post some of his later.
Arrow FOC (front of center) is an important concept that all archers should understand. Whether bowhunting, target or 3d shooting, the FOC of an arrow will affect the flight of the arrow. FOC becomes especially important the farther downrange the target is, and also if a broadhead (most notably a fixed blade broadhead) is used.
The basic definition of FOC is the percent difference between the physical midpoint of the arrow and the center of gravity (balance point) of the arrow as compared to the total length.
When an arrow is shot, the fletchings immediately begin working to correct the flight of the arrow. Any little imperfection in the initial launch of the arrow, either from shooting with fingers, imperfect release, improperly tuned bow, torquing the handle of the bow, etc. will cause the arrow to flex as it leaves the bow and/or come out crooked. The job of the fletchings is to correct this imperfect flight by straightening the arrow as air flows over the fletchings during flight.
In order for the fletchings to correct the flight of the arrow, they must move the arrow into a straighter flight path by rotating the shaft about the center of pressure. The center of pressure is the point along the shaft where the aerodynamic forces are balanced. Just like with a weather vane, the fletchings on the back of the arrow provide more surface area on which the air can act upon the arrow.
Fletchings are best able to steer the arrow when the center of pressure is farther behind the center of gravity. This is more easily done the longer the distance from the fletchings to the center of gravity. Therefore the farther the center of gravity is from the back of the arrow, the easier it is for the fletchings to correct the arrow flight. If the center of gravity and center of pressure are too close together, or worse if the center of gravity is behind the center of pressure, the arrow will become unstable. To witness this, take an arrow or a dart and try to throw it backwards! What happens?
For penetration purposes, the opposite phenomenon occurs. A larger FOC will prevent and arrow from flexing as much when coming into contact with the intended target. Hunters will want a higher FOC so that as the broadhead enters the the game, the distance from the front of the arrow to the center of gravity is lower, making it more difficult for the arrow to flex or deflect. Any flex or deflection of the shaft upon entry means that energy is being lost somewhere other than directly along the center of the shaft, lessening the overall penetration.
A larger FOC is necessary for shooting long distances where crosswinds tend to have a large effect on arrow flight. The same applies to shooting broadheads because the larger surface area of a broadhead is more greatly affected by launch imperfections as well as crosswinds. Broadheads effectively move the center of pressure closer to the point and tend to steer an arrow which can often have a negative affect that the fletchings must overcome.

Cookies
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Re: FOC or Front of Center

#2 Post by Cookies »

Good post Tony. That really helps to clarify things for guys like me!

Carpdaddy
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Re: FOC or Front of Center

#3 Post by Carpdaddy »

How much FOC does one need? The range of FOC recommended for different forms of archery varies. In their charts, Easton shows the following recommended FOC ranges, with calculations based on the AMO Standard formula: o FITA (Olympic Style) 11% to 16% o 3-D Archery 6% to 12% o Field Archery 10% to 15% o Hunting 10% to 15% FITA shooters, who compete at the longest ranges, use the highest average FOC’s; 3-D shooters the lowest; with field archers and hunters in-between.
Ed Ashby

Kybownut
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Re: FOC or Front of Center

#4 Post by Kybownut »

Most correctly spined arrows will fall into the acceptable ranges if you follow the 10 ggp rule, not always but most of the time! There is also a point of diminishing return. Good topic tony, I think higher foc is especially helpful with lower weight bows used for hunting

Captainkirk
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Re: FOC or Front of Center

#5 Post by Captainkirk »

Great post and a good reminder for those of us building arrows, esp. broadhead-tipped hunting arrows.
One can easily check "eyeball" FOC by simply noting where centerpoint is (between nock vee and end of pile end where tip meets shaft). See how far forward the arrow 'balances' on a fingertip, this will give a rough estimate. If you are balancing close to center you may need more tip weight (based, of course, on shaft spine)
Aim small, miss small!

Carpdaddy
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Re: FOC or Front of Center

#6 Post by Carpdaddy »

Two schools of thought concerning measuring arrow length. The AMO standard measures from throat of nock to end of shaft not including point. The other method measures from end of nock to tip of point whether it be broadhead or field point. Which is best? Either will benefit and there will not be a great deal of difference, either should help you determine what needs to be done. Unless your foc is borderline already either should put you in acceptable range.

Carpdaddy
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Re: FOC or Front of Center

#7 Post by Carpdaddy »

So how does one figure the foc on his arrows? Actually there is more than one way to do that also, first I will post the AMO method. Now my explaing may be fur too hillbilly to understand so feel free to jump in and correct me or better explain what I am trying to say. :oops:
AMO method
1. Measure length of arrow from throat of nock to end of shaft (not point). Divide by 2.
I mark that center point on my arrow with felt tip pen.
Mine; 30.5" long, so marked at 15.25 or 15 1/4.

2. Find balance point by balancing on back of knife or some other thin object, mark that point also.

3.Measure from throat of nock to balance point. Mine is 20.5"

4. Subtract center of arrow measurement (step 1) from balance point measurement.
20.50 (BP )- 15.25 ~ 5.25
(Sorry but my phone don't have an equals symbol so you get the ~ instead) .

5. Multiple that X 100 ~ 525.

6. Divide step 5 figure by total length (from step 1)
That will be your FOC.

525 divided by 30.50 equals 17.2% (mine)

Carpdaddy
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Re: FOC or Front of Center

#8 Post by Carpdaddy »

The easier for me way..

1. Measure length of arrow.
Mine are 30.50 or 30 1/2".
Divide by two, (mine 15.25.)
Mark that point on arrow.

2. Find balance point by balancing arrow on back of knife or whatever. Mark the balance point on arrow also.

3. Measure distance between the two marks, mid point of arrow to balance point.
Mine was 5 1/4" or 5.25.
(I take decimal out and use 525. (Or X 100)

4. Divide balance point difference by total length.
525 divided by 30.50 equals 17.2%

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Graps
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Re: FOC or Front of Center

#9 Post by Graps »

I use the Graps method to calculate the FOC .
( 1 ) Balance on finger , looks OK .
( 2 ) Shoot the arrow , yep shot OK .
"Maybe the truly handicapped people are the ones that don't need God as much." ~ Joni Eareckson Tada

Carpdaddy
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Re: FOC or Front of Center

#10 Post by Carpdaddy »

For the target or 3D shooter this subject may not be something to worry about as much so long as they are getting good arrow flight. But for those of us who shoot these arrows at live animals should certainly make the effort to make sure our arrows are equipped to perform as intended. Make a perfect shot on that deer and your okay, make a less than perfect shot like me, say shoulder blade, and foc can make the difference between a dead deer or a wounded deer. My arrows now range from 17-19% depending on the bow I am hunting with. If I were to be lucky enough to get drawn for an Elk hunt, or if I ever got to hunt even larger game, I would still feel confident using a 40-50lb bow. The only thing I would change for larger game would be to increase my foc to above 20%. If you doubt it makes that much of a difference just look up Ed Ashby's reports from many years of extensive testing on this subject.
On these cold winter days while there is not a lot more to do I can think of nothing better for tradhunters to do than to learn and apply what they learn on this subject. That's my opinion anyhow.

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