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FOC %

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BowEd:
Some experts say ideal is in between 11% and 18% for broadheads and 7% to 11% for target shooting.Easton reccommends an average somewhere between 10% to 15% for hunting arrows.
I was wondering after making some full length tapered privet shoot shafts [3/8" tip and 9/32" self nock] here what my FOC % was on them.A 125 grain field tip is on them now.28.25" distance from throat of nock to base of field tip.18.25" from throat of nock to balance point of shaft.After calculating it it was 10.83%.They shoot great like darts.They weigh an average of 695 grains.
That's putting balance point about 3" forward towards the tip of center of shaft.Balancing anywhere from 2" to 3" forward will achieve close to the same results for good flight and penetration.The farther the balance point towards the tip the higher the %.
To summerize the advantage of higher FOC % the farther the distance the flecthings have from the nock to balance point of shaft the easier it is for the fletching to correct an arrow shaft in flight.For better penetration reasons the shorter the distance of tip to balance point leaves shaft stiffer upon impact with less flex.
I figured mine out by looking up the FOC formulation on line.
It goes like this.I hope I have this right now.
1.First measure from throat base of nock [where string touches nock]to the back end of field tip or nail or blunt head or broadhead or stone point.Divide that number by 2.
2.Then measure from throat base of nock [where string touches nock]to actual balance point of the shaft.
3.Subtract no 1 from no. 2
4.Take resulting number from step 3 and multiply by 100.
5.Divide the resulting number from step 4 by the arrows overall length.
6.Resulting figure is your FOC %.
Taking advantage of this could make the difference in a successful hunt or unsuccessful hunt.
Bob Barnes sent me these privet shafts cut green.We trade off and on.He did an excellent job selecting these size wise.I straightened/rebundled these numerous times by hand while green.Leaving them to dry a few months.They were plenty long so I spined them by removing length.The bark was left on and dried tight to the shaft on all of these as a finished arrowshaft.Just some slight knot removal.Kind of unusual as no heat was used to straighten these.A rareity.They are staying straight.I now have 8 different types of shoot shafts from around the country I use.http://www.primitivearcher.com/smf/index.php/topic,67275.0.html Privet is 1 of the heaviest.

Finished pictures.

TimBo:
Those look great!  I was following the math until step 5...it seems to me that if the balance point of a 30" shaft was 3" front of center (not yours, just an easy number example), that would be 10% FOC.  If I am understanding your steps correctly, it comes out at 20%.  (30 divided by 2 = 15; 18-15 = 3; 30 divided by 3 = 10; 10 x 2 = 20%)  I found a formula (on the Easton Archery website) that changes step 4 to: multiply step 3 by 100 (maybe that is what you mean by moving the decimal?), then for the next step divide that by TTT shaft length.  If I plug your numbers into that, I also get 14.6%...so I am probably misunderstanding something with your formula.  The important thing is that they shoot well!  This makes me want to check some of my arrows.  Mine aren't anything close to a matched set though...

BowEd:
Yes TimBo I've corrected the process of the formula to Eastons formula.The FOC on the privet shafts shown is 10.83%.Too many experts out there.....ha ha.
My other concern is about momentum as you can have high KE but little momentum.Good speed/good FOC/along with a heavy arrow produces good momentum for good penetration.
Momentum is mass X velocity divided by 225400.At current the average momentum of my privet shafts from my 48# self bow is in the mid 40's with momentum being around 160 for KE.
 

TimBo:
Hi Ed,

I didn't mean to derail your post!  Hope I didn't come off as a know-it-all.  (I checked one of my multiflora rose arrows this morning and came up with 12%.)

Tim

BowEd:
No not at all,unless that was your intention.I am merely the messenger here.It's a good discussion if someone wants to talk about it.That's a good FOC %.What weight up front are on them?I have multiflora rose here too.30" TTT 50# spine wieghing at 600 grains with a 125 field tip.I like them too.
The more weight up front increases the FOC %.I went with Eastons' formula as I've seen others have the same formula.

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