Hunting Washington Forum

Equipment & Gear => Archery Gear => Topic started by: jrebel on July 02, 2011, 07:55:25 PM

Title: FOC (Forward of center)
Post by: jrebel on July 02, 2011, 07:55:25 PM: I have read three different ways to measure foward of center percentages. What is the proper way?

What is total length of arrow? Tip of nock to tip of broadhead, tip of nock to tip of incert, groove in nock tip of arrow?

Where does the measurement start after finding center of gravity? tip of nock or grove in nock?

What is Ideal FOC % for a hunting arrow?

Thanks

Johnny
Title: Re: FOC (Forward of center)
Post by: Todd_ID on July 02, 2011, 09:18:09 PM: AMO-Standard F.O.C. balance formula

F.O.C. % = (100 x (A-L/2))/L

L= Correct Arrow Length-Distance from bottom of nock groove to end of shaft
A=Distance from bottom of nock groove to finished arrow balance position (includes weight of point [+ insert], nock system and fletching)

If you end up with 10-15% FOC you're likely to see good arrow flight. If you have 12-15+%, then you're way more likely to have good broadhead flight. You'll have flight troubles at less than 10% and penetration increases and trajectory losses above 15%. The most recommended for a balance between stability and trajectory is 12-15% for broadheads.
Title: Re: FOC (Forward of center)
Post by: jrebel on July 02, 2011, 10:37:22 PM: Thanks Todd ID

If my measurements are right with 100 grain broadhead foc is 12% and with 125 grain broad head foc is 11%. Either will work but 12% may be better.

Thanks

Johnny
Title: Re: FOC (Forward of center)
Post by: RadSav on July 03, 2011, 12:01:51 AM: How are you getting better FOC with the lighter head? I'd double check that.
Title: Re: FOC (Forward of center)
Post by: Snapshot on July 03, 2011, 10:49:20 AM: ...or more simply put (for me, the mathmatically challenged):

((Balance point / Arrow length) -.5) x 100 = F.O.C.

Yes, as T.I. says, the distance from the bottom of nock groove to the place where the shaft meets the back of the arrow head is the arrow length.

Yes, R.S. is correct; re-do the math. A heavier broadhead will give you a higher F.O.C.
Title: Re: FOC (Forward of center)
Post by: jrebel on July 03, 2011, 12:09:04 PM: My thoughts exactly...I will have to recheck. I bet I misspoke and got the two mixed up.
Title: Re: FOC (Forward of center)
Post by: jrebel on July 06, 2011, 08:48:54 AM: Did the calculations again. 100 Grain broadheads on come to just shy of 12% and 125 Grain broadheads come to 14%.

I guess either will do.

Thanks everyone.
Title: Re: FOC (Forward of center)
Post by: buckfvr on July 18, 2011, 05:17:07 PM: If you decide to go with the 125s, just be sure your arrows are spined for it........you or your quary will not see the difference between an foc of 12 and 14..........
Title: Re: FOC (Forward of center)
Post by: jrebel on July 18, 2011, 07:50:39 PM: I went with the 125 grain broadhead and paper tuned my bow (with field point). They are punching perfect bullet holes. Arrow flight has always seemed better with the 125 grain tip...tighter groups and cleaner flight.

My spine should be good. I shoot Easton Axis 340 at 29.5 inches. I am pulling a 60 lbs Mathews DXT which I think turns the scale at just under 60 lbs (57-58). I am not sure I trust my bow scale though. I chrono'd it and my arrows are coming out of the bow at 267 fps.

This is why I don't trust my scale....My body has the same bow with a 70lbs draw but only a 27 inch draw. When we used the scale his showed the same draw weight as mine. His arrows are coming out at 265 fps. Eather his is not 70 or mine is is not 60???? I can't believe 2" of draw would make that big a difference? What do you think?
Title: Re: FOC (Forward of center)
Post by: RadSav on July 18, 2011, 10:06:19 PM: That's 10% more at peak power stroke! It makes a big difference! The Bowtech Destroyer 340 & 350 are basically the same bow with 1" difference in peak power stroke. Same thing with the Mathews MR6 and MR7 being the same bow with 1" difference in power stroke and 10 fps difference. Now reduce the efficiency of both the cam and the draw length and you should see the 2" of changed power stroke become an even greater percentage of difference.

Plugged the numbers into Archers Advantage and assuming his bow at 70/27 and a 28" Axis 340 and yours at 60/29.5 and a 29.5" Axis the speeds are forecasted at exactly the same 267 fps. If he were to shoot your 29.5" arrows his bow would actually shoot slower with a forecasted speed of 263 fps.

Remember, if your cam size is the same so too is valley size and zero to peak length. The difference in power stroke will be taken from peak energy during the draw cycle. 2" or 10+% is quite a lot! Especially in a soft single cam like the DXT.