Arrow Front Of Center Calculator

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easton2.gif (10087 bytes)
The above illustration would yield an FOC of 10%

For some interesting broadhead test that Andrew Middleton
performed using a crossbow on FOC, goto the following page

For more info on FOC, goto the following Model Rocketry pages
the same general rules of flight apply

The formula on my page is from Larry Wise's "Tuning & Silencing Your Bowhunting Shooting System, 3rd Ed" book. If memory serves me correctly he attributed that FOC formula to AFC(now part of Carbon Express I think). It will yield a slightly different result than Easton's method.

Inasmuch as neither the Easton method nor AFC method calculates the "real" FOC of the arrow(which would be a real hassle - explanation below), the AFC method I believe is truer to the actual method which inlcudes the entire length. In any case, I would use either calculation of FOC as a "figure of merit" for comparison between setups, rather than an exact measurement.

REAL FOC explanation - The actual FOC of an arrow is dependent on the distance between the arrow Center of Gravity(CG) in front of the arrow Center of Pressure(CP). It is the CP measurement that both FOC methods approximate by taking the halfway measurement of the finished or unfinished(with or without point, AFC vs Easton). To see the real method of calculating CP for an arrow click on the following link;
http://homepage.ntlworld.com/joetapley/cop2.htm

I think it's obvious from the diagrams that adding a broadhead, especially a fixed one upsets the real CP measurement considerably, in the Easton FOC method there's no effect while in the AFC there is a penalty because of the length(not really enough though). The diagrams from Joe Tapley's page should help explain why an arrow which flys great with field points suddenly goes to crap with broadheads even though the "apparent" FOC seems reasonable.