I am not completely sold on that idea.
Hi Grey... Looked up Easton FMJ crossbow arrow and found a 13.7 gr/in shaft. That's a pretty heavy (and slow) arrow for a 140 lb bow. We can address that later. I don't know what procedure you used for FOC, but using your ~10% number and matching the lever arms about the balance point with only a 1.5% difference suggests that the shafts had a 90 grain insert and no broadhead. If you include a 125 gr BH, 1 1/2" long, that adds 125*X in-gr to the right arm which must be balanced by 13.7*((21.5-x)/2) in-gr on the left. X=1.12 forward added to the 2.15 inches to start gives roughly 3.27 inches forward of center or ~ 15% FOC. NOTE: Math and logic subject to correction.
Bofos... I don't think Troy is going to take up crossbow activities any time soon, but a lot of his "hog research" with vertical tackle carries over. The process I used in post #5 (above) is taken almost directly from some of his work to find the "sweet spot" for a setup with minimum lateral waving of the arrow on impact. If your arrow is not "dead straight" when it hits the target, you are losing "penetration energy" of the broadhead.
Sorry for the late response, but with the eastons would that be a problem doing what I’m wanting to do a heavier foc for better penetrationHi Grey... Looked up Easton FMJ crossbow arrow and found a 13.7 gr/in shaft. That's a pretty heavy (and slow) arrow for a 140 lb bow. We can address that later. I don't know what procedure you used for FOC, but using your ~10% number and matching the lever arms about the balance point with only a 1.5% difference suggests that the shafts had a 90 grain insert and no broadhead. If you include a 125 gr BH, 1 1/2" long, that adds 125*X in-gr to the right arm which must be balanced by 13.7*((21.5-x)/2) in-gr on the left. X=1.12 forward added to the 2.15 inches to start gives roughly 3.27 inches forward of center or ~ 15% FOC. NOTE: Math and logic subject to correction.
On the heavy arrow issue. I use a 1/3 lighter arrow shaft and put 325 grains in the front end with a 200 gr insert and 125 gr BH. This gives me a FOC of roughly 22% in spite of a 36 gr lighted nock. A 9.1 gr/inch Black Eagle Zombie Slayer will easily handle your 140# draw.
I went through a similar process balancing increased FOC against loss of velocity as the total arrow weight increased. A heavy shaft spreads the weight over the entire length of the arrow instead of concentrating it at the tip for increased FOC. The difference between 13.7 gr/in FMJ and 9.1 gr/in (Zombie Slayer) is 4.6 gr/in or a total of 4.6x20=92 grains that I can put in my tip to increase FOC without further slowing my arrow, but your arrow has that 92 grains spread evenly over its whole length and not helping your FOC at all. The process noted in Post #5 included running the same arrow with each of the weighted tips through the chronograph and plotting initial velocity as a function of increasing weight of the arrow. I don't have the equation at hand, but suffice it to say for MY bow going from a 380 gr arrow at 324 fps (FOC~8.5%) to a 570 gr arrow (FOC~22%), I lost ~50 fps in initial velocity. The draw weight of my bow is only 10# greater than yours, so your velocity loss as you increase total arrow weight may be comparable. If you are going to estimate FOC for a "virtual" arrow, be sure you know the weight and size of each component (nock, vanes, wrap, shaft, insert, and tip. I think Easton has a very good calculator for imaginary arrow FOC on their website. Let me know if this stuff makes sense in your build.
