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Understanding FOC?

HuntNorthEast

Well-Known Member
Joined
Jul 24, 2020
Messages
1,027
Location
Southern Maine
Haven't dove too much into FOC. NEVER lost a deer bow hunt, yet. A buddy of mine is all over it and we are shooting similar setups, but he's getting 4" more penetration on targets. We both pass through most whitetails.

His overall weight is about 30 grains more than me (complete arrow).

Any experts on here care to chime in and give me the FOC for dummies version? (I mean really, really make me feel stupid.) :neutral:

Any input would be appreciated, thanks!
 
Let’s take 2 hypothetical setups to compare them, both 500 grain arrows total weight with head. Arrow A has a 50 grain broadhead and the arrow B a 250 grain broadhead, (not really realistic but go with me). A has 10% weight in broadhead and 90% in shaft. B is split 50% head and 50% shaft. We know that when you hit one end of your arrow shaft really hard the whole arrow flexes. This happens at shot and at impact. So, when you hit the deer with arrow A and the broadhead punctures the skin, you’ve got 10% of the weight pulling the arrow straight through the deer, while 90% of the weight is pushing into a deer and wasting energy flexing your arrow. I think you see where this is going... more head weight is less weight that is being wasted pushing your shaft and bending and an overall more efficient impact.
 
Let’s take 2 hypothetical setups to compare them, both 500 grain arrows total weight with head. Arrow A has a 50 grain broadhead and the arrow B a 250 grain broadhead, (not really realistic but go with me). A has 10% weight in broadhead and 90% in shaft. B is split 50% head and 50% shaft. We know that when you hit one end of your arrow shaft really hard the whole arrow flexes. This happens at shot and at impact. So, when you hit the deer with arrow A and the broadhead punctures the skin, you’ve got 10% of the weight pulling the arrow straight through the deer, while 90% of the weight is pushing into a deer and wasting energy flexing your arrow. I think you see where this is going... more head weight is less weight that is being wasted pushing your shaft and bending and an overall more efficient impact.
Transfer of kinetic energy, got it! So what's a good ratio would be my next question... Thanks!
 
Transfer of kinetic energy, got it! So what's a good ratio would be my next question... Thanks!
Google foc calculator and gold tip has a thing where you can type in your arrow and everything and it gives you a percentage. I think it should be above 15 and you won’t get too high without doing something really weird. The problem is with new bows pulling 70 lbs and more than 28” draw all the arrows get really long and heavy and even 1% more FOC is hard to get. If you just want the easiest route grab 100 grain heads and 100 grain inserts tune it and don’t worry about anything else.
 
DISCLAIMER: This is merely my hypothesis, we'd need an actual thorough scientific study to validate this.

Your penetration is maximized when 100% of your inertial mass is directly inline with the path of travel. Critters move, deflections happen. Higher FOC moves the fulcrum closer to the point of initial impact and helps steer the broadhead past those critical few miliseconds of initial penetration. The closer your fulcrum is to the point of impact, the harder it is for your arrow to be deflected off the path of travel (i.e. maximizing your penetration).

As a thought experiment, open a door and then try to close it by pushing it close to the handle. Pretty easy right? Now try closing it by pushing as close as you can to the hinges, a bit harder? Same mechanics apply to FOC.
 
Google foc calculator and gold tip has a thing where you can type in your arrow and everything and it gives you a percentage. I think it should be above 15 and you won’t get too high without doing something really weird. The problem is with new bows pulling 70 lbs and more than 28” draw all the arrows get really long and heavy and even 1% more FOC is hard to get. If you just want the easiest route grab 100 grain heads and 100 grain inserts tune it and don’t worry about anything else.
My set up is in my signature
 
Transfer of kinetic energy, got it! So what's a good ratio would be my next question... Thanks!

Dr. Ashby recommends 19% FOC. You can go the the Ashby foundation web site and read his findings, there is a lot of information to be shared on there regarding FOC and arrow construction.



Sent from parts unknown
 
My FOC is only 17.6. Do you think I should bump it up to 19%?
Thats really up to you. Sounds like you are pretty darn close. I think my foc is about the same,depending on me using regular nocks or lighted nocks. I am calling mine close enough and it is a world of difference from my 335 total weight flappers i shot last year.
 
I just did all the math out and I also come in around 17.6% FOC. I don't math good though so hopefully I did that right. :tearsofjoy: Should be mint!
I would be surprised if your foc is that high with your heavy arrow gpi. Look up the goldtip foc calculator. It is free.
 
I would be surprised if your foc is that high with your heavy arrow gpi. Look up the goldtip foc calculator. It is free.
I DID NOT MATH RIGHT... I forgot about vanes and stuff, came in at 7.1% w/ 125gr heads I'd be at 9.1%. Going to Use heavier inserts next season, as I am already ready to rock and my season starts in 30 days.
 
Haven't dove too much into FOC. NEVER lost a deer bow hunt, yet. A buddy of mine is all over it and we are shooting similar setups, but he's getting 4" more penetration on targets. We both pass through most whitetails.

His overall weight is about 30 grains more than me (complete arrow).

Any experts on here care to chime in and give me the FOC for dummies version? (I mean really, really make me feel stupid.) :neutral:

Any input would be appreciated, thanks!

If your setups are that close, your lack of penetration is probably more related to poor arrow flight.
 
My FOC is only 17.6. Do you think I should bump it up to 19%?

If your shooting well with what you got I wouldn’t Bill. That’s pretty much there and season is fast approaching.


Sent from parts unknown
 
If your setups are that close, your lack of penetration is probably more related to poor arrow flight.

Agreed that poor arrow flight robs penetration. But would be interested to know his buddy’s arrow setup and FOC vs his FMJ‘s at 7.1% FOC....

7.1% vs 15+% would be a big difference! It definitely would explain some of the differences they’re seeing in penetration.
 
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