Arrow F.O.C. what is it? How to calculate it.

Discussion in 'General Crossbow Discussion' started by urban legend, Feb 7, 2010.

  1. urban legend

    urban legend Administrator Staff Member

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    What is arrow Front of Center, and how do I figure out my Front of Center. F.O.C is a term used to express the percentage of weight forward from the center of the arrow. Having good F.O.C. is not only important in obtaining good down range accuracy, but crucial when it comes to achieving good broadhead accuracy. This is especially important when dealing with crossbows. Being that the crossbow arrow is much shorter and less stable than traditional vertical bow arrows, it will require more front of center.
    So how do we determine F.O.C. and what is “good” F.O.C for a crossbow arrow? To find the F.O.C. of any arrow, the first step is to measure the overall length of the arrow from the outside of the tip or broadhead to the outside of the nock. Once this measurement is achieved, divide it by two and mark the arrow shaft at the center point. The next step is to find the center of mass of the arrow. This will require balancing the arrow shaft horizontally on something, much like a fulcrum or teeter-totter. Once you have established the center of mass, mark its location on your arrow shaft too. Just a note, it should be somewhere between the center point of the shaft and the tip of your arrow. The last step is to measure the distance between the two marks and then divide that number by your overall arrow length. This will give you the F.O.C. of your arrow in its decimal form. Don’t forget to move the decimal point over two places to the right to turn it into a percentage. Congratulations, you have just measured the front of center of your arrow.
    Now that you’ve found out what your F.O.C is, how do you know if it’s “good” or not? There isn’t an exact percentage to go by, but more a range. Most guys feel they achieve best accuracy with crossbow arrows that fall somewhere between 12%-18% front of center. Having a crossbow arrow that falls into this range will provide you with excellent broadhead flight without compromising trajectory. As a rule, a higher than 18% front of center will not have a negative effect on crossbow accuracy, however arrow front of center below 12% can lead to wider than normal arrow groups at farther distances.
    So what if your arrow doesn’t fall into this range? Well, you’re in luck. There are a couple of things you can do to adjust the F.O.C. of your arrow. The first, and easiest is to change the weight of your field point or broadhead. Normally you will have too light of a F.O.C. so you will want to use a heavier point to increase it. Next, you can use a heavier “brass” insert or add screw in weights to the back of your existing insert. This works well if you have a favorite broadhead that you don’t want to change but need to add more weight up front. The last thing you can do is to get rid of those old 5” vanes and switch to lighter feathers, or shorter higher profile vanes. This will remove weight from the rear of the arrow resulting in a higher F.O.C. without changing the point weight.
    Finding the correct front of center for your crossbow isn’t rocket science, but with a little experimenting, you may find that you can achieve greater accuracy. One crossbow may shoot better at 12% and the next crossbow may shoot better at 19%. The most important thing you can do is to practice with your crossbow with both field points and broadheads. If your hitting with good accuracy, chances are you won’t need to change a thing.

    F.O.C. Example:
    A 21” overall arrow has a center of mass measurement 3” from the center of the arrow. 3” ÷ 21” = 0.1428 or 14.28% F.O.C.
     
  2. urban legend

    urban legend Administrator Staff Member

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    This is a common question so I thought I would make it a sticky for you noob's.:)
     
  3. Moe60

    Moe60 Senior Member

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    Excellent explanation!

    Thank you.
     
  4. bucknbill

    bucknbill Senior Member

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    thanks a bunch Urban..checked mine with laser,s and 125 grizz tricks and got 18.6..they are shooting in the same hole at 25yds. very good info
     
  5. TenpointTL-7 100#

    TenpointTL-7 100# Senior Member

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    Thanks for the information. I'll check mine out now. ;) Mine came out to be 8.75%. Last season I shot a 6 point at 30 yards and nailed it perfect and I had a pass threw and the deer only went 30 yards and fell over. I sighted the crossbow in and it shot great up to 40 yards. Can I keep everything the way it is or do I need to redo the arrows all over to get 12% FOC. Urban Legend. Thanks
     
  6. beervo2

    beervo2 Senior Member

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    Excellent info U.L. I know people will use it....

    Mike
     
  7. Sporty87

    Sporty87 Senior Member

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    Good post with excellent information. A sample formula like: 2.25" from balance point to center on a 17.25" arrow looks like this - 2.25 / 17.25 = 0.1304 moving the decimal over 2 places comes out to 13.04 percent. For people like me who are math challenged.

    Sorry UL I didn't see your example at the bottom - Old age isn't easy.
     
  8. urban legend

    urban legend Administrator Staff Member

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    I did put a sample formula up at the very bottom.:D
     
  9. urban legend

    urban legend Administrator Staff Member

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    The most important thing is if you're hitting with accuracy. Sounds like you are. I wouldn't change a thing. One question, what type of broadhead are you using. Sometimes you can get away with less FOC if shooting mechanicals.
     
  10. urban legend

    urban legend Administrator Staff Member

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    Thanks for the suggestion.:D
     
  11. Buck Buster

    Buck Buster Flying Dutchman

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    Thank you for this. This is very helpful.
     
  12. TenpointTL-7 100#

    TenpointTL-7 100# Senior Member

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    I'm useing 125Gr. Spitfires. :D [​IMG][/IMG]
     
  13. Mister B

    Mister B Member

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    foc

    excellent post...thanks
     
  14. AlexG.

    AlexG. Senior Member

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    I ordered a new 1/2 dozen arrows for the upcomming year and was pleasantly surprised when I figured out the FOC 2 days ago. I didnt have any issues with the way my older ones were flying but after reading all the posts on them I wanted to get better than the 12% that I figured them out to be.
    My new arrows are:
    20" GT Laser 2"s
    Bohning Blazers
    FireNocks
    NAP Bloodrunner 100gr.

    FOC measures out at 16.8
     
  15. TenpointTL-7 100#

    TenpointTL-7 100# Senior Member

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    I changed my arrows to get more FOC. I'm now using Easton Carbon Power C2 18" arrows with a 112gr. brass insert with a 100gr. Spitfire broadhead and a plastic flat nock and 4 " flo. yellow QuickSpins vanes. My FOC is now 13.9%. :D I'll sight the crossbow back in sometime in September. :thumbsup:
     
  16. Stant

    Stant Member

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  17. kbarr1

    kbarr1 Senior Member

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  18. kbarr1

    kbarr1 Senior Member

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    Thanks for the info and glad to have the site back.
     
  19. natedawg

    natedawg New Member

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    Thanks for the info very helpful
     
  20. cwa1104sab

    cwa1104sab New Member

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    This noob is learning stuff everytime I come on here...thanks
     

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