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 you're hitting with good accuracy, chances are you won’t need to change a thing.
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.