Mar 14 , 2013
At one point in our life, just about all of us have played a game of darts. For being such a small projectile, darts have the ability to fly with great stability and accuracy. Why? It has to do with the dart’s front of center (FOC) which makes it heavier in the front, than it is in the rear. By having most of its weight in the front, the dart is able to recover almost immediately in flight and it also requires very minimal fletching on the rear to guide the projectile. This same weight forward principle applies when trying to get superior flight from an arrow in archery as well. Granted, a dart has much higher FOC than a properly designed arrow requires for stable and accurate flight, but the same concept applies. What is FOC? First off, what exactly is FOC? In archery, to get the proper flight trajectory, you need the front half of your arrow to weigh more than the back half. An arrow’s FOC is simply a percentage reflecting how much more the front half of the arrow weighs, than the rearward half. While there is no hard and fast rule for what the FOC of a hunting arrow should be, most people would agree that it should be between 7-15%. Although most people you will talk to like being on the upward end of that scale, you can achieve excellent arrow flight if your arrow falls anywhere within that range. For instance, my current setup is right around 10% and I am getting great arrow flight. To calculate the FOC of your arrow is very simple. The first thing you need to do is measure your correct arrow length, which is the distance from the bottom of the nock groove to the end of the arrow shaft. Next, you will have to find the balance position of the arrow. To do this, simply balance the arrow on a pencil or similar object (with field tip or broadhead installed) and mark the balance point on the shaft. Lastly, you will need to measure from the bottom of the nock groove to the finished arrow balance position you marked earlier. Once these steps are completed, you simply do the calculation listed below: FOC% = 100 x (A-L/2) L A= Distance from bottom of nock groove to finished arrow balance position with point or broadhead installed L= Correct arrow length – distance from bottom of nock groove to the end of shaft How do you manipulate FOC? Now that we know exactly what FOC is and how to calculate it, now comes the fun part – manipulating it. I really enjoy experimenting with my archery equipment to find out what shoots best out of my bow. Finding the exact FOC that gives me the best down range trajectory is no exception. When attempting to manipulate FOC, one thing to remember is that your shaft weight has no impact on your arrow’s FOC. I often hear guys saying that they don’t like using heavy shafts because it messes up their FOC. This statement comes from a lack of understanding the concept. For example: if you built up two identical length arrows with the exact same vanes, nocks, inserts and points and used an 8.2 gpi shaft on one, and a 9.8 gpi shaft on the other, the overall mass of the arrow would increase, but they would have exactly the same FOC. The only thing that affects the FOC on those two arrows is the different weight components on the front or back of the arrow. Simply put, the more weight on the front, the higher the FOC. One of the easiest ways to increase the FOC is by going with a heavier broadhead. By simply changing my 100 gr broadhead out with a 125 gr, raises my FOC by 3%. With this comes a slight loss in arrow velocity, but a gain in kinetic energy, but if you are hunting deer size animals like I normally do, I would prefer having the slightly faster arrow, rather than the slight kinetic energy increase. Keep in mind, the option of adding a heavier tip is based on the spine of your arrow. Adding tip weight weakens the spine of an arrow since there is more weight on the front end to push causing the arrow to flex more. If your arrow is spined heavy enough to handle another 25 grains of tip weight it may very well help. If not, the arrow will flex too much and cause poor arrow flight, especially with broadheads. If you only want to add a few grains to the front of the arrow, you can apply small brass washers between the broadhead and insert. Most of these weigh approximately 5 grains each. Another option would be to go with a heavier insert. If your arrow spine is already on the weak side, adding weight to the front of your arrow may not be an option. In this case, the better option is to reduce the weight on the back half of the arrow. This can be accomplished by using a lighter nock, smaller and lighter vanes and eliminating wraps. I have reduced the weight on the rear of my arrow by installing the 2” Bohning Blazers vanes. At only 6 grains each, these small vanes are very rigid and do a great job of stabilizing my arrows. I have used them with both mechanical and fixed broadheads with great results. A couple of years ago, Carbon Express came out with a product that intrigued me. They created a weight forward design in their arrow shafts by using coatings on the exterior of their shafts. The Buff Tuff layer on the front of their weight forward shafts is slightly heavier than the Buff Tuff Plus on the back end of the arrow. Although the weight difference is not that significant, it is slightly heavier in the front half. The bigger and more important part of the "Built In Weight Forward" story is that the Buff Tuff Plus on the back end is stiffer than the standard Buff Tuff on the front. What that means is that the arrow is really flexing predominately in the front 2/3 of the arrow, rather than the entire arrow, so it is able to recover faster. The result, it stops oscillating quicker, which is more accurate and reduces "wind drag" which allows it to retain energy and speed better. As you can see, there are virtually unlimited options a guy can experiment with to get the optimum FOC that works best for your particular setup. While experimenting, it is important to remember that there really isn't a FOC % that is perfect for all setups. There are too many other variables in the equation that affect arrow flight and an arrow's ability to retain energy, thus speed and a flatter trajectory. How important is FOC? So exactly how important is FOC on a hunting arrow? I personally think that it is worth calculating and experimenting with to find out what works best with your setup. During my experimenting, I have found that my arrows fly the best with an FOC percent somewhere in the neighborhood of 9-11%. To help answer this answer this question, I decided to ask several of the top archers in the industry, as well as representatives from two of the major arrow manufacturing companies, for their opinion on FOC. Following are their responses: “I think it is fairly important in that it needs to be above a certain percentage. Most arrow manufacturers tend to put that number at around 7%. As long as you are above that number, I don't think the exact percentage is critical. I would guess my current arrow is a little north of 10% FOC.” – Randy Ulmer, Hall of Fame Bowhunter
“I believe adequate front of center is absolutely crucial to attaining the very best accuracy especially with broadheads, and also it is a necessity if you want your broadheads and field points to group together. I shoot only fixed blade heads and without proper front of center to pull the point of the arrow downrange, the best accuracy is not possible with any arrow. This is compounded when putting what is essentially a wing on the front of your arrow in the form of a fixed blade head. Remember, the fletching on the arrow is meant to add drag to the rear of the arrow to stabilize it in flight and by having the point "pulling" the arrow along, it balances the thrust in a straighter line and makes the job of the fletching much easier.
I usually shoot for 11% FOC but have also had good results with as much as 17% and as little as 7%. The spine of the arrow will also play a part. I have found that shooting an arrow that is traditionally looked at as too stiff is more forgiving to different FOC. If the spine is weaker it will take more FOC to achieve the best accuracy from that arrow and the exact FOC is more critical. I shoot 28" Gold Tip 22 series arrows for all my hunting and outdoor target events and even though they are stiffer than what is generally recommended, I can have great accuracy with these arrows with FOC from 7% up to 12% or more.” – Mike Slinkard, CEO Winner’s Choice Custom Bowstrings.
“I personally prefer a lot of FOC weight. This is especially true with fixed blade broadheads. I've done a lot of point weight testing for both target and hunting setups. Usually, the heavier points seem to out-group the lighter setups as long as the arrow is spined appropriately to handle the added point weight. I don't get wrapped up much with the actual FOC numbers, but rather just start with the 125 grain broadhead and see how it groups. Easton has produced heavier inserts for use in some arrow shafts (50 & 100 grains). I would recommend using these for exceptionally large game like moose, grizzly, and African big game to improve penetration. These inserts are also a great way to tame down the velocity on an exceptionally fast bow when shooting fixed blade broadheads where speeds above 280 fps might negatively affect accuracy. The heavier arrow setup will also make your hunting setup quieter. I have used two HIT inserts back to back with Easton's FMJ for an additional 16 grains up front. Two HIT inserts and a 125 grain broadhead yields a point weight of 157 grains - the perfect weight to slow my speedy 29" Alpha Max 35 down to about 280 fps where it drives tacks.” – Darin B. Cooper, Double D Bowhunting, Rokslide Archery Technical Editor.“FOC is very important on a hunting arrow. I feel on average it should be 9 to 11%. On a non Built-in Weight Forward arrow and or a slower speed bow, a higher FOC like 11% may be a better way to go; on a faster bow, a lower FOC or a lighter 100 grain point on a Built-in Weight Forward arrow. We find if you go to heavy FOC or heavier point on the front like 125 grain or more you run the chance of causing the arrow to be a little under spine and not forgiving under hunting condition shots.” - Lennie Rezmer, Carbon Express
“It is very important to have plenty if FOC with any arrow for guidance and stability in the wind. We recommend having at least 8% FOC.” – Jeff McNail, EastonConclusion Hopefully, after reading this article, you will have a better understanding of FOC and the role that it plays in establishing good downrange arrow flight. Although trying to achieve an exact FOC percentage is not all that critical, being within a certain range is essential to proper arrow flight. The nice thing about FOC is that it's a fun and easy thing to experiment with and if you are willing to devote a little time to it, your arrows will be grouping better than ever.