Ok so this one is for the nerdy archer type..
The term Forward of Centre (FOC) is in fact a measure of stability that calculates the distance between the arrow centre of gravity with respect to the aerodynamic centre of pressure, and it is usually expressed as a percentage.
Just doing a fast search over the web, you should find typical values for various types of archery, most commonly: 11% to 16% for Olympic competition and 10% to 15% for field shooting or hunting. But you have to remember that although these are tried and tested, they are not a universal rule, especially when it comes to high FOC.
So, let’s challenge these, but first a bit of theory, shall we?
Theory will say that with too low values of FOC, your arrow centre of gravity and your centre of pressure will be much closer, therefore providing almost a statically balanced arrow. Don’t forget that the centre of pressure is a dynamic point and it will shift with speed along the length of any body in motion. In fact, you need to imagine a still image of your arrow in mid-flight, then, imagine you could step into that still motion and hold the arrow from the centre of pressure. It should feel almost statically balanced.
The lower your FOC, the least efficient your fletching will be, which for some shooters that might be just their ticket, because it means you can get away with very small fletching with less air resistance, although less spin too. But remember the centre of pressure will shift with speed and you could easily find your arrows loosing directional stability at longer ranges as they slow down, when the centre of pressure does move forward. When your arrows centre of pressure moves closer to the centre of gravity, your arrows may become divergent or exhibiting an increased fishtailing, noticeable by your arrows resting at off angles on the long range targets. In archery terms, the flight of an arrow is never too long and even long range shooters might have to stretch to observe the effects of arrows with exceedingly low FOC.
If you manage to run your arrows with negative FOC, if the arrows simply flip forward to back and fall out from a radical increase in drag, resembling a paper ball opening in midair, then you know your centre of gravity went too far back. Of course, it is possible to have slight negative FOC on an arrow, but rest assured your arrows will keep flying straight only due to arrow drag.
At the opposite end of the spectrum we find that exceedingly high values of FOC will be caused by a greater gap between the centre of pressure and the arrow centre of gravity; and as a result, your arrows will heavily rely on the fletching to keep them stable. This means that arrows with high FOC will require either large draggy vanes or those providing high spin rates that will stabilize it dynamically. It also means that the effect of any field tip or broadhead installed on the arrow will be minimised, which is something to consider when hunting or shooting outdoors in general.
Consider the still motion approach from before, now applying to a high FOC arrow in mid-flight and holding it by the centre of pressure, would show an apparently unbalanced arrow, statically, simply because in this still-motion concept, the effects of the fletching would be missing. Better think of it as an old Roman Steelyard, where the hanging point is the centre of pressure, the counterbalance is the centre of gravity and the opposite end is the combined force from the fletching. During the dynamic event of the arrow in mid-flight, the fletching on a high FOC arrow would provide the balancing force and therefore get a proper workout, and that is a good thing too.
Spinning broadheads will tend to reduce any stabilising effect from your fletching, resulting in less than perfect groups at the target. Therefore, every inch of moment arm you give to your fletching to counteract this will be very very useful.
Keeping in mind that for hunting purposes you want to obtain the maximum possible penetration, I am a great believer of a centre of gravity as much forward as reasonably possible with respect to the arrow centre of pressure; in other words a high FOC. Popular wisdom dictates that 15% is tops for hunting, but I have found that going to values as high as 19 and 20% delivers very hard-hitting arrows, stable throughout the flight and with vanes in full command, therefore also offering a stable spinning rate. A hunting tip of 300 grain would give me an FOC of 24%.. That’s got to be tested some other time!
Good carbon arrows don't come in cheap, so I wanted one same shaft that would allow me to shoot tips of 125 grain through 190 grain, or even beyond. I settled for the Carbon Express Kevlar using a manufacturer spine stiffness rating of 350, cut at a length of 30.0 inches from the nock groove to the edge of the insert. These would give me the right stiffness arrows I needed, also offering weight-balanced arrows with an FOC ranging from 13% to 19%.
The first time I set my arrows with a 145 grain tip, I found a couple of interesting developments. Firstly, the offset between the each pin sighted from 20m through 60m was constantly smaller than when using a tip of 125 grains. This means that pound for pound of velocity, my arrows kept the energy better during flight, loosing less to aerodynamic drag along the way. The second finding was even more interesting, in the form of increased accuracy, my arrows would more often than not, consistently kissing on groups at a range of up to 30m, and I’d be able to hit bull’s eye with small groups all the way to 80 m.
At this point, some of you may be experiencing flashbacks at my mention of 300 grain tips, of course, why should anyone shoot 300 grain broadheads with modern compound bows? With the speed frenziness that comes with every offering in the market and virtually all the commonly available broadheads being of 100 or 125 grain, why would anyone want anything heavier than the absolute minimum, in other words, did industry get it all wrong?
Of course not, because light tips will give you longer range, and that’s fine too. But heavy broadheads also have a reason of being. These heavy broadheads came in long ago when bowhunters were practicing instinctive archery, and a good recurve bow would be incapable of shooting beyond 200 fps or so. Then, it was all about penetration, and since speed was not there, heavy arrows and heavy heads were providing the energy required. Such heavy tips do have a reason to exist when it comes down to hunting the largest or meanest game, but in fact the same purpose also occurs when hunting even moderate whitetail: accuracy and penetration.
Again, it is all a balance between range, trajectory and ultimate penetration. But at least for me, a 145 grain field tip gives me a lot of consistency, accuracy, penetration and more importantly, confidence.
The picture I include in this post is of last Saturday at the range, using field tips and aiming for a pinned folded paper mark of about 2x1.5 inches, at a range of 70m. My arrows have a length of 30.0 inch and with the 145 grain tips, the FOC is 15%. Notice the consistent penetration all the way to the fletching. In contrast, my previously longer arrows (same arrows just longer before I cut them) would use 125 grain tips, they were 31.5 in long, would score an FOC of 11%, and shooting at the same target, would not penetrate more than 50% of the arrow length.
All right, enough with all that jabbering and let’s get right down to food! This posting special is a classic recipe that works great on any occasion:
Rabbit with Onion Stew
Serving for four civilised people or two hungry hunters, a deep skillet and a campfire.
- A good size rabbit of about 3 lb.
- White onions, 6 lb (yes you read it right – don’t worry, they reduce to caramel at the end).
- Crushed salt and black pepper (freshly ground if possible).
- Four bay leaves.
- Olive oil.
- 1 cup of extra dry sherry.
- A square of dark chocolate (85% pure or unsweetened cooking chocolate).
- Mash potatoes.
Salt and Pepper the rabbit and golden the cuts in the oil with the bay. I recommend adding more salt than you would normally to counteract the sweetness from the onions over a hot heat. Add the sherry and take it down to medium-low heat. Add the sliced onions and the chocolate. Cover for 15 minutes to help the onions break to sweat, and then uncover it and have it cooking slowly for another 90 minutes. Stir lightly or shake the deep pan every now and then. The dish is ready when the meat separate from bones effortlessly or when the onions look slightly caramelised. I like it served with abundant onion, mash and a little cooking juice.
Enjoy it.
