FOAC — the number your broadhead breaks

This article starts with a hunting story and ends with a differential equation. You can get off the train at any station and still leave with something useful. Part One needs nothing but a bow. Part Four has calculus in it. The arrow doesn’t care which one you read.

Two weeks before season

You shot all summer with field points. Your groups were tight enough to be smug about. You checked your FOC because that’s what you’re supposed to do, and it came out somewhere respectable, and you felt good.

Then you screwed on broadheads.

And your arrows started leaving. Not wildly — that would at least be diagnosable. Just… drifting. A couple inches at twenty yards. Six at forty. Enough to matter on an animal. So you did what everybody does: you started chasing it. Rest a hair left. Nock point a hair up. Maybe it’s the cams. Maybe it’s me. Maybe I need to shoot more.

Here is the thing nobody told you. You changed the aerodynamics of your arrow more violently than any tuning adjustment you have ever made, and the one number archery uses to describe arrow balance reported that nothing happened at all.

Not “a small change.” Nothing. Let me show you.

You have felt this happen. You just haven’t had a number for it. Now you do.

What FOC actually measures (and what it doesn’t)

FOC — Forward Of Center — is a balance measurement. You lay your arrow across your finger, find the spot where it sits level, and compare that to the middle of the shaft. If the balance point is ahead of the middle, you have positive FOC. That’s it. That’s the whole thing.

It’s a see-saw with a ruler under it.

Now here is the part that should bother you: there is no air in that measurement anywhere. You could do it on the moon. You could do it in a bathtub. The number would be identical. FOC knows about mass and it knows about a ruler, and it knows nothing whatsoever about flying.

The reason this normally doesn’t matter — and the reason FOC has survived for decades as useful advice — is that for most of the changes archers make, mass balance happens to move in step with the thing that actually matters. Add point weight, balance moves forward, arrow gets more stable. The two track each other so closely that you can use one to talk about the other and never get caught.

Until you change the shape of the front of the arrow. Then the two come apart violently, and FOC keeps right on reporting a number as if everything were fine. That is the dangerous part. It doesn’t fail loudly. It fails silently, and it fails at exactly the moment you care most: two weeks before season, with a broadhead on the front.

What actually keeps an arrow pointing forward

Forget arrows for a second. Think about a weathervane.

A weathervane has a small arrow head at the front and a big flat tail at the back, and it pivots on a post in the middle. Wind hits it, the big tail gets shoved harder than the small head, the tail swings downwind, and the pointy end ends up facing into the wind. Every single time.

The reason it works is that the pivot point is ahead of where the wind pushes. Move the pivot behind the wind-push point and the thing spins around backwards and stays there.

An arrow is a weathervane that isn’t attached to a post. It pivots around its balance point — its center of mass, the spot you found with your finger. And the wind pushes on it at a spot engineers call the center of pressure: the single point where all the air pressure on the whole arrow can be treated as one shove.

Those two points are what the entire sport of tuning is secretly about.

When your arrow gets knocked sideways a degree or two leaving the bow — and it always does — the air catches the back of it and shoves it straight again. That’s the fletching doing its job. The bigger the gap between balance point and pressure point, the harder that correction pushes.

So the honest measurement of arrow stability isn’t “how far is the balance point from the middle of the shaft.” The middle of the shaft is a ruler mark. It has no physical meaning. The honest measurement is how far is the balance point from the center of pressure — because that gap is the actual lever the air gets to work with.

Call that FOAC: Forward Of Aerodynamic Center. Measure from the real thing instead of the ruler mark.

This is not us being clever

Worth saying plainly: we did not invent this. Rocket and missile engineers have used exactly this measurement since the 1950s, where it’s called static margin. Every model rocket kit you ever built was designed around it. It is one of the most thoroughly settled ideas in flight.

Archery is the odd one out for using a pure mass number instead. Not because archers are wrong — because until you have a way to actually calculate where the pressure point sits on a real arrow with real vanes and a real broadhead, static margin is a nice idea you can’t use. That calculation is the work behind this article.

The equation that connects everything

Here is where it gets satisfying. Write both numbers out as fractions of arrow length, and something falls out.

Subtract one from the other and the balance point — the mass, the point weight, the inserts, everything you fiddle with — cancels out completely:

Read that carefully, because it’s the spine of this whole piece.

G contains no mass. None. It is built purely from the shape of your arrow — how much vane you have, where it sits, whether there are blades on the front. It is a fixed bonus that your fletching hands you, and it does not care what you screw into the insert.

Call it the geometry bonus. On our reference arrow with three Blazer 2″ vanes, G is +28.5 points. Your fletching is quietly donating twenty-eight and a half points of stability that FOC never mentions.

And now every confusing thing about FOC explains itself at once:

• Why FOC works so well normally. Keep the same fletching and the same head shape, and G is a constant. Add point weight and both numbers move together, in lockstep, forever. They’re the same number with an offset. (We measured it across 252 configurations: within a fixed fletching, FOC and FOAC correlate at r = 0.9994 to 1.0000. That is not “closely related.” That is a straight line.)
• Why broadheads break it. Blades are area at the very front. They drag the pressure point forward, which shrinks G. Same FOC, smaller G, less stability. FOC literally cannot see it, because the change happened entirely inside the term FOC doesn’t have.
• Why bigger vanes help. More area at the back pushes the pressure point back, which grows G.

One equation, three answers. That’s usually a sign you’ve found the right way to look at something.

Your arrow is wildly overstable, and that’s the point

Before the bad news, some perspective that will change how you think about FOC tinkering forever.

Engineers measure stability margin in calibers — how many shaft-diameters of gap you’ve got. A model rocket is designed for 1 to 2 calibers. Fighter jets and guided missiles often run slightly negative and only fly because a computer catches them thousands of times a second.

Our reference arrow with a field point? 45.8 calibers.

This is why FOC advice feels simultaneously true and unsatisfying. Chasing FOC does help — a bit, at the margins, for reasons more to do with mass and penetration than stability. But the reason your arrows fly straight is that you glued three plastic wings to the back, and those wings are worth twenty-eight points of margin all by themselves.

Hold onto that 45.8 calibers. It’s about to get demolished.

The broadhead cliff

Every fixed-blade broadhead is, aerodynamically, a set of small wings bolted to the wrong end of your arrow.

That’s not a metaphor. A broadhead blade is a flat plate at an angle to the airflow whenever the arrow is even slightly crooked, which is constantly. It makes lift. It sits as far forward as it is physically possible to be. Your vanes are pulling the pressure point backward from about 2 inches off the nock; the blades are hauling it forward from 28 inches. Same tug-of-war, and the broadhead has a much longer rope.

Here is the same arrow with everything in our library screwed onto the front. Field point is the baseline.

Look down the FOC column. It barely moves — and where it does move, it moves for boring reasons (some of those heads are heavier). Now look at the margin column. Between a third and two-thirds of your arrow’s stability, gone, depending entirely on blade geometry that FOC has no way to represent.

The Montec swap moves the center of pressure from 6.01″ off the nock to 12.47″. Six and a half inches forward, on a 28-inch arrow, from a part that weighs the same as what it replaced.

And notice the pattern, because it’s a buying guide:

• Blade count hurts more than cut width. The Slick Trick Magnum (4 blades, 1⅛″) costs more margin than the Montec (3 blades, same 1⅛″). Four blades means more area regardless of how wide the cut is.
• Short ferrules win. The QAD Exodus is a wide 1¼″ cut but a famously stubby head — and it lands mid-pack rather than at the bottom, because blade length along the shaft matters as much as width.
• Two blades are genuinely gentler. The single-bevel 2-blade gives up 37% where three-blades give up 57–65%. There is a real aerodynamic reason single-bevel traditional guys report easy broadhead tuning.

The free lunch nobody talks about

Go back and look at the top three rows again.

The reason is almost stupidly simple once someone says it out loud: the blades are closed while the arrow is flying. A mechanical in flight is a slim ferrule and a tip. The air never sees the cutting diameter. The 2″ cut you paid for happens in the last thousandth of a second, on the animal — not on the way there.

So the endless argument gets a clean answer. “Do mechanicals fly better than fixed blades?” Yes. Measurably, substantially, and for a specific physical reason. Whether they penetrate the way you want them to is a completely separate argument that this math has nothing to say about — energy on impact is a different question from stability in flight, and anyone who tells you one answers the other is selling something.

The hybrids sit exactly where you’d predict: the bleeder blades are real, fixed, forward area, so they cost a real but modest 17%. The model doesn’t know what’s written on the package. It just counts what the air can push on.

Can you fletch your way out of it?

Yes. Partly. Here’s the honest accounting.

If blades at the front steal margin by pulling the pressure point forward, more vane at the back should push it back. It does. But you are fighting from a long way behind: your vanes work on a 2-inch lever, the blades on a 28-inch one.

Same arrow, same Montec, different fletching. “Keeps” is the fraction of the field-point margin still standing once the broadhead is on.

Three things worth taking to the fletching jig:

Micro vanes and fixed broadheads are a genuinely bad marriage. Ten percent margin retention. If you shoot 1.75″ spin vanes because they’re fast and quiet and forgiving indoors, and then you bolt a three-blade on for elk, you have built the single worst combination in the table. It will still fly — 3.1% is positive — but every tuning flaw you own is now amplified.

Going from three vanes to four is the cheapest fix available. Blazer 2″ goes 43% → 52% for six grains. That is a better return than almost any component swap you can buy.

Feathers punch above their size. The 5″ shield feather tops the table, and there’s a bonus the numbers here don’t show: feathers also damp the wobble harder, so the arrow settles sooner. Traditional archers shooting big feathers with big single-bevel heads worked this out empirically decades ago. They were right. This is why.

From here on the language gets more technical. If you got what you came for, the calculator above is the practical takeaway and you can stop clean. If you want to know how the pressure point is actually located — and why two popular pieces of tuning wisdom are half wrong — keep reading.

Where the steering force actually comes from

The center of pressure isn’t measured, it’s computed: every surface on the arrow contributes a normal-force slope (how hard it pushes per degree of yaw), and the center of pressure is those contributions averaged by position, weighted by strength.

For our reference arrow wearing the Montec, the breakdown is:

Anything acting behind the balance point steers the arrow straight. Anything acting ahead of it steers the arrow off course. A broadhead converts nearly a third of your arrow’s total aerodynamic authority into a force actively trying to turn it around.

The bareshaft paradox, solved

Everyone tells you to bareshaft tune. Nobody explains why it works so well. The geometry bonus explains it in one line.

Strip the vanes off. G doesn’t just shrink — it goes negative. With no fletching, the only things left are the shaft’s own cross-flow lift and the point, and the point is ahead of center. The pressure point lands slightly forward of the shaft’s midpoint, and the arrow’s comfortable 40% margin collapses to about 6%.

It is still, technically, stable. It is barely stable. And the restoring spring is weaker than you would ever guess:

A bare shaft has about one twenty-third of the self-correcting authority of the same arrow fletched. That is the entire reason bareshaft tuning is the gold standard: you have removed the system that hides your mistakes. The vanes aren’t just less able to cover a bad launch — they’re gone, and what’s left barely corrects at all. Whatever the bow did to that arrow at the shot is still written on it when it hits the target.

The bareshaft doesn’t lie to you. It also won’t save you.

Two things this number will not fix

Now the part where intellectual honesty costs us the tidy story. We tested whether FOAC explains two classic archery complaints. It doesn’t. Both are real; both have different causes; and the arrow-balance number you use is irrelevant to each.

Wind drift is not a stability problem

The intuition is universal: high FOC arrows “cut the wind better,” low FOC arrows get pushed around. The mechanism everyone imagines is that a stable arrow resists weathercocking.

Work it through, though. A stable arrow in a crosswind yaws until it lines up with the apparent wind — the vector sum of its own velocity and the wind. That’s what stability does; it’s the whole job. Once it’s aligned, the angle of attack is zero, and the aerodynamic side-force goes to zero with it.

So where does the arrow end up pointing? Pure velocity geometry:

There is no stability term in that equation. No FOC, no FOAC, no vane area, no margin. Every arrow in the sky today, at 280 fps in a 10 mph crosswind, is cocked 3.00 degrees into the wind. The overstable one and the twitchy one, identically.

Drift itself then comes from drag acting over time of flight — mass and speed. High-FOC arrows genuinely do drift less, but because they’re typically heavier and carry velocity better, not because the balance point is doing anything clever. The mechanism is mass, not margin. Buy heavier arrows, not different balance points.

How fast the wobble dies has nothing to do with margin either

This one genuinely surprised us, and it is the single most counterintuitive result in the whole study.

Intuitively, a stiffer restoring spring should settle the arrow faster. Model the arrow as a standard damped oscillator — inertia I, damping c, restoring stiffness k — and the wobble decays with a half-life set by ζω_n. Then substitute the standard definitions:

The stability margin cancels out of the decay rate entirely. How fast your arrow stops wobbling depends only on its rotational inertia and its aerodynamic damping. Static margin sets the frequency of the oscillation — how fast it swings — and contributes exactly nothing to how quickly it stops.

We confirmed this numerically to four decimal places across six arrows before we believed it, because it looked wrong.

Which resolves the strangest thing in the broadhead table. Go back and look: the Montec arrow, with 57% less margin, actually settles sooner than the field point arrow — 19.9 yards against 25.8. That looks impossible until you have the equation. Blades way out front on a long lever are excellent dampers, and damping is the only thing that matters here.

So should FOC be thrown out?

No. And we’d rather say that plainly than oversell our own metric.

Within a fixed fletching and head type, FOC and FOAC correlate at r = 0.9994 or better. FOC is cheap, needs no software, is unambiguous, and is standardised across the industry. Replacing it would cost archery a common language and buy almost nothing.

What FOC cannot do — at all, ever — is tell you what changing the shape of your arrow costs you. The moment you change vane size, vane count, or head geometry, G moves, and FOC is measuring a different arrow than the one you’re holding while reporting the same number.

FOC tells you where your arrow balances. FOAC tells you whether that matters. Most days they agree. The day they don’t is the day you’re standing in the dark with a broadhead on the front, and that is precisely the day you want the right number.

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Published 2026-09-09  ·  Axial Bowstrings

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