Broadhead Flight & Tuning Stability Checker

A field point is a smooth cone. Point it slightly off from the direction it's travelling and almost nothing happens. A fixed-blade head at the same angle has one or two square inches of flat blade hanging in the airflow, well ahead of the balance point, and it pushes — hard — toward whichever side it's leaning.

So the head steers the arrow while the fletching is still busy straightening it out. The fletching wins that fight, usually within the first ten or fifteen yards. But by then the arrow has picked up a small sideways speed, and nothing takes that back. It carries on drifting in a straight line for the rest of the flight.

That's why the gap grows the way it does. It isn't that the arrow keeps getting worse — the damage was done in the first few yards, and distance just multiplies it. It's also exactly why so many people say their setup is fine at 20 and unshootable at 50, and why checking broadhead flight at 20 yards tells you almost nothing.

Check it: load the default setup and slide the distance. The gap roughly doubles between 20 and 40 yards, then again by 60. If your own two groups behave that way, the model is describing your arrow.

A paper tear shows you where the tail went. The broadhead is on the other end, so it's aimed the opposite way — and the head is what steers. So:

• Tail left → point aimed right → broadheads land right of the field points.
• Tail right → broadheads land left.
• Nock high → point aimed down → broadheads land low.
• Nock low → broadheads land high.

This piece of it is pure geometry and it does not change if you shoot left-handed. The tail goes one way, the point goes the other, every time. What handedness does change is the likely cause of the tear — and that's a separate argument, covered two panels down.

Vertical tears get one extra complication: your sight is zeroed with field points, so a nock-high tear costs you both a planing miss and a slightly different trajectory. The model handles the planing part only.

Both ends of the arrow are turning it. The margin is simply how much more turning force the back has than the front:

margin = (fletching area × its distance behind the balance point) ÷ (head area × its distance ahead of the balance point)

Area is the side profile each end shows to the air, averaged over the arrow's roll — a two-blade head spends part of every turn edge-on, three and four blades always have something in the wind, and a mechanical flies with its blades shut so it shows barely more than its ferrule.

Leverage is where FOC earns its reputation. Weight up front drags the balance point toward the head, which shortens the head's lever and lengthens the fletching's. Both move in your favour at once, which is why high-FOC arrows have a real reputation for taming big heads — and why the effect is bigger than people expect from such a small change in balance point.

A margin above about 2.6 means the fletching is comfortably in charge. Below 1.4 the head has enough authority to fight back for a long way downrange. But read it for what it is: margin describes how fast the arrow recovers, not whether it flies straight. A beautifully fletched arrow with a bad tear still throws broadheads a long way off — it just settles down sooner.

Check it: switch from 3 fletch to 4 and watch the gap close by roughly a fifth. Then set the tear to zero and watch it go to nothing regardless of fletching. The tune is the bigger lever, every time.

This is the genuinely unresolved part of broadhead tuning, and it's worth being straight about it.

The paper-tune convention says chase the tear: a tail-left tear means the rest goes left, toward the riser. Follow that through to impacts and it means moving the rest away from where the broadheads are hitting.

The broadhead-tune convention says chase the broadhead: if the heads hit left, move the rest left. You'll find this stated just as confidently, by people who tune bows for a living, in long ArcheryTalk threads where both camps turn up with results.

They cannot both be right for the same bow, and the reason the argument never ends is that people are describing different setups — different rests, cam lean, how far off centre shot they started, whether the shaft was even close to the right spine. Published charts contradict each other on this too.

So don't settle it by reading. Settle it by shooting:

• Move the rest 1/32 in in either direction. Just pick one.
• Shoot three field points and three broadheads at 30 yards or more — 20 is too close to show you anything.
• Measure the gap. Smaller means you guessed right; carry on in 1/32 in steps. Bigger means go the other way, and you now know which convention your bow follows.

Two moves tells you everything an afternoon of forum reading won't. And keep the moves tiny — centre shot changes work in hundredths of an inch, and it's easy to run past the answer and conclude the whole idea doesn't work.

You'll read that a right-handed shooter with a tail-left tear has a weak arrow. You'll also read, from people with just as much time behind a bow, that a horizontal tear is a centre-shot problem and says nothing about spine at all — and that reading it as spine has sent plenty of archers out to buy shafts they didn't need. Both positions turn up in the same tuning threads, year after year.

What isn't in dispute:

• The weak/stiff reading flips for left-handed shooters. The impact direction in the panel above does not.
• If a tear won't close after several small rest moves in the correct direction, the shaft is a genuine suspect — that's the point to check spine against a manufacturer chart with your real draw weight, draw length, point weight and cut length.
• A shaft badly wrong for the bow cannot be tuned out by any rest position, and broadheads are what will tell you, because they magnify what the fletching was hiding.

This tool takes no position on your spine. It works from the yaw you measured, whatever caused it, because the arrow's flight doesn't know the reason either.

If the margin panel says you're short of steering, there's a sensible order to fix it in.

• Height comes first. A rule that gets repeated on ArcheryTalk and holds up well: the vane should stand at least as tall as the blade stands out from the shaft. This page checks that for you under the bar.
• Twist is next. Helical beats offset beats straight, and straight vanes with a fixed head is the setup people most often describe as ungovernable past 30 yards. Helical costs a few fps and buys back far more than it takes.
• Count is third. Going from three to four is a real gain, but a modest one — and a fourth short vane won't rescue an arrow that was under-fletched to begin with.
• Length matters least of the four for its cost in drag, though it's the easiest thing to buy. Feathers behave like slightly taller vanes for their size and take a hard helical without complaint, which is a large part of why traditional rigs shoot big single-bevel heads so cheerfully.

Every one of these slows the arrow. That's a real cost and it shows up in the Trajectory Visualizer — but a few inches of extra drop at 40 yards is a much smaller problem than six inches of sideways.

A mechanical in flight is a ferrule with its blades folded against it. Its sail area is a fraction of a fixed head's, so it barely reacts to yaw and lands close to your field points. That's not marketing — it falls straight out of the same area calculation.

What it doesn't do is fix your tune. Switch to a mechanical on this page and the gap shrinks a long way, but it doesn't go to zero, because the yaw is still there. It's just no longer being amplified. Everything else that yaw costs you — energy bled off in the first few yards, an arrow arriving slightly sideways at the animal, penetration paying for it — is unchanged.

So the honest framing is that a mechanical buys you tolerance for a tune you haven't finished, and if you're taking a marginal shot on a big animal that tolerance was worth something in the first place. What the KE, Momentum & Pass-Through shows is the other side of the bargain.

• The tear: shoot through paper at 6–8 ft with a field point, three arrows, and use the tear you get twice. One shot is a coin flip.
• The gap: shoot three field points and three broadheads at 40 yards at the same spot, not the same distance on different days. Measure centre to centre.
• The comparison: feed your tear into this page and read the predicted gap. Close to what you measured means the tune explains your problem and the rest is where to work. A measured gap much larger than predicted means something on the checklist above is still in play.
• FOC: balance the finished arrow on a straight edge and measure it — two minutes, and most people are surprised.

If your numbers disagree with this page, believe your numbers — and leave a comment below so the next reader gets the benefit.