What spine do I need?

Read it off a manufacturer chart (Easton, Gold Tip, Black Eagle, Victory all publish them). Cross-reference your peak draw weight, your finished arrow length, and your point weight. Most 60-70 lb compound hunters with a 28-30" draw and a 100-grain point land somewhere around 340 spine, but use the chart — heavier points and longer arrows shift you weaker (higher number); shorter arrows shift stiffer (lower number).

Good arrow weight for whitetail?

425-475 grains is the modern norm for whitetail-class hunting and works well at typical bowhunting ranges. 400 grains is fine. 500+ adds penetration margin with little real downside on whitetail-distance shots under 40 yards. The 5-grains-per-pound floor (so 350 gr at 70 lb) is the manufacturer minimum for safety, not a hunting recommendation.

Is high-FOC actually better?

For penetration on big game, yes — within reason. Higher FOC concentrates mass forward, improves recovery from launch flex, and steers broadheads more cleanly. Diminishing returns kick in around 20%. For target and 3D, modest FOC (around 10-13%) flies flatter at distance. For whitetail-distance shots, the difference between 12% and 18% FOC is small in practice.

Why don't my broadheads group with my field points?

Almost always a bow-tune issue, not a broadhead defect. Broadhead blades amplify any setup imperfection — cam lean, weak or stiff spine, fletching contact, or rest centerline error. The fix sequence is paper tune → bareshaft tune → broadhead tune. Don't blame the broadhead until you've bareshaft-tuned.

100, 125, or 150 grain points?

100 if your spine chart points there and you don't want to re-tune. 125 is the strong default for modern hunting setups — better FOC and momentum, broadhead selection is still excellent, modest spine impact. 150+ for heavy-arrow builds; expect to drop a spine class to keep the dynamic spine in range.

How does arrow length affect spine?

Longer arrows are weaker in dynamic spine, shorter are stiffer. Cutting an inch off a shaft is enough to shift it noticeably toward stiff. This is why every spine chart has both a draw-weight axis and a length axis. Cut to your draw length plus 0.5-1" of overhang past the rest for safe clearance.

Heavy arrow or fast arrow — which is better for hunting?

Fast arrows forgive range-estimation errors (flatter trajectory). Heavy arrows forgive shot-placement errors on bone or large game (more momentum, deeper penetration). For most North American hunting under 40 yards, a balanced mid-weight setup around 450-500 grains wins. The heavy-arrow argument grows stronger as game size and bone risk increase.

What's the minimum arrow weight my bow can safely shoot?

5 grains per pound of peak draw weight is the universal manufacturer floor. So a 70-lb bow needs a 350-grain minimum. Below that voids your warranty and stresses the bow toward dry-fire territory — there isn't enough mass to absorb the stored energy of the limbs.

Do I need to re-tune if I change point weight?

For any swing larger than about 25 grains, yes. Going 100 → 125 grain softens dynamic spine; you may need a stiffer shaft, a small rest adjustment, or both. Re-paper-tune and re-broadhead-tune after any point-weight change.

Guide · Arrow setup

Arrow setup for a compound bow

Spine, total weight, point weight, FOC, kinetic energy, momentum — what each one is, what it does, and the numbers worth caring about.

Open the arrow lab →

What an arrow actually is

An arrow is six parts: shaft, point (or broadhead), insert, nock, fletching, and the glue and wraps that hold it all together. Every spec choice you make picks tradeoffs across these — pick a heavier point, dynamic spine softens; pick a longer shaft, dynamic spine softens further and trajectory drops faster; add weight forward, FOC rises, momentum rises, drop increases.

Shaft

The tube. Three numbers describe it: spine (stiffness — lower number means stiffer in the AMO/ATA system), GPI (grains per inch — raw mass density of the bare shaft, typically 7.5-11 for hunting), and length (measured from the throat of the nock to the end of the shaft, before insert and point).

Point or broadhead

The business end. Comes in standard weights — 75, 85, 100, 125, 150, 175, 200+ grains. Field points are essentially uniform mass forward; broadheads add steering surface and amplify any setup error. Heavier point means more FOC and weaker dynamic spine.

Insert

The threaded sleeve bonded into the front of the shaft that the point screws into. Standard aluminum inserts run roughly 12-16 grains. Brass FOC-heavy inserts can run 50-100 grains and are how the heavy-arrow camp loads weight forward without going to a 200-grain point.

Nock and fletching

Nock weight is small (around 8-10 grains for a standard pin nock; 16-25 for a lighted nock). Fletching is plastic vanes or feathers — typical hunting setups use three 2-3" vanes weighing 5-10 grains each. Helical or offset orientation imparts spin, which stabilizes broadheads.

Spine — and how to pick it

Spine is the stiffness of the shaft. The published number is the static spine: how far an unfletched shaft deflects under a standardized weight (per the AMO/ATA test, an 880-gram weight hung at the center of a 28" supported span). Lower number means stiffer.

What you actually care about when tuning is dynamic spine — how the shaft flexes when launched. Dynamic spine depends on static spine plus point weight (heavier softens), arrow length (longer softens), bow draw weight (more force softens), cam aggressiveness, and release type.

Use the chart

Don't eyeball spine. Open the manufacturer chart for the shaft you're considering — Easton, Gold Tip, Black Eagle, Victory all publish them as PDFs. Cross-reference draw weight × finished arrow length × point weight. The chart returns a spine bucket (commonly 250, 300, 340, 400, 500). Most modern 60-70 lb compound hunters with a 28-30" draw and a 100-grain point land around 340 spine, but the chart is authoritative.

Over and underspined

Arrows that are too weak (underspined) for your setup typically produce a tail-left paper tear for a right-handed shooter. Tail-right is the stiff (overspined) signature. Vertical tears are almost always nock-point or rest-height issues, not spine.

Total arrow weight — the heavy vs light debate

The single most-argued question in modern bowhunting. Both schools shoot dead deer; the honest answer depends on what you're hunting and how far.

Light (350-400 gr)

Faster, flatter trajectory, more kinetic energy at the muzzle, less margin against wind drift downrange. Below 5 grains per pound of peak draw weight is unsafe per every major bow warranty.

Mid (400-500 gr)

The historical North American whitetail standard. Best balance of speed and momentum. Most hunters end up here without thinking about it.

Heavy (500-650 gr)

Slower, more momentum, deeper penetration on game, less wind-affected once flying, quieter bow at the shot. More drop at distance, which raises the cost of a range-estimation error.

Very heavy (650+ gr)

Ashby-thesis territory. Targeted at very large or thick-skinned game (cape buffalo, moose, brown bear). Outside that context it's overkill.

The two schools

The heavy-arrow / high-FOC argument leans on Dr. Ed Ashby's penetration studies. The thesis: penetration on big game correlates with momentum, structural integrity, and forward weight bias far more than it correlates with kinetic energy. This argument grows stronger as game size and bone risk grow.

The flat-trajectory school argues that modern compound bows have plenty of energy to kill whitetail-class game cleanly with mid-weight arrows, and that flat trajectory reduces range-estimation error, which is the dominant cause of bad shots in the field.

FOC — Front of Center

FOC is the percentage of your arrow's length by which its balance point sits forward of physical center.

FOC% = ((balance_point − arrow_length / 2) / arrow_length) × 100

Typical bands across published sources:

Target / 3D / indoor: 7-11%
General hunting: 10-15% — most modern setups land here
EFOC (extreme FOC, Ashby term): 19-25%
Ultra-EFOC: 30%+ — rarely seen outside dangerous-game setups

What FOC actually does

Higher FOC means the arrow recovers from launch flex faster (cleaner downrange flight, especially with broadheads) and concentrates more mass forward of impact (deeper penetration on game). The cost is trajectory: front-loaded arrows nose down sooner.

How much FOC is enough — the actual debate

Where the curve flattens is contested. The Ashby Bowhunting Foundation position is that there's effectively no upper limit for hunting performance — pushing setups into the 19-25% band and beyond. Practical archery instructors and target shooters tend to call diminishing returns earlier — somewhere around 12-18%. Both sides agree setups under about 8-10% FOC steer broadheads worse and penetrate worse than setups in the 12%+ range.

Kinetic energy and momentum

The two ways to talk about how hard an arrow hits — and they don't say the same thing.

KE (ft-lb) = (grains × fps²) / 450,240
Momentum (slug-fps) = (grains × fps) / 225,218

Kinetic energy scales with velocity squared. Momentum scales with velocity linearly. So a light, fast arrow looks better on a KE chart than a heavy, slow arrow of the same kinetic energy — but the heavy arrow has more momentum.

Penetration on game correlates more closely with momentum, sectional density, and broadhead profile than with kinetic energy alone. This is the physics core of the Ashby argument.

Commonly cited KE thresholds

Small game / turkey: 25 ft-lb minimum
Whitetail / antelope / mule deer: 40 ft-lb
Elk / black bear / wild boar: 50 ft-lb
Moose / large African plains game: 65 ft-lb
Cape buffalo and dangerous African: 80-100 ft-lb

These appear in Easton's hunting setup material and most state hunter-education curricula. They're guidance, not regulation.

Velocity, IBO, and ATA

IBO speed is measured with a 350-grain arrow (5 grains per pound at 70 lb peak), 30" draw length, 70 lb peak draw, mechanical release, and no string accessories. ATA speed is similar but slightly more conservative.

Real-world fps drops from those numbers as soon as your setup deviates:

About 10 fps lost per inch of draw length below 30"
About 2 fps lost per pound of peak draw below 70 lb
About 1 fps lost per ~3 grains of arrow weight added beyond 5 gpp
A few fps lost for each string accessory (peep, kisser, silencer)

Field points, broadheads, and tuning

Field points have minimal aerodynamic surface ahead of the shaft. Launch errors don't affect them much. Broadhead blades act as forward steering surfaces and amplify any setup imperfection.

A clean field-point group at 30 yards with broadheads drifting four to eight inches off the same point of aim is almost always a tuning issue. Standard fix sequence:

Paper tune to a clean tear (or close to it).
Walkback or French tune to confirm rest centerline.
Bareshaft tune at 15-30 yards.
Broadhead tune: micro-adjust the rest until broadheads hit the field-point group.

The paper tuning guide covers steps one and two in detail.

Practical starting points

Whitetail-class

Total: 425-475 gr

Point: 100-125 gr

FOC: 12-14%

Elk / large NA

Total: 480-550 gr

Point: 125 gr

FOC: 13-16%

Heavy / big game

Total: 600+ gr

Point: 150+ gr

FOC: 15-20%

Step-by-step

1. Pin down your bow setup first. Confirm your peak draw weight (in pounds) and your actual draw length (in inches). These two numbers anchor every chart and calculator from here on.
2. Pick a target total arrow weight. Decide what game you're hunting. Whitetail-class: 425-475 gr is the modern norm. Elk and large North American: 480-550 gr. Heavy-arrow setups for big or thick-skinned game: 600+ gr. The 5-grains-per-pound floor is a safety minimum, not a hunting recommendation.
3. Choose point weight and broadhead style. 100 gr is the historic standard; 125 gr is increasingly the default for hunting (better FOC and momentum, plenty of broadhead selection). Heavier points soften dynamic spine — pick the point first, then size the shaft to it.
4. Read the spine off a manufacturer chart. Open the Easton, Gold Tip, Black Eagle, or Victory chart. Cross-reference draw weight × arrow length × point weight. The chart returns a spine bucket (e.g., 340 or 400). Don't eyeball it.
5. Cut to the right length. Measure your draw length and add roughly 0.5-1 inch of overhang past the rest. Longer arrows are weaker in dynamic spine; cutting an inch off shifts the shaft noticeably stiffer.
6. Pick fletching. 2-3 inch hunting vanes for typical setups. Helical or offset orientation imparts spin and stabilizes broadheads. Larger vanes steer broadheads better; smaller vanes give marginally higher FOC at the cost of steering authority.
7. Run the FOC, KE, and momentum numbers. Build the arrow on paper (or in a calculator). Confirm FOC lands in the 12-15% range for typical hunting setups. Confirm KE and momentum exceed the guidelines for your target game.
8. Tune and confirm with broadheads. Paper-tune, then bareshaft-tune, then broadhead-tune at 20-30 yards. Broadheads that drift off field-point group are a tuning issue, not a broadhead defect. Re-tune any time you change point weight, arrow length, draw weight, or shaft.

FAQ

What spine do I need?: Read it off a manufacturer chart (Easton, Gold Tip, Black Eagle, Victory all publish them). Cross-reference your peak draw weight, your finished arrow length, and your point weight. Most 60-70 lb compound hunters with a 28-30" draw and a 100-grain point land somewhere around 340 spine, but use the chart — heavier points and longer arrows shift you weaker (higher number); shorter arrows shift stiffer (lower number).
Good arrow weight for whitetail?: 425-475 grains is the modern norm for whitetail-class hunting and works well at typical bowhunting ranges. 400 grains is fine. 500+ adds penetration margin with little real downside on whitetail-distance shots under 40 yards. The 5-grains-per-pound floor (so 350 gr at 70 lb) is the manufacturer minimum for safety, not a hunting recommendation.
Is high-FOC actually better?: For penetration on big game, yes — within reason. Higher FOC concentrates mass forward, improves recovery from launch flex, and steers broadheads more cleanly. Diminishing returns kick in around 20%. For target and 3D, modest FOC (around 10-13%) flies flatter at distance. For whitetail-distance shots, the difference between 12% and 18% FOC is small in practice.
Why don't my broadheads group with my field points?: Almost always a bow-tune issue, not a broadhead defect. Broadhead blades amplify any setup imperfection — cam lean, weak or stiff spine, fletching contact, or rest centerline error. The fix sequence is paper tune → bareshaft tune → broadhead tune. Don't blame the broadhead until you've bareshaft-tuned.
How much kinetic energy do I need for whitetail or elk?: Commonly published guidance puts whitetail-class game at 40 ft-lb minimum, elk and black bear around 50, moose and very large game 65 or more. These thresholds appear in Easton's hunting setup material and most state hunter-education curricula but aren't legally binding. Almost any modern 60+ lb compound setup clears the whitetail and elk numbers comfortably; momentum and broadhead choice matter more once you're in that range.
100, 125, or 150 grain points?: 100 if your spine chart points there and you don't want to re-tune. 125 is the strong default for modern hunting setups — better FOC and momentum, broadhead selection is still excellent, modest spine impact. 150+ for heavy-arrow builds; expect to drop a spine class to keep the dynamic spine in range.
How does arrow length affect spine?: Longer arrows are weaker in dynamic spine, shorter are stiffer. Cutting an inch off a shaft is enough to shift it noticeably toward stiff. This is why every spine chart has both a draw-weight axis and a length axis. Cut to your draw length plus 0.5-1" of overhang past the rest for safe clearance.
Heavy arrow or fast arrow — which is better for hunting?: Fast arrows forgive range-estimation errors (flatter trajectory). Heavy arrows forgive shot-placement errors on bone or large game (more momentum, deeper penetration). For most North American hunting under 40 yards, a balanced mid-weight setup around 450-500 grains wins. The heavy-arrow argument grows stronger as game size and bone risk increase.
What's the minimum arrow weight my bow can safely shoot?: 5 grains per pound of peak draw weight is the universal manufacturer floor. So a 70-lb bow needs a 350-grain minimum. Below that voids your warranty and stresses the bow toward dry-fire territory — there isn't enough mass to absorb the stored energy of the limbs.
Do I need to re-tune if I change point weight?: For any swing larger than about 25 grains, yes. Going 100 → 125 grain softens dynamic spine; you may need a stiffer shaft, a small rest adjustment, or both. Re-paper-tune and re-broadhead-tune after any point-weight change.

Glossary

Spine: Static stiffness of the arrow shaft. Lower number means stiffer.
Dynamic spine: How the shaft actually flexes when launched. Affected by point weight, arrow length, draw weight, cam type, and release.
GPI: Grains per inch — mass of the bare shaft per inch of length.
FOC: Front of Center. Percentage of arrow length the balance point sits forward of physical center. Typical hunting: 12-15%.
KE: Kinetic energy in foot-pounds. KE = (grains × fps²) / 450,240.
Momentum: Mass × velocity, in slug-feet-per-second. M = (grains × fps) / 225,218. Better predictor of penetration than KE alone.
IBO speed: Bow speed measured with a 350-gr arrow, 70 lb, 30" draw, no string accessories.
gpp: Grains per pound. 5 gpp is the universal manufacturer minimum for safety.
Insert: Threaded sleeve bonded inside the front of the shaft to receive points and broadheads.
Broadhead: Bladed hunting point. Fixed-blade or mechanical (expandable).

Run the numbers

Open the arrow lab →