Arrow Spine, Weight and FOC Explained
Arrows that fly well start with three numbers: spine, total weight and front-of-center (FOC). Get them close and tuning is easy. Get them wrong and no amount of sight adjusting will fix your groups.
Spine: how stiff the shaft is
When you release, the string pushes the back of the arrow forward and the shaft flexes. Spine is a measure of how much it bends. On most carbon shafts it's printed as a number like 300, 340 or 400. Lower numbers are stiffer.
Your bow's draw weight, your arrow length and your point weight all decide the spine you need. Heavier draw weights, longer arrows and heavier points all call for a stiffer shaft. Every arrow maker publishes a spine chart. Start there, and if you're between two sizes, a pro shop can help you choose.
Total arrow weight
Total weight is the shaft plus nock, fletching, insert and point, measured in grains. Lighter arrows fly faster and flatter, which forgives small range-estimation errors. Heavier arrows are quieter off the bow and carry more momentum, which helps penetration on big animals.
A common rule of thumb for whitetails is at least 6 to 8 grains of arrow per pound of draw weight. Many elk and moose hunters go heavier. Never shoot an arrow lighter than your bow maker's minimum, because that can damage the bow.
FOC: where the arrow balances
FOC describes how far forward the balance point sits compared with the arrow's center. More weight up front helps an arrow stay stable and steer through tissue.
To measure it, find the balance point and use this formula: FOC % = 100 × (balance point − half the arrow length) ÷ arrow length, measuring from the throat of the nock. Most hunting arrows land around 10 to 15 percent. Heavier inserts or broadheads raise it.
Crossbow bolts
Crossbows are different. Spine isn't something you choose. Use the bolt length, weight and nock type your crossbow maker specifies. The wrong nock or a bolt that's too light can cause a dry fire, which can destroy the crossbow and injure you.
Putting it together
- Pick a shaft from the maker's chart for your draw weight, arrow length and point weight.
- Choose your hunting point weight first, then build the arrow around it.
- Weigh a finished arrow and check FOC.
- Paper-tune or walk-back tune, then confirm broadheads hit with field points.
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