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Arrow Spine and FOC

What Is Arrow Spine?

Arrow spine is a characteristic that describes how much an arrow shaft bends when a force is applied. Spine values like 500, 800, or 1100 are used to indicate this flexibility.

Each number refers to how far a 28” shaft bends when a 1.94 lb (880 g) weight is suspended from its center.

Arrow Spine

For example, if the shaft bends 0.7 inches, that corresponds to a 700 spine. The lower the spine number, the stiffer the arrow.

This measurement is called static spine — it describes the shaft’s behavior at rest. But when a shot is taken, the arrow bends dynamically, which is influenced by other factors — that’s the dynamic spine.

What Is Dynamic Spine?

Dynamic spine depends on:

  • bow draw weight
  • arrow length
  • point weight

The longer the arrow or the heavier the point, the more it bends under load — meaning it behaves as if it were softer.

For example, a 28” shaft with a static spine of 700 will behave stiffer with an 80 grain point and softer with a 100 grain point

Why? Because at the moment of release, the force from the bowstring pushes the nock end forward. A heavier point resists acceleration more strongly, causing the shaft to bend more.

The same goes for length: a longer shaft is easier to bend, so dynamic spine decreases as arrow length increases.

How to Choose the Right Spine

It's important to understand: there is no exact formula to calculate the perfect spine. Instead, experienced archers and manufacturers use proven charts and guidelines.

For example, with a 27” arrow and a 37 lb draw weight, a spine of 700–800 is usually appropriate.

If the spine is wrong:

  • the arrow won’t stabilize in flight
  • groups will scatter
  • and tuning becomes nearly impossible

Every arrow brand provides a spine chart, which recommends shaft stiffness based on shaft length and draw weight.

You can also use the spine calculator to calculate the ideal spine based on:

  • shaft length
  • bow weight
  • point weight
  • fletching weight

What Is FOC?

If you realize your arrows are too stiff, you might be tempted to just add more point weight to soften the dynamic spine.

That’s possible — within limits. Why? Because of FOC.

FOC (Front of Center) is a measurement that shows how far forward the center of mass is from the arrow’s physical center.

The formula is:

FOC (%) = 100 × (Balance Point − (Arrow Length / 2)) / Arrow Length

Arrow FOC

So FOC tells you how far the arrow’s balance point is shifted toward the tip. Recommended FOC for recurve bows: 10–15%

If the point is too heavy, FOC increases beyond optimal range. This can affect:

  • trajectory
  • arrow flight stability
  • and consistency at longer distances

Conclusion

Spine and FOC are two of the most important aspects of arrow tuning.

When choosing new arrows or modifying existing ones, always:

  • consider bow weight
  • arrow length
  • point weight
  • and calculate the final specs

My website offers spine and FOC calculators to help you find the right arrows.

A well-tuned arrow is not just about stiffness — it’s about balance. When you tune your arrows, you’re tuning your entire bow system.