If you watch video of an arrow leaving a bow in super slow motion, you’ll see the arrow flexing during those first few yards of flight. That’s normal, as long as the flight is stable.
That in-flight flexing is the effect of the arrow’s spine, which is a measure of the arrow’s stiffness. Matching the arrow’s spine to your equipment is critical for accuracy. You must also understand how you can affect the arrow’s spine as you build your arrows.
Arrow manufacturers offer a spine chart that details their arrows’ stiffness. Typically, the higher the number, the weaker the spine. Common spine ratings are 300, 400, 500 and 600, with 300-spine arrows being the stiffest.
Manufacturers recommend which spine should work best for various draw weights, arrow lengths and point weights. All three factors affect arrow flexing. Higher draw weights require stiffer arrows, as do longer arrows shot by archers with long draw lengths.
Let’s say you have a 50-pound compound bow that shoots 27-inch Gold Tip arrows. Gold Tip’s spine chart recommends an arrow with a 500-spine rating, assuming the arrow carries a 100-grain point.
Take that same 50-pound bow, but increase the arrow length to 30 inches, and the chart calls for a 400 spine. Increase the bow’s draw weight to 60 pounds and shoot that 27-inch arrow, and the chart again calls for a 400 spine. But if you increase the arrow length to 30 inches for the 60-pound bow, the chart calls for a 340 spine.
New, raw arrows measure around 33 inches when shipped by the manufacturer. As you cut those arrows to fit your draw length, you make them stiffer. That’s why Gold Tip recommends the 400 spine for a 27-inch arrow shot at 60 pounds, as compared to the 340 at 30 inches.
Understand that the manufacturers’ charts refer to arrow length, not draw length. Archers often confuse the two when picking arrows. Archers with a 27-inch draw length might shoot arrows anywhere from 26 to 29 inches long, depending on where your arrow rest is mounted, and how much arrow extends in front of the riser when you’re at full draw. Use the arrow length you intend to shoot when consulting the spine charts.
Your arrow point’s weight also affects spine. Adding weight to the front of the arrow weakens its spine. That’s why Gold Tip recommends a 400 spine for a 27-inch arrow with a 100-grain point shot from a 60-pound bow, but also recommends a 340-spine arrow if its length and draw weight stay the same, but its point weighs 150 grains.
Point weight is a big factor, because many bowhunters try to boost their arrows’ penetrating power by adding weight to the front. They can do that by using extra-heavy point inserts, increasing the weight of their points, or both. Archers who don’t consider the effects of heavier points on spine might wonder why their arrow suddenly flies erratically.
Archers selecting arrows should stick to the manufacturer’s spine recommendations. Moving away from those recommendations can hurt accuracy. Arrows that flex too much or not enough can fly erratically.
Archers occasionally change their arrow spine incorrectly because they want to change their arrows’ overall weight. Arrows with weaker spines generally weigh less than those with stiffer spines. The Gold Tip Hunter XT in a 500 spine, for example, weighs 7.3 grains per inch, while the same arrow model in a 300 spine weighs 9.3 grains per inch.
Archers who want to increase or cut arrow weight sometimes think changing spine is the only way to achieve their goal. Instead, stick with the spine recommendations, and change the arrow’s inserts or points to adjust the arrow’s weight. Never forget that adjusting weight at the front of the arrow affects its spine.