Arrow spine is an important part of target archery. The measurement of the “stiffness” of an arrow is what is known as the “spine”. Weaker arrows have a larger spine number, and stiffer arrows have a smaller spine number. This sounded funny to me at first, because the arrow that is harder to bend has a smaller spine value.
Spine vs Archery Background
Before writing too much about what the spine is and how its measured, I think it’s important to understand why we care so much about spine. The reason is twofold: it is one part safety and two parts performance.
Arrow Spine and Safety
It may be hard to believe, but arrows flex quite a bit as they exit a bow.
In this video, the arrow flexes quite a bit as the string pushes against the end, causing the nock end to accelerate faster than the tip. This action is called the “archer’s paradox”.
If an arrow is too weak, the force of the string may cause the arrow to flex so much that the arrow will break. With modern carbon arrows this causes a large quantity of small carbon slivers to be ejected from the break point. Wooden arrows can split along the grain. When failed, either arrow can pierce the bow hand and effectively end your participation in the sport of archery
Arrow Spine and Performance
Arrows need to be correctly matched to the draw weight of the bow, the draw length and skill of the archer. Arrows that are too stiff will tend to shoot toward the bow arm (left for right-handed archers), and the opposite is true for arrows that are too weak. This is because of how the string moves around the fingers and flexes the arrow. As the archer releases the string, the string makes a subtle “S” shape around the fingers then back to midline at about brace-height. During this traversal an arrow that is too stiff will not correct for this “S” movement, and travel toward the bowarm, where an arrow that is too weak will overcompensate for the S movement and travel away.
An ideal spine for an archer will travel straight down the sightline from the string, to the tip of the arrow to the target.
How to Measure Arrow Spine
Most simply, the spine is a measurement of how much deflection an arrow will make from straight at its midpoint when a load is applied
The basic method is a 1.94 pound weight is attached to the midpoint of an arrow that is suspended by its ends.
Factors That Affect Arrow Spine
My observation is that arrows that have a lower spine tend to have a thicker “wall” of carbon fiber in their outer diameter. The net result of this is that lower spine numbered arrows are heavier than their higher spine rated siblings even though the outside diameter is the same.
What also affects the arrow stiffness is the length of an arrow. If you need to cut an arrow down from its full length, you will lower the spine of the arrow. Through experimentation we have learned that a good rule of thumb is that for every inch removed from an arrow, the spine is increased by one group — a 900 spine arrow becomes 800 and so on.
The competitive archer can use this to their advantage if they notice the 940 spine arrow is too weak, but the 870 is too strong, it may be a good idea to cut the arrow by 1/4 inch to “split the difference” and get the correct strength of arrow.