The Spine of Arrow and How to Choose It

1. What is the spine?

The spine is defined in mechanics as follows: the linear displacement of the centroid of the cross section along the direction perpendicular to the axis when bending and deforming is called the spine. For arrows, it is an indicator of toughness. Specifically, it refers to the quantitative value of the degree of deformation of the middle of the arrow shaft after being subjected to the rated force at a certain length. The larger the spine, the greater the deformation, the softer the arrow shaft, and the smaller the value, the harder the arrow. For example, an arrow with a spine of 700 is much softer than an arrow with a spine of 350. This is actually very easy to understand. Just remember that the spine corresponds to the degree of deformation. Under the same force, the greater the degree of deformation (spine), the softer the arrow shaft.

2. Why do we need to match the spine?

Matching the spine has two main functions, one is to improve accuracy, and the other is to ensure that the arrow is not damaged.

Let's talk about ensuring accuracy first. The harder the arrow, the smaller the degree of bending, and the softer the arrow, the greater the degree of bending. Arrows with suitable spines will not come into contact during the process of passing through the arrow rest or bow handle, and the arrows will be shot to the target regularly. However, if the arrow has a mismatched spine, it will hit the arrow rest or the bow handle, causing the arrow to deviate greatly, destroying the arrow's directionality, and the accuracy cannot be guaranteed.

Let's talk about safety. I believe everyone is familiar with the pictures of exploding arrows and injuring hands (because it is too bloody, I won't post the pictures here, and those who don't know can search for it by themselves). There are roughly two reasons for exploding arrows. One is that the arrow is damaged but not discovered, and the other is that the arrow is too soft relative to the poundage and breaks. The pressure on the arrow shaft at the moment of release with a higher poundage will cause the soft arrow to deform greatly. Even if the arrow does not explode, it is easy to injure the arrow. This is because the instantaneous force may cause the deformation of the arrow to exceed the safety range of the material.

In addition, when shooting a very hard arrow with a high-poundage bow, in addition to causing the arrow shaft to hit the bow window or the arrow rest when it comes out, it is also easy to split the arrow tail. Matching the right arrow, the deformation of the arrow shaft is also a buffer for the instantaneous force of the arrow tail. If the arrow is very hard, the deformation buffer of the arrow shaft becomes smaller, and it is easy for the arrow tail to be unable to withstand the impact force and break or break.

3. How to match the spine?

When matching arrows, you must remember one thing first. No matter what bow you use, your posture must be standardized to reduce the influence of human factors on the judgment of the spine.

Simply put, the larger the poundage, the lower the spine of the arrow should be, that is, the harder the arrow should be; the smaller the poundage, the opposite is true. This is basically known to everyone, so I won't go into details. However, how to match your arrows with the bow you use? There is a simple method that you can learn about.

For American hunting, a 600 spine is suitable for a 40-pound bow, a 550 spine is suitable for a 45-pound bow, a 500 spine is suitable for a 50-pound bow, and so on. The poundage of the bow and the spine are inversely proportional, and there is a certain rule, that is, for every 5 pounds increase in poundage, the spine must be reduced by 50. In this way, I think you should have a preliminary concept of how many spines you should use for your bow (why is it preliminary? Because it is a 40-pound bow, but because everyone's draw length is different, your draw force may not be 40 pounds, so how do you know your draw force? Please read on).

Next, it is the key point. There is a simple truth that we all know. The longer the bamboo pole is, the easier it is to bend. The shorter the pole is, the less likely it is to bend. In fact, the same is true for arrow shafts. Therefore, the spine of 500, 550, etc. that I mentioned above must have a condition: a certain length of the arrow shaft! Otherwise, even if the arrow shaft is made of the same material, its deflection is different at different lengths. Therefore, according to international practice (actually the practice of the United States), when we usually say that a certain arrow shaft has a certain spine, we actually refer to the spine of the arrow shaft at a length of 28 inches.

According to this rule, we can also master the matching of the arrow spine and the bow we use by cutting arrows. If I have a dozen arrow shafts with a spine of 510, although my pulling force is relatively large at 55 pounds, but the draw length is small, only 26, is there any way to make a softer arrow shaft match my bow? It is also through cutting arrows. I only need to cut the arrow shaft to 27 inches, which is 1 inch shorter than 28 inches, and the spine will be reduced from 510 to 460, so that it matches my bow.

4. Some factors that affect arrows

1. Arrow weight: The weight of arrows should be paid attention to. Arrows cannot be too light, which can easily damage the bow pieces. They are close to being released in the air. Generally, they should not be less than 5 grains/pound (one grain is about 0.065 grams). For the same bow, heavy arrows have slow speed, high kinetic energy, and high penetration, but the throw line is large, that is, the arrow path is curved, and the distance must be calculated accurately. Light arrows have low kinetic energy and low penetration, but the throw line is small, and the arrows are closer to flying in a straight line, and the tolerance for distance is large.

2. Straightness: Whether an arrow is straight or not has a great impact on accuracy. Up and down, left and right, and the bending direction are inconsistent, resulting in the accuracy of the scattered flowers of the sky. Some arrows, such as bamboo poles, are wavy and uneven, but they can still fly normally in a straight line. Some people also correct a small number of arrows by sticking spiral feathers.

3. Consistency of components: The length and weight of the head and tail shaft feathers should be consistent. Generally, the main problem is the shaft. For example, bamboo shafts have different diameters and densities due to natural growth. The weight consistency of the shafts is different, and the thickness of the two ends is also different. They must be carefully selected. Wooden shafts are also natural materials. On the surface, the diameters are the same, but the weights vary greatly. For example, the 8X800 white wood shafts are 23 grams for the lightest and 36 grams for the heaviest. They must be carefully selected. In addition, wooden arrows should be bent first. Some wooden shafts have twisted fibers. Fiberglass arrows also have obvious problems. Some are hollow and some are eccentric, which means one side is thick and thin, resulting in one side being soft and hard. The consistency of other pure carbon, aluminum, and mixed carbon shafts is relatively high.

4. Overall consistency: length, weight, center of gravity, and feather length will cause accuracy problems. Different lengths are not conducive to archers who aim at their target. Inconsistent weights, inconsistent casting lines, different distances of arrow landing points, inconsistent center of gravity, in addition to inconsistent landing points, inconsistent directions, inconsistent straightness, and no idea where it will go up, down, left, and right. Inconsistent feather lengths are not obvious at close range, but will be magnified at a distance, especially in strong winds.

VII. Comments on various commonly used arrows

1. Wooden arrows: A: Advantages: traditional style, easy to use; B: Disadvantages: easy to break, easy to scratch the surface, poor straightness, poor consistency

2. Bamboo arrows: A: Advantages: traditional style, strong and durable; B: Disadvantages: poor straightness, poor consistency

3. Fiberglass arrows: A: Advantages: slightly better straightness, cheap; B: Disadvantages: heavy arrows, no transverse fibers, easy to crack and burst

4. Mixed carbon, pure carbon: straightness, consistency, and firmness are all good. The difference is that mixed carbon is heavy and pure carbon arrows are light. There are no obvious disadvantages.