The proper spine for primitive arrows

I wrote this years ago and am just re-editing it now for this blog.I’ve grown older and wiser, but many of my points are still based in physics and not conjecture, as many websites and books reflect. I know a few have taken what I think of primitive arrows as heresy, in their minds, as their thoughts on primitive archery are the law and physics has no place for them. Hopefully, you are more open minded and we can have a conversation about technical points on the true physics of archery. No matter your beliefs, physics does not lie, or care what you think. 😀

I have spoken with many archers over the years that like to use primitive equipment about their bows and arrows. The one thing that is clear is that some of them know how to match their arrows to their bows and some do not. Some have learned to shoot very well and some have not. What I have learned is that I may be stepping on the “Holy Grail” by suggesting this highly technical aspect of matching your arrows to your bows since Indians did not have spine testers. But what they did have might have been almost as good. They had an arrow maker that was usually an elder. That arrow maker made all or most of the arrows for the entire tribe.They didn’t let the guy that just got interested in archery make arrows for the tribe and let him expound his wisdom about how it was all down wrong in previous years. No, they took the elders knowledge and advice on what made a good arrow. Experience and knowledge are the same thing and even primitive people knew this.

I know I can flex an arrow between my hands and closely guess weather the spine is close to what I want. I may be off ten pounds but a shaft can be shaved to drop spine once made into an arrow. The fact is we really do not know what they did and we are only guessing at what they did know sometimes. What we do know is that arrow spine really does matter to be able to shoot consistently and since primitive people could hit what they were aiming at, they clearly understood the physics of it.

One thing that is true about Archery is that the spine of your arrows is the most important thing you can match or learn to make your shooting better. Arrows that are unmatched in spine will shoot much worse than arrows not matched in any other way. I know most Archers have used arrows that were matched in spine and weight and all the suppliers and arrow makers sell shafts grouped this way. It would be much better for you to get the correct spine arrow shooting out of your bow first before you even think about matching the weight of those arrows. Shooting arrows that are closely matched in weight can also help you shoot better but is not nearly as important as how closely they are matched in spine.
Ok, here is where we get technical and start getting into physics and all that stuff. One thing we need to go over before we go any further is spine measurements. Spine measurements are covered in two different measurements. They are referred to as Static and Dynamic measurements. Static is of course an arrow measured at rest or on a spine tester. A Dynamic measurement would be the arrow under movement like when it is shot out of your bow. We need to understand how an arrow performs in dynamic spine because that is how we are using it. Unfortunately, I do not have a way of measuring an arrow dynamically so I have to get a static measurement and apply what I know to figure how it will work dynamically.

I have had many conversations with Archers about long verses short arrows. Most Archers favoring long arrows are doing so usually to compensate for their problems with the spine of their arrows and having shooting problems caused by arrows that are over spined. When you increase the length of your shafts two things will happen. The first is the shaft will loose 5 pounds in Dynamic spine for every inch added to the length of the arrow. The second is that the weight of the shaft will also increase. One thing that is true is that everything has a trade off. If you’re shooting a longer heavier arrow of the same dynamic spine as a shorter arrow, the shorter one will be faster, weight less and have better cast due to its lighter weight. The heavier arrows will be a little slower, have less cast but might have a little more Kinetic Energy or punch due to its increased weight. You may have increased your hitting power slightly but you would have decreased your effective range more than you increased your hitting power.

Now with that being said, I can get a shaft that is of heavy wood to give me good weight for penetration when hunting. I can used a static measurement to match the arrow to my bow keeping the arrow short as possible and light enough to have good cast and really get those arrows to zip out of the bow, but what are the factors that really affect my arrows and the spine that they have.

I am glad you asked that question as I have the answer. The things that change the spine of any arrow are how thick the shaft is, how long the shaft is, how stiff or weak the materials is, what the weight of the bow is, how heavy the point on the arrow is and how center shot the bow is.
The first one is simple enough. We all can understand that a sapling will bend in the breeze much easier than a tree with a one-foot diameter would. So the thickness of the wood make a difference on how much the shaft bends in shooting. This would also be similar to the stiffness or weakness of the material. It easier to understand that oak is stiffer than cedar and, of course, would spine and shoot that way. An oak shaft of the same diameter as a cedar shaft will shoot stiffer and would need to be taken into account.

9 - shafts

The picture shows the top shaft made from hazel and the bottom shaft made from a ramin dowel. The hazel shaft is a little larger in diameter but has the same spine due to being a stiffer material.

Next factor for change in arrow spine would be how long the arrow is. The rule of change is 5 pounds of spine change per inch of arrow changed. If you cut an arrow that is sold to you spined for 40 –45 pounds to 29” it will now have a dynamic spine of 35 – 40 pounds. You lost 5 pounds in the spine by adding one inch to the arrow length. Consequently, if you cut the shaft to 27” it would have a dynamic spine of 45 – 50 pounds and shoot as if it was 5 pounds stiffer in spine.

Now let’s cover the weight of the point. The general rule used for spine calculations is for the arrow to have a 125-grain head. Once you step out of that weight you start changing the dynamic spine of the arrow again. The rule is 5 pounds for every 25 grains of change. You remember science class where an object at rest tends to stay at rest. Well this is part of that. A heavier head requires more energy to put it in motion. So when the arrow is released that shaft will not move the head until it gets enough energy stored to move the head. When the shaft is storing this energy it is bending and the heavier the head the more it bends. So you can see a lighter 100 grain stone head will make your arrows shoot 5 pounds stiffer than you might have actually thought they should have. So in turn you could see that a 150-grain head would make the arrow react as if it was 5 pound weaker in spine that it actually is.

The last two items are how heavy the bow is and how center shot the bow is. The weight of the bow is a simple thing but center shot is not as simple. The more off center shot a bow is the more the arrow needs to flex around the bow to avoid hitting the riser when released.

Shooters of a traditional recurve bow will shoot arrow spine more closely to their bow weight to actually exceed their bow weight because the bows are very close or completely center shot. Their arrows do not have to flex around their bow so having an arrow that is stiffer makes the arrow recover quicker.

You can take all these points and apply them to your arrows and make yourself a better shooter by making your arrows better and consistent. Before you become a good shooter you must become a good arrow maker or at least hire a good arrow maker. Hopefully I have given you enough information to become a good arrow maker and be self-sufficient as every Primitive Archer should be.

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