Matching Arrow Shafts
There is nothing that is more critical to getting good, consistent flight from arrow to arrow than having them properly matched. In this case matched means that the arrows are matched to the bow and that the arrows are also, within limits, matched to each other.
Wood is a material that has properties such as density, bending elasticity, and grain that can vary substantially from shaft to shaft. One has to be much more cognizant of these variations than you would if you used materials such as aluminum or carbon where these properties are generally very consistent and uniform.
The weight of a shaft depends on the density and the moisture content of the material. If you ordered a thousand wood shafts that came directly from the manufacturer without the benefit of any sorting, you would find that weight variations would run as much as 200 grains from the lightest to the heaviest. That means that you could have as much as a 70% weight difference between two shafts made from the same wood species.
The best and most practical way to get weight-matched shafts is to purchase them from a reliable supplier who has already performed this sorting function. Most major shaft suppliers will purchase shafts in quantities of 10,000 or more. They then sort these shafts according to weight and spine (next subject). Normally, suppliers will weight-sort shafts in increments of 10 grains. If you purchase these shafts you might get one shaft that weighs 400 grains and another that weighs 410 grains, which is only a 2.5% weight variation. Several arrow suppliers will provide a card showing the weight range of your shafts. Keep these for your records and refer to it the next time you buy shafts so that you can match your existing arrows.
Spine is a term that refers to the relative stiffness of a shaft. Sometimes you hear people incorrectly call it "spline". There is a standard for the measurement of spine which is the amount of deflection that a shaft will assume when loaded with a 2 pound weight mid-point between supports located 26" apart. Its been nearly 30 years since I took Mechanics of Materials in Engineering School, so bear with me for the engineering explanation of what properties affect a shaft’s stiffness. There are two basic properties that affect deflection: the section modulus and the modulus of elasticity. The section modulus is a function of the geometry of the shaft. Simply put, the more material there is from the center of the shaft, the higher the section modulus. So, larger diameter shafts will have a higher section modulus and will resist bending more than a smaller diameter shaft. This is one of the reasons why higher spine shafts usually are 23/64" diameter. If you are shooting a barreled shaft, the greatest diameter is at the center of the shaft where the bending stress is the highest. The other factor affecting stiffness (resistance to bending) is the bending modulus or modulus of elasticity. Bending modulus is a measurement of the material’s resistance to bending. The bending modulus for steel as an example is much higher than the bending modulus of wood. The bending modulus is different for different species of wood. But since wood is not an isotropic material, there are variations in the bending modulus within the same species of wood and even from shaft to shaft.
If you place the same arrow shaft in a spine tester, you can get different spine readings depending on the orientation of the grain of the shaft. The shaft is weaker when the load is placed perpendicular to the grain and is stronger when placed parallel with the grain. This variation can be several pounds of spine and is thus important to consider when measuring spine and when installing nocks.
- Make sure that the nocks are aligned so that the parallel grain (stiff orientation) is perpendicular to the riser of the bow.
How do you know what spine arrows to purchase? Shafts are normally sold in 5# spine increments such as 55#-60#. A good shaft supplier will hand spine shafts using the same orientation with the grain. The spine charts developed over the years have been established for 28" draw lengths for near center shot bows. For draw lengths greater than 28", the spine will need to be increased. If you plan to shoot heavy broadheads (heavier than 125 grains, then you may need more spine. Normally, for longbows and flatbows that are not center shot, then the shaft needs a little more flex to bend around the bow and a little lower spine is needed. There is no real magic here, I have seen two people shooting nearly identical setups shooting different spine arrows. The best way to determine the proper spine weight for your set up is to shoot different spine arrows and see which shoot the best for you. If you are starting out and don’t have a clue where to begin, start with a spine chart or ask your shaft supplier, they will usually have a good idea of where to start. If need be, get a few shafts of different spine and shoot them, or borrow a few arrows from a friends. One thing that I really enjoy about the traditional bowhunting fraternity is that people are really willing to help others and share information.
There has been a lot of discussion in recent years about bare shaft tuning and shooting very stiff shafts. Some of this has come from the tuning video produced by Black Widow. Personally (I will probably get flack from this), I am not a fan of bare shaft testing nor of shooting ultra-stiff shafts. I tend more towards shaft testing using my normal hunting setup, 5" helical feathers and a 140 grain broadhead. Have somebody (who knows what good arrow flight looks like) stand behind you when you shoot and watch your arrow flight at different distances.
- If your arrows wag left and right that is usually a pretty good indication of improper spine - usually too little spine.
- If you are getting pretty good flight but your arrows are shooting to the left that it usually and indication of too much spine.
One thing is for certain, spine is very important. In fact, it is probably the most important part of getting good arrow flight. Find out what is right for your setup and then stick with it and insist on good quality, matched shafts.