Choosing the ideal arrow is a common problem for any archer. The length of the arrow depends on your physical size, your flexibility, and your technique. The correct arrow spine can depend on the draw weight of the bow (at full draw), and also the bow type. An arrow with the correct spine rating and length will recover perfectly from the archer’s paradox and give you a good arrow flight. An incorrectly spined arrow doesn’t recover from the paradox as quickly and is much more likely to show up inconsistencies in an archers form.
Why the bow type?
Recurve bows and longbows behave similarly during release. The power is greatest at full draw, and the acceleration of the string diminishes as it moves closer to the bow. For compound bows, this is different. The cams allow for a low weight at full draw, and this corresponds to a small acceleration immediately after the release. Then the closer the bowstring comes to the bow, the faster it moves and the greater the resulting arrow speed. These differences in the application of force to the arrow also have an impact on the correct spine choice.
What Is Arrow Spine?
Arrow spine refers to the stiffness of an arrow. You’ll often find arrow spine measurements printed on the side of arrows by manufacturers. They will also usually be specified when purchasing arrows. But what do these numbers for ATA and ASTM spine actually mean?
Without knowledge of how arrow spine is measured, it’s just a number and not a very useful one at that. The reason for this is that different standards measure the arrow spine in a different manner. Therefore, the following article provides a short overview of these measuring methods.
Note: Arrow spine tables are only a recommendation and sometimes the number provided by the manufacturer does not really correspond to the measured values. There are several mobile apps which provide precise information about different arrow types, including diameter, weight, and spine. Two examples for free apps are “B4 Shaft Selector” and “Arrows”.
ATA, AMO, and ASTM
ATA (AMO) is the Archery Trade Association, formerly the Archery Manufacturers and Merchants Organisation. When they measure the arrow spine, they record the deflection in thousandths of an inch. An arrow is attached to two supports, 26 inches apart, and pressed in the middle with a weight of 2 pounds (907 grams). A deflection in the arrow of 0.4 inches gives an arrow spine of 400. The arrow spines of wooden arrows are measured according to the ATA system.
ASTM is the American Society for Testing and Materials. In their Test Nr. F2031-05, they record the deflection in thousandths of an inch. In this test, an arrow is attached to two supports, 28 inches apart, and pressed in the middle with a weight of 1.94 pounds (880 grams). The weight is smaller, but the distance between the supports is greater, which should give a similar deflection. Arrow spines of carbon fiber and aluminum shafts are specified according to the “modern” ASTM standard.
ATA and ASTM values can be converted as follows: ATA / 0.825 = ASTM, therefore ASTM*0.825 = ATA
Example: A carbon fiber shaft has an arrow spine of 500 according to ASTM (28″ distance, 1.94 pounds). 500*0.825 = 413 spine according to ATA (26″ distance, 2 pounds).
In other words, an ATA shaft will always be stiffer than an ASTM shaft with the same “number”. Carbon fiber arrows measured with an ATA device will appear slightly “softer” than the manufacturer listed value.
Note: These values are static and relative. They relate only to the aforementioned conditions. They are a good starting point, but no silver bullet for every situation. The perfect stiffness of the arrows can depend on:
- Bow type
- Shaft material
- Arrow length (longer arrows are softer than shorter ones)
- Arrow build (weight and shape of the arrowhead, weight, and shape of the fletching, possible tapering of wooden shafts)
- Shooting technique (Mediterranean archers and thumb archers have different arrow dynamics)
Wooden arrows are usually not marked with a spine measurement. Rather, they carry the corresponding draw weight, with which they should be shot. The draw weight for an arrow with a certain deflection is calculated by dividing the ATA test distance (26 inches) with the measured deflection. If a shaft deflects at 0.900 inches, the corresponding draw weight is 26/0.900 = 29 pounds, for a 26-inch long arrow.
If the archer is looking for matching arrows for a certain draw weight, 26 is divided by the draw weight at full draw in order to calculate the desired deflection. An archer with 30 pounds at full draw requires arrows which deflect at 26/30 = 0.867 inches. However, experienced wooden arrow archers are recommend to use arrows which are 5-10 pounds “stiffer” (26/35 = 0.743 or 26/40 = 0.650 inches). Additionally, it is recommended to add or subtract 3-5 pounds for every inch over or below 26, respectively.
The ATA/AMO table below lists deflections in inches and their corresponding spines. The matching arrow spines of carbon fiber shafts according to ASTM are also provided.
ATA to ASTM
|Deflection in inches||Corresponding draw weight in pounds at 28 inches of draw||Corresponding ASTM arrow spine||Deflection in inches||Corresponding draw weight in pounds at 28 inches of draw||Corresponding ASTM arrow spine|
Example: An archer has a 35 pound draw weight at full draw and is shooting 32 inch arrows. Matching wooden arrows for that archer would have a spine of 35+10+6*5 = 75 pounds. This corresponds to a deflection of 0.341 inches. Matching carbon fibre arrows would have an ASTM spine of 421, rounded down to 400.
Courtesy of ARCO Vienna
Created in collaboration with Konstantin Tomanov from ARCO Vienna. One of the best places for archery, in a penthouse, right in the centre of Vienna.