How do you convert ATA to ASTM arrow spine measurements?

Anthony Cote
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Why the bow type?

Because your needs and wants are different from everyone else’s, your choice of bow needs to be.

Archery is an individual sport, whether you’re hunting or shooting on a target range.

The two most common categories of archery are target shooting and hunting, but do you need a tournament bow or a bow meant for the field? How do you know what kind of bow to get?

It’s always a good idea to take a beginner’s class on bow use and handling from a certified professional.

This will give you a good idea of whether you have a natural talent for archery or whether you should consider another sport more suited to your abilities.

Before even thinking about which bow to buy, you need to figure out how you’re going to use it. Are you interested in hunting or will your interest in archery lie solely with target shooting?

The different types of bows reflect those two typical sports.

Take a look at the world’s record for distance in and endurance in either category and you’ll see that tournament archers shoot lighter and faster bows than those who are out in the field.

What Is Arrow Spine?

Arrow spine is a measurement of straightness in an arrow. The spine is expressed in either force pounds per inch or as a straightness tolerance. The low end of the straightness tolerance is expressed in an absolute value, such as +/- 0.002 in., which means the maximum overall deviation of the arrow from the straight line is 0.002 in. A higher tolerance of +/- 0.003 in. expresses a greater deviation (three times as much) from the aim on only one side.

Arrow spine is usually given in three forms: straightness tolerance, force pounds per inch, and deflection (warp) weight.

Straightness Tolerance – the size of the error a high quality arrow is allowed to have relative to the straight line in the direction of arrow flight. It is expressed as a limiting measurement in an absolute value, such as +/- 0.002 in., or as a percentage of mean arrow diameter, which is expressed as a percentage. For example, plus or minus 1% of the mean diameter means that the straightness tolerance is plus or minus 0.002 in.

Force Pounds per Inch – an expression of the amount of force required to deflect the arrow in an inch when the arrow is given a specified impulse. For example, 30 lb. means that the arrow must withstand a force of 30 pounds measured at one inch deflection.

ATA, AMO, and ASTM

What do they mean?

First, measure the shaft's arrow spine. This is the arrow's spine measurement. Arrow spines come in a range of measurements. Choose whichever measurement is most convenient for your needs. If you have to adjust to a larger measurement, chances are that some arrows are on the larger side already and will not need to be readjusted.

The National Bowhunting Organization of America or the NBOA, uses arrow spine measurements in inches. The NBOA measures the spine of the arrow at twenty inches from the lip end of the nock to the other lip end of the nock.

Micro-Dyneema or Dyneema arrows have their own numbering system from the European Hystory and Organization for Standardization (ISO). Substitute the ISO number of the Micro-Dyneema arrow for the arrow's spine measurement. For example, if your arrow has an arrow spine of 746.3, replace it with a Micro-Dyneema arrow's ISO number of 743.

Arrow spine measurements can also be abbreviated by entering the part of the arrow spinedown a scale from the diameter of the shaft to the diameter of the arrow at twenty inches from the end of the nock. Another helpful abbreviation is for buckhunting arrows. This abbreviation highlights spine sizes, but it also highlights arrow weight and size.

Wooden arrows

Wooden arrows are an interesting affair. Heavier arrows, from a technical point of view, are usually considered better. The two most common standards for arrow physics interaction are the ATA and ASTM.

The American Taping Association (ATA) is the prevailing standard for measuring arrows. The ATA value is comprised of the spine, shaft and point weight respectively. Typically heavier arrows are placed into the ATA standard than the ASTM standard. The one downfall with the ATA standard is that it does not take into account any arrow accessories.

For this reason, the ASTM (American Society for Testing and Materials) has come into play. The standard is comprised of the same values but does take into account the accessories on the arrows you will be measuring.

The ASTM value differs from the ATA value because the ASTM feature takes into account the mechanical advantage and string characteristics. If you have an arrow of each standard measurement, you can use a chart to convert ASTM into ATA value.

ATA to ASTM

The arrow spine unit can be confusing to some archers who like to use the governing body’s system rather than an arrow manufacturer’s. They need to convert from one to the other. We feel the process is cumbersome, and created an easy to use chart to solve this common question. Our conversion chart simplifies the process of converting ATA spine measurements to ASTM. Arrows used in international competition need to be measured in both ASM and NAB spine units. The chart below will help you make the conversion.

Courtesy of ARCO Vienna

The “ATA”, a special form of arrow spine, is the official hardness measurement standard of the American bowhunting association. All the ATA publications are freely available on the internet.