Arrows Series – Part 4: Spine, Flex and Stiffness

Before we discuss the topic of arrow spine, for proper safety and best performance, arrows need to match your entire bow setup. If you change draw weight, draw length, limbs, riser size, etc. this will affect you arrows. You may need to adjust the arrow length or require different arrows.

Arrow Myth:  An arrow is always straight.

Arrows SHOULD be perfectly straight when not in motion. However, when an arrow is released the force applied from the string causes the arrow shaft to be compressed against the resistant static weight of the arrow point and therefore bends.

Newton’s First Law of Motion: Every object in a state of uniform motion (rest) tends to remain in that state of motion (rest) unless an external force is applied to it.

Since the force applied is greater than the  resistance (weight of the tip), the arrow is propelled forward, and the shaft continues to flex and oscillate as it straightens itself. Arrow shafts that are either too stiff or too flexible will not fly well and will impact the accuracy of your shots or fail causing damage and/or injury. Therefore, we need to manage the flex properly so the arrow does not make any contact with the bow, or your arrow flight will be affected.

Important Tip: Arrow spine refers to the arrow shaft’s degree of stiffness (how much the arrow resists being bent) and is called spine deflection.

Basic Rule 1:  Shorter arrows act stiffer and longer arrows act more flexible.

Basic Rule 2: Powerful bows require stiffer arrows and less powerful bows require shafts that are more flexible.

Basic Rule 3: The heavier the tip equals greater the resistance, therefore the greater amount of compression.  So, a heavier tip causes the arrow to flex more and a lighter tip increase the stiffness.

According to the American Society for Testing and Materials (ASTM) the modern standards (ASTM F2031-05) an arrow’s official spine deflection is measured by hanging a 1.94 lb. weight in the center of a 28″ suspended section of the arrow shaft and is used for aluminum and carbon fiber arrows. (I believe original AMO standard has a basic guide to use for wood arrows spine determination and uses of 2 lb. and 26” section for standard measurement.) The actual distance the 1.94 lb. weight causes the shaft to sag down is the arrow’s actual spine deflection

For example, if a 1.94 lb. weight causes the center of a 28″ arrow to bend down 1/2 inch (.500″) the spine deflection would be .500″.  Stiffer arrows will bend less and more flexible arrows with bend more.

Almost every arrow manufacturer has its own numbering system and there are no universally agreed spine sizes among the various arrow manufacturers. Simply, the lower the deflection measurement equates to a stiffer arrow and higher the deflection measurement the more flexible the arrow. Manufacturers can number, size, and market their arrows anyway they want, as long as they provide the deflection data and test using the industry standard method.

Fortunately for us the engineers have already done the math for us and manufacturers provide spine selection charts. You are able to select an arrow based on your draw length and draw weight.

So when selecting arrows a good rule of thumb is that lighter draw weight, shorter draw length and/or lighter tip weight equals LESS arrow spine OR heavier draw weight, longer draw length and/or heavier tip weight equals MORE arrow spine.

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Mission Accomplished

No one can experience things for you. They can describe them in detail, draw pictures, give you a slide show, but you need to be there to truly understand what it’s like. That describes my trip to World Indoor Archery Championships in Las Vegas, NV.

I have been back for a week now, and thought I should blog about my experience. My goals before leaving were very simple; learn and enjoy the complete experience. If all I bring back is the knowledge of what it takes to compete that the World level, I will succeed.

Well mission accomplished, I gained the experience I was looking for and I now know what it takes to compete at the international level.

Although, I really enjoyed the learning experience and met several new friends from various other countries, it was extremely nerve racking. Prior to the competition beginning, I was shooting extremely well right up to the minute the announcer came on and declared “Welcome to the World Championships” and this set the wheels in motion. I was finally here competing for my country, and the whole world is watching.

My coach, Kathy Millar, tries to prepare us for this type of thing. Every couple of weeks in class we practice with distractions however nothing can prepare you for that level of distraction with loud fans, multi-lingual teams, and the constant commentators. It started with the introductions, world champion here, junior world champion there and me. Then the distraction of the commentator constantly announcing scores and who is shooting and what they did or needed to do. He even declared the winner in our of the bronze medal match even before we shot our final arrow.

Although individually I finished tied for 17th in the recurve junior women division; my junior women’s team set a new national record, even though we lost in the bronze medal match on the final end to the host Americans. Also congratulations to all my Canadian team members, although none of us medaled in this competition, we all preformed well and showed the world that the Canadians can compete at any level.