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.

Want to get better at Archery?

I have been asked by many people, how do I get better at archery? The answer is simpler than you think. Practice properly with expert guidance. Target archery is all about consistency since the target is not moving and you are in the same spot. The more consistent you are the better your scores will be.

If you want to get better then there are five areas of archery that deserve your attention.

Equipment

Ensure you have the right equipment for you. Find the right bow, proper mass weight and draw weight for you and your size. If you are unable to hold or draw the bow you are not going to be able to practice very long. Also arrows need to sized according to your draw/bow weight and draw length. Shooting arrows that are incorrectly sized will not perform well and will be very frustrating.

“It’s not the size of the dog in the fight, it’s the size of the fight in the dog.” Mark Twain

Form

Focus on consistent archery form, pay attention to the minor details such as your exact stance, exact hand position, your exact anchor point, etc. Replicating the exact same form every time will improve your performance and accuracy. Some people blame their equipment for malfunctions which can be true however more often than not it is your form. Focus on to your form not your equipment.

“There is no spoon … Then you’ll see, that it is not the spoon that bends, it is only yourself.”  Spoon Boy. The Matrix

Practice

Practice, Practice and when you are done practicing, practice some more. It takes thousands of arrows to make a difference.

“Practice is the best of all instructors” Publilius Syrus

Remember when practicing adjust one thing at a time until you have corrected it, and then work on the next thing.

Guidance

Nothing can replace proper guidance from a good certified coach with years of experience. Coaches are another set of eyes with knowledge to help you make those adjustments that will improve your shooting. So find a local archery shop and sign up for regular lessons.

“A coach is someone who can give correction without causing resentment.” John Wooden, basketball coach

Fun

Remember the reason you got into archery in the first place; it is fun.

“Just play. Have fun. Enjoy the game.” Michael Jordan