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.

Arrows Series – Part 2: Measurements

As we start to dive deeper about arrows, my goal is to simplify the information so young archers can have a good basic understanding; it is not meant to be all in compassing or a physics lesson, remember I am still learning too. Therefore, the next step is to understand how arrows are measured and how to determine draw length.

The Archery Trade Association (ATA) formerly the Archery Manufacturers and Merchants Organization (AMO) has the following standard for measuring draw length:

AMO DRAW LENGTH STANDARD

For Manufacturers

Draw length is a specified distance, or the distance at the archer’s full draw, from the nocking point on the string to the pivot point of the bow grip (or the theoretical vertical projection of a tangency line to the pivot point parallel to the string) plus 1 3/4”. Draw length from pivot point shall be designed at DLPP (Draw Length Pivot Point) and shall be called TRUE DRAW LENGTH.

EXAMPLE: 26 1/4” DLPP plus 1 3/4” is the equivalent of 28” draw.

For Dealers and General Use

For practical reasons not requiring precise terms, draw length is the distance, at the archer’s full draw, from the nocking point on the string to the back of the bow at the arrow rest.

EXPLANATION: The standard Manufacturers is consistent with the Bow Weight Standard as related to the pivot point. The DLPP plus 1 3/4” is compatible to previous concepts of draw length. Draw length for Dealers and General Use relieves the burden of preciseness not required for general use and facilitates determining arrow length. THIS STANDARD SUPERSEDES THE PREVIOUS STANDARD.

This can be technical and confusing, however with most recurve risers, the distance of the draw length pivot point (DLPP) to the front edge of the riser is 1 ¾”. Therefore, in general terms…

Your approximate draw length is equal to the distance from your string to the front edge of the riser at full draw.

You could use a measuring tape to measure this distance however unless you have a very consistent anchor point and good form you will have varying results, since you should measure several times. Therefore, any archer who is at the point they are fining tuning arrows for high-level of performance they should consult a professional. Otherwise, I would suggest you use the following simple method to determine your draw length.

Arms Length method

Using a ruler (or other straight stick), place one end in the “V” of your neck (where your neck meets your chest ) and relaxed, reach straight out until your palms touch the ruler. At the point for your fingers is your approximate draw length. Your arrows should always be at least 1” to 2” longer than your draw length for safety reasons and young archers often need to set their arrow length little longer ( 2” to 3” ) to allow for growth.

This gives you a basic understanding of draw length and how arrows are measured, however, things get very complicated for young competitive archers who are still developing and growing in size once you add-in arrow stiffness and flex, arrow cost, type of equipment and bow weight.

It is important to understand how things are measured so you can understand why you are using the arrows you have.  I highly recommend you visit your local archery professional for assistance.

Twisted Limbs

Arguably, the MOST important part of the bow are the limbs, since the movement of the limbs transfers energy to drive the arrows to the target. Investing in well-manufactured limbs can be the best move any archer can make. Since consistency is the number one requirement for any archer, you want to avoid any twists. Wood/Fibreglass limbs perform well however can be prone to warping in areas where temperature changes a lot. Carbon fibre layers help strengthen the limb and reduce the tendency to twist.

After purchasing a new set of limbs, one of the first things an archer should do is align their limbs to their riser. Alignment of the limbs means the string should appear to run right down the center of the limbs thru the center of the riser.

I highly recommend the purchase of a new set of limbs for any archer; however, the purchase of a good used set can be an option for a new archer who is still learning the sport. When purchasing a used set remember any twist will cause inconsistent flight of arrows, check for limb distortion (often called limb twist). Viewing the bow strung with the limbs from either end of the bow, if one or both of the limbs have a slight twist, you may have limb twist.  If the distortion is only slight sometimes it can be cured with one of three methods.

1)       Cold Bump Method

Attempt to straighten the limb using the “cold bump” method. If a limb has developed a slight twist from improper stringing, this method often will correct the problem. Grab the bow by the riser with your dominant hand. Use your other hand to grip the bow limb slightly above the twist. Slowly twist the limb in the direction opposite of the distortion, and then quickly release the limb. Repeat this process several times until the limb properly is aligned.

2)       Warm Wet Submerge Method

Submerge the twisted limb in a tub of hot water for 3 to 4 minutes. The water should be no warmer than a hot bath, around 105 degrees Fahrenheit. This is just hot enough to mildly loosen the glues in a laminate. Remove the limb from the water, then slowly twist the limb in the direction opposite of the distortion. Release the twist slowly and examine the limb for straightness. Repeat the twisting procedure several times until the limb is straight.

3)       Low Heat Method

Twist the limb in the direction opposite the distortion and hold it in place. Have an assistant heat the limb using a hair dryer. Hold the hair dryer 6 to 10 inches from the limb, and slowly move it up and down the length of the limb. Heat the limb slowly for 2 to 3 minutes, then release your hold and examine the limb for straightness. Repeat the heating procedure as necessary until the limb is straight.

In my opinion, it is not worth playing with twisted limbs or possible physical injury and especially where ranking is important, competitive archers should not risk the possible performance dip . New well-manufactured limbs are very forgiving of a poor release, feel smooth and are more affordable than ever. Cartel Doosung and Bow Korea offer a complete line of limbs for everyone including introductory limbs, the new Midas MPS limbs for intermediate archers and MK Archery Vera/1440 limbs for competitive archers.