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.


Adjusting your sights

As a recurve archer, your sight is a vital piece of your equipment. Recently, I had a request to do a blog about the anatomy of a sight and how to adjust it. Although every sight is different, they all have similar components. We will use the Cartel Mighty sight in this example.

A recurve sight is typically made of four components; the riser mount with knob, horizontal extension bar, vertical sight bar, and scope. The riser mount is used to attach the sight to the riser and the knob is used to hold one end of the horizontal extension bar. The vertical sight bar is mounted to the other end of horizontal extension bar with the scope attached to it.  The vertical sight bar and scope have thumb screws that are used to adjust and micro-adjust the scope in relationship to the target.

Most sights have three knobs, one to adjust the length of the horizontal extension bar, one to adjust the height of the scope and one to adjust the left/right location of scope. The latter two usually have both large and micro-adjustments available. When you are ready to start adjusting your sight, remember the simple rule, “Follow the arrow”. Check out my earlier blog called Follow the Arrow: Adjust your sights.

To adjust the length of the horizontal extension bar simply loosen the knob and extend it to the desired length and tighten the knob to secure it. The marks on the bar to help you remember the length so you can set it up again.

To make an adjustment of the vertical location of the scope, loosen the thumb screw fastener and either depress the thumb lever for a large move or turn the micro-knob for small adjustment. Once complete then tighten the thumb screw fastener to secure it.

To make a left or right adjustment of the scope, loosen the thumb screw fastener and either twist the scope in or out for a large change, or twist the tension knob for micro-adjustments and then tighten the thumb screw fastener to secure it.

Once you have adjusted your sight make sure to shoot several arrows to confirm your adjustments. Once you are satisfied with the results, record the location of the sight marks on the vertical sight bar. Remember as you grow, develop muscle and change equipment your sight marks will move. One trick is to add a thin piece of white tape over the sight marks. This allows you to adjust the markings without permanently damaging the sight itself.

Follow the Arrow: Adjust your sights

Unless you are a traditionalist, one of the first things you will need to do as an archer is adjust your sight. Unless you are a compound shooter with the same setup for ever, you will need to make adjusts again and again. For recurve archers there are many reasons why you will need to make adjustments including…

Environmental Differences : Every location is different, wind, rain; check out my earlier blog about Weather Conditions here.

New Distances : As you develop and change categories, distances change. Junior women need to shoot 70M and junior men 90M, if you are not there yet, do not worry, you will be.

Growth : As you get older, your body changes, you get taller and stronger and your draw length changes, using a clicker can help with consistency, so check out my Clicker blog.

Equipment Changes : As you get stronger and you are required to reach longer distances you need to change equipment, such as higher poundage limbs. Eventually equipment wears outs and we all want the latest and greatest technology.

Archery Form Changes : As you develop as an archer, you will perform better as an archer and your archery form will get better.

For young recurve archers adjusting a sight is a frequent event and is actually quite easy, the hard part is resisting the temptation to adjust it after every shot. Remember that consistency is your ultimate goal, so track your arrows before making any adjustments. You can do this on paper or there is an awesome free application for your IPod Touch called Archery Score Free by Yakoob Ali.  Once you are warmed up begin to track your arrows and determine the centre of your arrows grouping and then move the sight accordingly. Remember, if your groups were good yesterday and are not today, evaluate your form first. Also, if you have one arrow consistently out of group, check the arrow for defects.

When you are ready to adjust your sight, apply this simple rule, “Follow the arrow” or in this case the centre of the group of arrows. If the centre of your arrow grouping is to the left, then move your sight towards the left or opposite if your group is to the right.  The same principle applies for the group’s height; move your sign up or down if the group is not centered.  By moving the sight towards the arrow, the trajectory of the bow is altered to better centre the arrows on the target.

Remember, consistent form is essential to archery, and before you start micro-adjusting be sure you are grouping consistently first. Otherwise, if you are always making changes to your sight you will never truly know if you will consistently hit the bulls-eye.

New Tournament Target Faces for 2012

On December 27, 2011, FITA Council approved the introduction of new target faces mandatory for World Archery Indoor Championships.

The change includes the removal of the extra rings on the 40cm vertical (and triangular) triple face. There is a distinction between regular triple faces and Recurve (R) and Compound (C) 40cm triple faces. The 40cm triple face (both triangular and vertical) will differ in the size of the 10 ring: the 40cm-R has a 40mm 10 ring, the 40cm-C has a 20mm 10 ring and the combined triple face has the two 10-rings.

The changes also affect tie-breakers, now for both team and individuals tie breakers will be determined by the greatest number of 10’s and then the greatest numbers of 9’s. If a tie remains, athletes still tied will be declared equal. For position in match play, a coin toss will determine declared equal archers position.

I am interested in seeing and using these new targets since the new rule took effect as of January 1, 2012 it makes the Vertical R and C triple faces mandatory for the upcoming World Indoor Archery Championship in February.

To review the rule changes and decision announced by the FITA council, please check out FITA website.