Spine Chart Arrow: Why Can’t Arrow Spine Charts Be 100% Foolproof?

Spine Charts Arrow

WHY CAN’T ARROW SPINE CHARTS BE 100% FOOLPROOF?

Many bows do not match the expectations of the chart! FOR EXAMPLE. 

Few if any ever pick up on the fact that there is virtually no allowance made for the actual draw length of a compound bow when it is not directly related to the cut length of the arrow. 

  • Tradition dictates that an arrow will generally be cut somewhere between one-inch longer and one inch shorter than the draw length rating of a compound bow.
  • Therefore, if you choose to use a longer or shorter arrow than which is expected to be used in your bow there are no provisions to explain how to allow for it.
  • To make it even worse, it can vary in both directions, i.e. the bow may actually be more capable (powerful) than you expect or it may be less powerful, each of which will cause to choose a dramatically inadequate spine.
  • For example, there is absolutely no reason why you can’t load-in and shoot a 32-inch arrow in a 26-inch draw bow or more commonly, shoot about a 26-inch arrow in a 32-inch draw bow. Believe me that every combination between these 2 extremes is in wide use this very minute. 

DIFFICULT TO ADDRESS. 

Why Can't Arrow Spine Charts Be 100% Foolproof

To the defense of every arrow shaft provider, let me say that it is an extremely difficult task due to the fact that there is simply not a suitable method available to determine the exact capability of each individual’s specific bow.

There is one ‘given’ that can be readily addressed, such as the fact that releasing with your fingers means the bowstring and arrow nock actually start-out moving dramatically to one side rather than ‘straight ahead’ as with a release aid. Such action immediately flexes the shaft hard to one side but can easily be compensated for with a slightly increased stiffness.

The Easton chart variable: ‘Release aids, subtract 3-5 pounds’ from your peak weight figure indicates that a finger release shooter will require a little more spine to compensate for this situation. But you first need a basic shaft size to make the allowance on. 

Trying to figure out EXACTLY (‘close’ is great in horseshoes and hand grenades) how hard that specific bow is going to sock the tail of the arrow during the trip is the overwhelming problem. 

Obviously, the harder the shaft will be hit, the more stiffness it must have to avoid reacting adversely. For those who are not aware of it, I believe and loudly profess that attempting to achieve an acceptable arrow spine is the; 

Number 1, Paramount, Biggest, problem in the sport. It probably causes most bow setup (tuning) frustrations and misunderstandings, particularly when trying to control winged broadheads on the leading end of the arrow. Probably nothing else has caused more people to give up the sport in total frustration. Confusion by uninformed archers can result from their interpretations of advertisements as well. They often take literally such statements that a certain shaft type ‘can be used from 25 to 80 pounds.’ You and I understand that they merely omitted; ‘if cut to the proper length with the proper point weight attached’ but the neophyte does not and often wastes time and money based on his understanding.

For example, the 2312 is listed for peak weights from 26 to 106 pounds at various points on the Easton chart, but we know it is extremely conditional, not a magic shaft size. LIKEWISE, I assure you that 98 or 99 of every 100 archers could improve their arrow spine from where they are right now. This is NOT TO IMPLY that it would produce any tangible benefit what-so-ever in many cases, I only use it to emphasize that it is not an easy thing to accomplish. 

EASIER TIMES. 

When we were all shooting longbow and recurve type non-mechanically-assisted bows it was a snap to find an adequate arrow spine. Simply because every bow of this type has virtually the same CAPABILITY. That is, every 60 pound bow drawn back the same distance would shoot a given arrow about the same speed. Using a shorter-than-normal arrow with an overdraw arrowrest on one of these was not in style although the Chinese armies did it thousands of years ago. Then, it didn’t take many shaft sizes to cover the spectrum. In the days when most hunters were adult males shooting 50 pounds with a 30 inch cut arrow we all used about the same shaft size. 

EARLY COMPOUND BOW DESIGN. 

For many years after the compound bow was introduced the situation was about the same for them, they all contained about the same capability. Then, for the first time the draw length of the bow became a factor, but it was basically ignored, possibly either because it was not understood or because it may have been entirely too complicated to chart and considered as not enough of a factor to warrant the added confusion. At best, arrow charts have only been offered as an ‘educated best first guess’ and should never be expected to be the absolute final word. 

SPEED CRAZE BOW DESIGNS. 

In more recent years compound bows have become far more complex in the sense that the laws of physics have been maxed out in every governing area of bow design. Few models contain equal amounts of the several tricks of the trade. That is, to obtain the higher capability (velocity) ratings they have been ‘souped up’ in every possible area, in fact to the point that how good or how comfortable a bow feels to draw is no longer a factor. Such crude and uncomfortable feeling bows ‘could not be given away with a $10 bill taped to them’ in the earlier days, but are now readily acceptable, simply ‘because they are fast.’ The round eccentric design does the most work for the archer, requiring the archer to do less of the work but the resulting easier and smoother feeling draw means less energy storage, therefore less arrow velocity. The models which contain the fewest number of ‘advantaged areas of design’ are the slowest and those with the greatest number are rated the fastest. Now, with these higher levels of energy storage available, the fact that one bowstring is drawn back 2 inches more than another with the same peak weight has a significant impact. That is, both test bows may be a 30 inch draw length to the archer but one may require the string be drawn back up to 4 inches farther than the other to accomplish it. Each additional inch means more energy is available and will be applied to the shaft so if additional spine is not available it will be underspined. This is typically not addressed on the charts with no allowances made for the 32 inch shaft in the 26 inch draw bow or the 26 inch shaft in the 32 inch draw bow example. As is, the chart can only presume a 26 inch cut shaft will be used in about a 25-26 or 27 inch draw bow, not one that has more energy applied to it during an unexpected 5, 6 or 7 more inches attached to the string. Still a broad range which in itself can contribute to an inadequate spine. 

HOW DRAW LENGTH EFFECTS BOW CAPABILITY. 

As the ‘Archery Engineer’ responsible for using every bow component and all cabling to hit both the draw lengths and the peak weights required for the many new models each year, I was obliged to run many conclusive and duplicable tests and evaluations. By keeping same arrow-weight velocity result data on every draw length version (personal records only) of every model, particularly after the original round eccentrics began to give way to more radical designs in an effort to do more in the same allotted space, I soon realized and then began to use a very helpful rule-of-thumb of 5 FPS per inch of string travel. That is if the 60 pound 30 inch version of that family/model shot a 540 grain arrow at 225 feet per second the 29 inch version could be expected to shoot that same weight arrow about 220 FPS (due to an inch less storage area) and the 31 inch shoot it at about 230 because of the extra inch. Regardless of how accurate it actually is in all cases, the philosophy is the same and the fact that you can put more water in a box that is an inch longer but the same (width & ) height is self-evident.

The reason most people never pick up on the fact that the draw length of a bow has a profound effect on it’s capability is due to the fact that the charts provide them with shorter, smaller and therefore lighter arrows. Therefore, their reduced capability bow can still produce a velocity about equal to their longer draw length counterparts, because every bow can propel a lighter arrow faster than a heavier one. Their ‘delivered kinetic energy’ (potential for impact) is proportionally slightly reduced but again few ever notice, or care. (If you are unhappy with this situation, just hash it out with your parents, whom developed your draw length, or Sir Isaac Newton of ‘Newton’s Laws’ fame concerning the laws of physics here on planet Earth).

Here is how 2 different archers with the same bow model and same peak weight can be effected by this phenomenon.

  • Archer ‘A’ has a 32 inch draw version set at 60 pounds from a model/family which has an advertised AMO rating of 225 FPS. (AMO ratings are only for the 30 inch version of the model using 60 pounds peak with a 540 grain arrow).
  • Archer ‘B’ has a 26 inch version of the exact same model/family set at 60 pounds.

Each interpreted the chart identically and ending up on the same line across the chart, one picks a 2212 in the 26 inch cut column the other a 2512 in the 32 inch cut column. Using the rule of thumb of 5 FPS difference for every inch of power stroke it means the 32 inch draw model would probably shoot that 540 grain test arrow at about ( 2 times 5 plus 225 ) near 235 FPS. With a 4 inch shorter string power stroke, the 26 inch version ( 225 minus 4 times 5 ) can be expected to shoot it only about 205 FPS. That is, if both were to load in and shoot that 540 grain arrow, a capability difference of about 30 FPS may exist between these two 60 pound peak bows. 

Fig.1Typical set-back arrowrest which permits the use of arrows much shorter than ‘usual’. Thousands of years ago armies discovered that using a lighter arrow permitted them to use the faster arrows to shoot greater distances. 

THAT 26 INCH ARROW IN THE 26 INCH DRAW BOW. 

Since the 26 inch 2212 weighs approximately 100 grains less than the 2512 cut at 32 inches, this shorter draw bow will be able to make up most of it’s inherent inadequacy and still manage to propel it about the same as the longer draw version storing more energy over a longer area. In these instances, if the allowance made for using a release aid was acceptable and above all the best possible one of the 3 ‘bow capability’ sections ( labeled ROUND-ENERGY-SPEED) was used there is a good chance the shaft size chosen will be useable. PARTICULARLY when using field points, in which case any spine inadequacy may not become apparent until same-weight broadheads are added. At which time, due to not understanding the situation most archers tend to immediately blame the broadhead, or the bow, rather than their arrow shaft spine. 

UNDERSTANDING THE 3 BOW CAPABILITY SECTIONS. 

Allow me to make this revelation in this fashion: AMO bow ratings (for the 30 inch 60 pound versions only) are only found in an area of about 195 to 235 FPS for all practical purposes, or a range of about 40 FPS. It should be painfully obvious that the model that propels the test arrow at 235 FPS must surely be applying significantly more energy to it during it’s time on the string than the one achieving a mere 190 FPS with the same mass weight arrow. Obviously all of these bows cannot possibly ‘get by’ with an identical spine stiffness simply because they are all labeled and set at the ‘same’ 60 pound peak.

Looking at the Easton chart and coming down any ‘cut column’ note that between the ‘slowest’ 60 pound bow listed and the ‘hottest’ listed on the left, there is an average of at least 5 significant shaft sizes from the slowest to the fastest listed in each column. 

Just for the sake of argument, let’s presume that could be an indication that a stiffer spine is needed for about every 10 FPS of your bows’ capability. Note also that if you compare the peak weights listed in the 3 sections on each line across you’ll see a 4 to 9 pound allowance between sections. That’s their best shot at breaking these ‘190-235 FPS’ rated bows into; Slow-Medium & Fast groupings. 

CONCLUSION ON IDENTIFICATION & USE OF THESE 3 SECTIONS. 

Their names can be misleading, it has nothing specifically to do with just the appearance of the eccentric, but with the total capacity of the entire bow. Finding out what the AMO Rating (of the 30 inches 60-pound version) of your brand and model is and from that determining which of the 3 sections to use is my best advice. 

BOWS WITH OVERDRAWS, ‘USING THE OTHER GUY’S ARROW.’ 

Now let’s put an overdraw on that 32-inch draw bow and use a 26-inch arrow in it. Sounds simple enough, it’s the same routine as before but instead, you use the 26 inch cut column, we’ll even pick the same 2212 as the friend with the real 26-inch draw bow. Wow, we suddenly can load in and shoot an arrow that weighs 100 grains less and chances are the bow will be able to propel it about an additional 25 FPS just because it is lighter. 

WHOOPS, HOLD IT JUST A MINUTE, that’s propelling the very same 26 inch arrow 25 FPS faster than that shorter draw bow can do it, 

how can it now suddenly absorb that much more energy and still be the correct spine? 

Now you’ve got it, IT CAN’T, at least not perfectly. This situation is the reason so many bows end up being called ‘temperamental’ or ‘unforgiving’ or ‘on the edge’ because people think they can use them just because ‘the ‘chart said it was correct.’

Wrong. ARROW CHARTS CANNOT BE EXPECTED TO BE THE FINAL AUTHORITY. 

Now can you see why so many people with high AMO rated bows or using overdraws, or worse BOTH, won’t accept the fact they may well be struggling with underspined shafts? It may be they are just not using the ‘hot bow’ (labeled ‘Speed Cam’) section of the chart when they should, or are not adding 5 or 10 or 15 extra pounds to their peak weight figure (the equivalent of moving ‘down’ one or 2 or 3 selection boxes). The simple fact is that if the hottest bows around were rated at 230 fps when the chart was made, these newer designs rated up nearer and nearer to 250 are simply ‘off-the-charts’ and some additional peak weight should be added to compensate for it. 

ALTERNATIVES TO BUYING DIFFERENTLY SPINED ARROWS. 

Understanding all this better should make it more clear why in the past you may have had to either; reduce your point weight, and/or shorten the shaft all you could, and/or lower your bow peak weight (all three is often done) in order to effectively stiffen and be able to still use the first shaft size you picked with your best guess from the chart. Easton is the first to realize how difficult shaft selection can be for you and includes this disclaimer notice for clarification; (excerpt) ‘- IN THESE CASES, YOU’LL NEED TO EXPERIMENT AND USE STIFFER OR WEAKER SPINE SHAFTS TO FIT YOUR SITUATION.’ Since there simply is no absolute perfect patented test that I know of which can positively assure you that you are even close, much less perfect, we can only make a personal final decision based on experience. By the ‘seat-of-our-pants’ if you will. Commonly used are either;

  1. The visual arrow-in-flight appearance.
  2. The arrow grouping conclusion.
  3. Comparing bare shaft impact points.
  4. Comparing same-weight-broadhead impact points.

Actually, I suggest that the less experienced you are in this field the more you should stick to the methods at top of the list. Virtually everything I have to offer on each of these techniques is already posted as a response to one or more questions somewhere in my ImproveHunting section. To better understand all of the reasons why all bows marked with identical draw lengths and peak weights do not shoot identical velocities, enter: BRACE HEIGHT and then POWER STROKE. There’s my contribution to help you better enjoy your recreational time in archery. Enjoy.

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