Be honest with yourself, do you really know what the term "spine" really is on a technical level? Most folks would answer and say arrow spine is the measurement of the arrow shaft stiffness. That answer would be correct on a topical level. However, arrow spine and what arrow spine actually is within flight goes much deeper. So together let's take a look at what arrow spine actually is.
STATIC SPINE
STATIC spine is measured by the amount of flex in the arrow when an 880-gram (1.94 lbs.) weight is on the center of the arrow. The arrow must be 29″ in length and supported by two points, at 28″ apart. The number of inches the arrow deflects or bends due to the weight, is the spine size or measurement of an arrow. In our opinion this only creates a standard to know what spine shaft a bowhunter should be shooting pending is setup. It is not what the arrow should be built around. If you were to cut your arrow shaft into cross sections, it's possible each section could have different spines. Meaning each part of the arrow is going to have different properties, strengths, and weaknesses. This comes from both material properties and manufacturing processes. Due to the potential of have multiple spines within a single shaft, static measurements have no place in arrow building, only arrow purchasing.
DYNAMIC SPINE
DYNAMIC spine describes the way an arrow reacts from the stored energy of a bow as it is shot. Several factors determine the way an arrow is going to react when shot out of the bow, including human executing the shot, the power stroke of the bow, weight of the arrow, STATIC SPINE of the arrow, length of the arrow, point weight, nock weight and fletching weight. Even the nock relationship with your string/serving can influence the arrow DYNAMIC SPINE. Due to the nearly unlimited variables in determining dynamic spine, arrows are usually measured using static spine. However, when thinking about arrow flight, dynamic spine is king and can be measured very closely before being built!!! This can be done by applying pressure at the center of the arrow shaft when the arrow is supported at two points 28" apart and slowly rotating the arrow. Finding the "valley" or the arrow shafts first dynamic bend provides the axis where the arrow will first react to the work applied by your bow.
After the force from you bow is applied to the arrow, dynamic spine dictates what the arrow actually does in flight! This is why arrows should be build from the shafts dynamic spine or the first dynamic bend of the arrow shaft. As bowhunters we should not care about an arrow at rest, only an arrow in flight!
Beyond the first dynamic bend, influences such as point weight, draw length, draw poundage, and power stroke of your bow will all factor into dynamic bend and how your arrow shaft reacts in flight. In essence, the point weight variables have been the driving factors behind the rising need to shoot high FOC setups. Folks have figured out how to change the dynamic spine based on FOC to build an arrow that reacts better in flight out of their setup.
While these cannot be taken to the bank each and every time below are some general notes to keep in mind.
Arrow manufactures may measure/label STATIC SPINE differently. Because of this a 350 spine arrow from one company may be comparable to another company's 340 spine arrow. When purchasing make sure you dive into their spine charts.
A heavy GPI arrow will usually have a stiffer spine. Typically as shaft weight increases the spine gets stiffer.
STATIC SPINE measurements are recorded on a 28" arrow shafts. Arrow shafts longer than 28" will have a spine weaker than the advertised metric.
As point weight increases spine weakens
At the end of the day, you can use STATIC SPINE metrics to purchase arrows but those same shafts should be built around the arrow's first dynamic bend. As bowhunters we should only care about what the arrow is doing in flight, not at rest and this is why dynamic spine is KING! And if you're into the high FOC trend, keep tinkering until you find something that reacts well out of your bow or save yourself time and find the first dynamic bend of your arrow before you build!
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