Andrew Tippins explains exactly what is meant by the spine, as comprehensively as he can in one easy to follow article!
This is a very technical subject that could take up this whole magazine! In simple terms spine or “Static Spine Deflection” is how much flexibility an arrow shaft has or, in simple terms, how stiff it is. The quality Static Spine is measured by clamping shaft material 28 inches apart and putting 1.94lbs of pressure directly on the middle. The amount of bend or “deflection” is then measured in 1/1000 of an inch. Shafts that have one full inch of bend will be labelled as 1000, if they bend half an inch they would have a 500 tag put on them.
The lower the number, the stiffer the shaft. Dynamic spine deflection is a different matter. Dynamic spine still refers to the amount of flexibility of the shaft, but this time it’s while the arrow is actually in flight, under the power of the bow. As the power is always changing from the moment an arrow is released, dynamic spine is incredibly difficult to calculate and quantify. Arrow selection charts always refer to the static spine deflection measurement.
The spine stiffness will increase with bow poundage and the archer’s draw length. During the shot cycle an immense amount of energy is placed into the bow at full draw, and this energy is then transferred into the arrow upon release. This energy transfer causes the arrow to flex and oscillate as the rear constantly tries to catch up with the front as it slows down. Too much flex and the arrow will not recover and settle onto the central line to the target. If the shaft is too stiff it could fail to clear the bow cleanly and won’t be as forgiving down-range. Spine, draw length and peak holding weight are all matched to produce the perfect arrow with a blend of just the right amount of flex to produce good flight characteristics which enables it to be easily tuned to the bow.
There is a potential danger to shooting under-spined arrows, as constant excess flexing of the arrow can cause fatigue which in turn could lead to total failure causing harm to the archer, bow or both. Under-spined arrows might not have the correct amount of overall mass to absorb all of the stored energy in the bow as they will have a lower GPI. The residual energy left in bow could shorten the lifespan of the equipment or lead to failure. If you’re unsure about what you need, consult a professional.