271 - Making River Cane Arrows 1/9
Channel: Jack Crafty
Date: 2013-10-18

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Spine is one of those things that everyone talks about, but few completely understand. I'll try to provide some basic information and hopefully won't make too many mistakes.
Spine is basically a measure of the stiffness of an arrow shaft. More accurately, it is a measure of the deflection a shaft exhibits when a two pound weight is suspended from the middle of the shaft when the shaft is supported at two points 26 inches apart and the shaft is rotated so that the grain of the wood is vertical. This measurement is generally made with a device called, surprise, a spine tester. It is important that the grain of the shaft be oriented properly since a grained material exhibits different stiffness with and against the grain. Arrow shafts are measured to determine the greatest stiffness and that measurement is across the grain. Note that this is important because it tells you how to orient the fletching on the shaft.

(Note:  River cane does not have grain, so it doesn't really matter how the cane is placed on the spine tester.  If I want to be extremely picky, however, I usually measure spine where the cane is the weakest.  This requires rotating the cane on the spine tester to find the orientation that produces the most deflection.)

There are tables that convert shaft deflection into pounds, a more usual description of the stiffness. This conversion can also be done using the simple formula Spine (in pounds) equals 26 divided by the deflection in inches. This says that if a shaft under test bends half an inch then is it has a 52 pound spine measurement. I suspect that the formula is simple because of the selection of the measurement method (the two pound weight seems to be the item most easily used to tune the conversion). In any case, the above information should be enough to allow a handy archer to jury rig a spine tester if one was needed.

The spine of an arrow is important because of a phenomenon called Archer's Paradox. Archer's Paradox is essentially the process by which an arrow shaft bends around the bow and shoots straight. Note the word bend. The force of the bow string on the arrow causes the arrow to bend during the process of accelerating the arrow off the bow. The amount of bend affects the flight of the arrow and the accuracy and consistency of the shot. This is particularly important where the arrow rest is significantly offset from the path of the string such as with longbows shot off the hand. It is less important in center shot bows where the motion of the arrow is essentially along the path of the bow string upon release.

For the bow and arrow combination to work well, the arrow must bend just enough to get around the riser of the bow. If it doesn't bend enough, the back end of the arrow will impact the riser and the arrow will deflect. If it bends too much, the back end of the arrow will fly off to the side and the arrow will deflect. Just right and the arrow goes straight and everyone is happy.

Selecting arrow shafts for your bow is a little more complicated than simply buying shafts with the same spine rating as your bow weight. There are a couple more things that affect the effective spine of a completed arrow. The first is the weight (mass) of the arrow head or point. The heavier the point of an arrow, the lower the effective spine of the shaft. This is because of the increased inertia provided by the greater mass. With a heavier point, acceleration of the arrow will be slower, and more energy will accumulate in the arrow shaft reflected in greater bending and a lower effective spine. All other things being equal, an arrow with a 30 grain target point on it will act like a more heavily spined arrow than the same arrow with a 125 grain field point.

(Note:  The length of the arrow also influence the "effective spine" or the "dynamic spine" of the arrow. Added length in front of the arrow has a similar effect to adding weight to the front and will make the arrow bend more when shot.  This effect is in addition to the actual weight of the added length.)