The Archers Word
Spine - bending displacement measure
Spine is the displacement measure of how much an arrow bends under a given load.
In archery there is the important distinction between "dynamic" and "static" spine.
Arrows are never stiff, but rather bend around the bow in order, if well callibrated, to shoot forward straight to the target. This phenomenen is called "dynamic bending". This however is a rather difficult thing to observe without highspeed cameras. Look up the high speed video to see this phenomenon. Do NOT confuse though "dynamic bending" with the famously called "Archers Paradox".
Since the prediction of the dynamic spine - how much it displaces as the arrow is shot - requires knowledge in advanced mechanics of materials (engineering field of study) and is even then rather complicated, a simpler alternative has been used to experimentally calibrate the arrows. This is called the "static" spine.
To measure the static spine, an arrow shaft is supported at two points, while a weight is suspended in the middle. The deflection is the measure of the spine.
The standard of this setup is a 28 inch span (71.12cm) of the two supporting points and a 880 gram weight. The deflection is measured in inches. Therefore, a "520" arrow will deflect 0.520 inches (1.32 cm).
Friendly notes:
- Having well calibrated arrows with their proper spine is extremly useful and important. (The arrow will perform as you shoot, and all "mistakes" cannot be the cause of the arrow anymore.) A professional archery shop will provide you with the service of getting proper arrows for the bow upon request. They will usually make use of the Easton-archery arrow chart, which provides all necessary numbers to size the right arrow for the right bow and archer.
- Want to make your own spine tester? Its a simple construction, and you can use the information from above to build your own. In case you want a reference, here one built by Poorfolkbows.
Useful sources:
Meta-synthesis on describing spine: http://www.meta-synthesis.com/archery/archery.html
Archery advice on selecting your arrow: http://archeryadvice.weebly.com/arrow-101.html
Easton-archery for further interest: http://www.eastonarchery.com/