The spine of an arrow is a measurement of its stiffness. It's is a number given in 1000ths of an inch of deflection given a certain amount of pressure.
There is a great article detailing the specifics here: http://www.huntersfriend.com/carbon_arrows/hunting_arrows_selection_guide_chapter_3.htm
Arrow spine refers to the arrow's degree of stiffness - how much the arrow resists being bent. Some arrows are very stiff, others are very limber. And if you ever intend to achieve serious accuracy with your (...) bow, you'll need to choose an arrow that's just stiff enough
- but not too stiff for your particular bow setup.
Most people think an arrow flies just like it looks when at rest - perfectly straight. But nothing could be further from the truth. Once fired from a bow, an arrow immediately begins flexing and oscillating. That's not a defect. Each arrow bends and flexes in a particular cycle as it leaves the bow (archer's paradox). If the timing of the cycle is correct, the tail of the arrow clears the bow without making contact with the arrow rest, riser, or cables. If the timing of the cycle is not correct due to improper arrow spine, the over- or under-oscillation of the arrow results in serious fletching contact and/or paper-tune tears which cannot be corrected. So we have to get this one right.
There are three main ingredients which determine an an arrow shaft's general spine characteristics:
- the stiffness of the actual shaft material
- the length of the shaft
- the tip weight that will be used
