Poluke, 'spine', as applied to arrows, is really a misnomer. Spine actually refers to the orientation of the arrow where the arrow is stiffer than at any other place around the shaft. That still applies to wood and carbon arrows, because they do have a spine. So do aluminum and fiberglass, but to a much lesser extent. You often can't even find the spine on an aluminum arrow.

What we really mean when we say spine is the stiffness of the arrow. We place the arrow across a pair of supports, suspend a 2-pound weight in the center of the arrow and measure deflection - how much it bends - in thousandths of an inch. Obviously, the less the arrow bends, the stiffer it is. We got the term 'spine' from the old days of wood arrows when you oriented the arrow to check it's stiffness across the spine of the shaft.

The things that affect how stiff the arrow needs to be are: draw weight, draw length, cam type (round wheels, medium cam, hard cam), release method (release or fingers) arrow length and tip weight. You have to take all those things into account to determine what size arrow you need for your bow.

Most arrow charts are set up with draw weight on one side and arrow length across the top. You find your draw weight and follow that line across to your arrow length and Bingo! that's your arrow. Problem with that is, you have to know what tip weight and cam type they used to set up that chart. If they set it up using a mild cam and 75 grain tips, and you shoot a hard cam and 125 gn tips, you'll likely need to go at least 15 pounds heavier on the chart to get the arrow you need.

Easton and Beman have very good interactive arrow selection programs where you plug in your information and it spits out suggested arrow spines. You can then look up the deflection for those arrows and maybe correlate that info to other brands. Say you need an arrow with .340" deflection, you can find out which arrows from other manufacturers have .340" deflection and you should be close.

Hope that helps.