SELECTING THE CORRECT SPINE
When we talk about an arrow’s spine, we’re talking about how much it flexes. We’re talking about its stiffness. Every arrow should flex when it leaves the bow, however the flex should be limited within the safe operation of the product. If it flexes too much (weak spine) then its flight will be erratic. If it doesn’t flex enough (stiff spine) then the arrow will have no forgiveness. Consistent accuracy usually suffers in either case.
Choosing the correct arrow spine for your setup depends on your draw length and draw weight. Draw length is important, as it that determines how long of an arrow you need to shoot. The longer a shaft is, the more it’s going to flex. Draw weight is factored in, because that determines the amount of force pushing the arrow.
Every arrow manufacturer has a spine-selection chart, so you know which shaft to choose for your draw weight and arrow length. (Some even factor in the bow’s speed rating, since faster bows exert more force on an arrow.) And every shaft bears its manufacturer’s spine rating.
Unfortunately, the numbering system for spine ratings is not uniform from manufacturer to manufacturer. So don’t assume the numbers you see on shafts across manufacturers are comparable. Check the below chart to see what fits your style.