What is the spine weight on an arrow?

Answer 1

The spine of an arrow is not expressed in "weight" - it is the bow's pulling power that is expressed as a weight figure. The arrows must suit this weight.

Any bow takes a certain amount of force to draw back to the normal shooting position (usually with the string touching the chin and nose); this force is measured in x number of pounds - my own ash longbow is lighteweight at just 45 pounds draw weight.

In order for the arrows to fly true, they must have a certain amount of flexibility ("spine") to match the draw weight of the bow - if they are too flexible they will shoot off to the right, if too stiff they will go left of the target. This is due to the so-called "archer's paradox" where the arrow first bends itself around the bow handle and then continues to fly straight.