Spine does matter!
If you’ve ever shot a bare-shaft test, you know it absolutely matters. And using a bare-shaft test is the only reliable way to determine the correct arrow parameters—it immediately shows whether an arrow is dynamically correct, too weak, or too stiff.
The old rule of 5# per inch of arrow length applies only when the bow’s draw weight is around 65–75#. The rule was “created” by Saxton Pope, who shot bows in that range. For weaker bows, the rule simply doesn’t hold. In reality, the correlation is non-linear. For example, with 35–40# bows, the “rule” is roughly 2.5# per inch. And so on.
It’s also commonly assumed that a 28" arrow requires spine matching the bow weight—for example, a 30# bow needs a 30# arrow. But again, in practice this varies from archer to archer. One shooter may need 35# or even 40# arrows, while another may need 25# arrows for the same 30# bow. A bare shaft test will tell the truth.
The cut-in window has no effect. The required arrow spine is the same for wide-handle bows, narrow-handle bows, or windowed bows. If you don’t believe it, do a bare-shaft test using the same bow with different handle widths.
And finally, arrow point weight has almost no effect on required shaft spine. A 25-grain change influences spine by about 1# or less. Again—shoot a bare-shaft test to see for yourself!
