In the Lancaster Archery Supply Pro Shop, we hear it all the time.“I want to switch to a lighter arrow to get more speed.”That’s usually not a problem, as long as you don’t confuse arrow weight with arrow spine. Too often, that’s exactly what archers do. Someone shooting a bow with a 70-pound draw weight and a 29-inch draw length will grab a 400-spine arrow to replace the 340-spine arrow they’ve been shooting. They think that’s the right way to reduce arrow weight.It’s not.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. But it should only flex a certain amount. 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, because that determines how long of an arrow you need to shoot. And 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.Beside or below the spine rating, most shafts usually also are stamped with their weight in grains per inch. And this is where archers can get confused.Let’s take the Easton Bowfire, for example. The 330 shaft weighs 9.6 grains per inch. The 400 shaft weighs 8.5 grains per inch. Logic might tell an archer that, in order to lighten their arrows, they should switch from a 330 to a 400. Bad move. The 400 arrow is lighter, but it’s also weaker, and so tuning could be a real problem.If you want a lighter arrow, stick with the recommended spine rating, but switch to a lighter shaft. Again, that 330 Bowfire shaft weighs 9.6 grains per inch. A 330 Easton Hexx, however, weighs 7.9 grains per inch. Same spine – lighter shaft.Just for comparison, take a look at these arrows. All measure 29 inches from the insert-end to the bottom of the nock throat. All include 100-grain points, and three Blazer, 2-inch vanes. And all are the correct spine for the archer shooting a 70-pound bow, according to the manufacturer’s chart.Shaft Weight340 Easton Full Metal Jacket 482.2 grains330 Easton Bowfire 433.6 grains330 Easton Hexx 382.1 grainsSo you can see here, there are opportunities to change the weight of the arrow, without deviating from the spine chart.