The Truth About Straightness, Spine, and Accuracy
I think one of the most common questions I get regarding our arrows is "What straightness should I buy?" Its a tricky questions. So much of it depends on bow tuning, arrow length, broad head choice, etc. There is a lot of misinformation about straightness, and even more misunderstanding about what really matters for accuracy: Spine Deflection.
Straightness The straightness of an arrow is usually measured by placing the arrow on a 28" span with a dial indicator gauge in the middle and measuring how much difference there is around the circumference. Here at DCA, we also use a RAM Spine Tester and the Pine Ridge Archery arrow spinner or spin tester. We prefer the Pine Ridge Arrow Spinner because it's method focuses our attention on the ends of the arrow, provides tighter specs for our customers.
Many people choose to purchase +/-.003 or +/-.006 arrows and cut the ends off to get a +/-.001. Usually this works pretty well. With field points or low profile broad heads (expandables) you should see great accuracy. At most distances out to 40 or 50 yards.
So it makes sense, the straighter the arrow, the more accurate the arrow. If you are a short draw guy that shoots a 26 inch arrow, you can probably buy +/-.006, trim the ends (or even just one end) and get a significantly more accurate arrow.
But there is another thing that effects accuracy. Something that is difficult to measure, but is intimately tied to arrow straightness. That thing is called Spine Deflection.
Spine Deflection and Deviation
Spine deflection measures how consistent the stiffness of the arrow is around the entire circumference. It is measured with the same gauge style machine by hanging a weight from the arrow so it flexes, and then measuring the straightness all the way around at full stress.
Spine deviation is how much variance there is in the spine deflection. The lower the spine deviation, the more accurate the arrow. For example, a 350 spine arrow that has a deviation of +/-.005 will flex as 345 at its stiffest and 355 at its weakest. This little variance will have little effect on accuracy.
But some brands have deflection as great as +/-.020. This means a 350 spine arrow can be 330 or 370 or anywhere in between. As you might imagine, if you have a dozen arrows with that much variance between spines, your long range groups will be all over the place. Particularly with fixed blade broad heads that provide steering surfaces on the front of the arrow.
One Indicates the Other
Now this is the most important part. Arrows with tighter straightness tolerances will also have lower spine deviations. This means that the guy who buys a +/-.006 arrow and cuts at both ends may in fact have an arrow that is +/-.001. But the spine deviation will not get any tighter. The inherent way that the arrow flexes at full stress will not become more consistent the more you cut off.
For this reason, I often steer archers (particularly long range and large broad head shooters) towards our +/-.003 arrows. And for archers that want to shoot long range or large fixed blades AND have a long arrow (anything over 29") I usually recommend +/-.001 arrows.
So what to buy?I usually say to buy the best thing you can afford. Straightness never mattered to me much until I started looking at the relationship between straightness and spine deviation. If you can afford the upgrade to +/-.003 or +/-.001 arrows, its something worth doing. That way, if you have to stretch your range out to get that trophy of a lifetime, you can do it with the confidence that the arrow coming out of your bow or crossbow is the absolute best that it can be.