I’m making a batch of arrows for the upcoming archery season– with my own shafting from lumber.
Whoop-de-do you might say, but for some reason I’ve always bought my arrow shafting and just assembled them, something I intend to remedy this time around! However, I know how important it is to have matching arrows. . . which led me to making a spine tester. Another project to build something that will assist in making something else!
What is Arrow Spine?
Spine is simply a measurement of how much an arrow flexes. When you shoot your bow, the arrow absorbs the energy from the string and flexes dramatically as it launches from the bow. If it is the incorrect spine for your draw weight your arrows will not recover as quickly and accurately as they should. If your arrows are impacting the target on an angle (the back is either to the left or right) you have arrows that are not spined correctly for the bow you are shooting them from.
Traditionally, spine is measured by bracing your arrow shaft exactly 26 inches apart and suspending a 2 pound weight from the center. The distance that the shaft deflects when the weight is placed on it will tell you your arrow spine– you then match your spine to your bow draw weight. There are several modifications to this principle that you can do to get perfectly tuned arrows for your set-up, but in general, if you get them close and have a matching set that are all similar that should be good enough. Or at least that’s my opinion. That said, I’m not a world class archer winning shooting competitions so I might not be the person you want to listen to. Basic other principles though: the further from centershot your bow is the lighter spine arrow you’ll need since it will have to flex more to get back to a straight line so if you have an extra wide handle maybe go down a little to accommodate. This assumes you have a 28 inch draw length, if you shoot significantly longer or shorter arrows you will need to shoot a different spine– look up information on dynamic spine to figure out how much, and to see a list of a couple other factors that could change the arrow spine that works best for you.
Design and Materials
First off, don’t get hung up on trying to make yours exactly like mine. All you need is to suspend the arrow shaft 26 inches apart, you need a 2 lb weight to put in the middle, and you need a way to measure how much it has moved. How you do that is entirely up to you, this is simply how I made mine. It was inspired by this one over at poorfolkbows.com and if you want to see a few other testers people have made he has compiled sample pictures of a few on the next couple pages following that link. It’s a great resource to see a few different options.
As you’ll see, I built mine attached to the side of the shelf that holds the scrap wood in my garage. Why? Good question. Mainly because it was convenient and right about the height that I wanted plus I was too lazy to fight past the junk in my garage to get to the wall. You could attach it directly to the wall or build a small stand for it as well. I almost suspended mine from the bottom of the shelf that is above my workbench as well…. And may move it there next time I want to use it. We’ll see.
I used eye screws to balance my shafting on (or run through in this case)– a 5 pack was about $3 at the hardware store. My 2 lb weight? An empty glass salsa jar filled with miscellaneous items: bag of copper powder, allen wrench set, chunk of antler, extra eye screw, etc. The only downside to this arrangement was that the salsa jar is somewhat taller than would be ideal if you were to build a tester that doesn’t have tons of room under it for the jar to sit. Of note would be the fact that I took one of my eye screws and cut out the side of it so it could hook over the arrow shaft and then screwed it into the center of the lid. You need to make sure that your method of hooking the weight onto the shaft is accounted for when weighing. To measure deflection I used a digital caliper because I already had one. Total cost for this project: $3 for the eye screws. Time: maybe 15 minutes (to set it up, longer to actually test all my shafts). Well worth it to ensure I have a matching set of arrows.
Making the Spine Tester
To begin with, I measured out and marked the end points (26 inches apart) as well as the center mark (at 13 inches). Then, I drilled a pilot hole at both of the end points and installed my eye screws. Don’t worry about how far in they are at the moment, just screw them in until they are solid. Above the center point I now installed a ¾ inch thick piece of scrap wood that will give me something to clamp my caliper to. Now run an arrow shaft through your eye screws (obviously, you will need them to be oriented so that the holes face each other….) and clamp your calipers to the scrap board above the center mark and bring it down until it touches the shaft. Does it hit it exactly on the center? If not, screw your eye screws in or out as needed until the tip of the caliper hits the center of your arrow shaft.
Using Your Spine Tester
Using the spine tester is pretty straightforward. Begin by setting your caliper (if you have a digital version like mine) mode to decimal inches. Then insert an arrow shaft into the two eye screws that mark your end points and orient it so that the grain on your arrow shafts is going vertically. Now, slowly bring the tip of your caliper down until it lightly touches the arrow shaft, then zero it out. Attach your weight as close to the center as you can and let it hang freely. Carefully extend the caliper until it just barely contacts the shaft and read the deflection amount. From here, your arrow spine = 26 / deflection. . . Or, you could go the simple route and just look at the spine chart I have posted at the bottom of this page. No sense in doing the math for each shaft, just get your deflection amount and compare it to the chart. Actually, in the long run I would like to make one that’s a little fancier and has a dial/pointer that simply tells arrow spine in 5# increments. That way I wouldn’t even have to look at the chart. Maybe in the future when I have time to think about it. For the time being however, as a $3 last minute way of double checking my shafts to select ones that match, I’m pretty happy with what I’ve got!
Here is the spine chart as promised. As you can see from my picture above, I had a deflection of 0.56 inches and when we look at the chart that corresponds to about a 47# spine (ie for a bow with a 47# draw weight).