How To Make a Spine Tester

By Sam Harper

It has been difficult writing this build along because there's a lot of information I want to include, but I'm having trouble deciding how to organize it. This is my third re-write. I guess the simplest way to do this is to show you one simple way to build a spine tester. Then explain how to use it and read it (page 1). Then explain how to account for the difference between static spine and dynamic spine (page 2). Then show you some other ideas for spine testers (page 3). Does that sound good?

First, let's give credit where credit is due. The spine tester I'm about to show you was inspired by a fellow whose handle on The Leatherwall is "Sailor." He came up with the idea of making a homemade spine tester with a caliper. Mine works exactly like his does; it's just built a little differently.

Almost all of the spine testers basically work like this: an arrow sits on two rests spaced 26 inches apart, and a 2 pound weight is applied to the middle. Then there's some mechanism to measure how much the arrow bends, and that is correlated to some spine.

There are various ways to make two rests 26 inches apart, various ways to apply weight, and various ways to measure the deflection, so you can use your imagination and come up with your own design. But here's how I did it.

I started off with a piece of plywood 3/4" thick.

I can't remember where I got it, but I can promise you I didn't pay a dime for it. People are giving away free scrapwood on craigslist all the time, and you can find this stuff in piles of rubbish at construction sites or in their garbage. The only requirement is that it be at least 28 inches long and maybe 10 inches wide. Mine was bigger than it needed to be, so I cut it down to 30 inches and left the width.

Then I made three pencil markes along one edge spaced 13" apart. So...0", 13", and 26". I used a square with my aluminum yard stick and drew verticle lines with those points.

That's my big toe in the bottom left.

I wanted to make sure that when I put the arrow on the tester and hung a weight from it that the weight would have room to hang, so I set my weight (which I'll show you later) on the plywood and measured a few inches up to make room for arrow deflection, and made marks on the 0" and 26" lines for my rests. That resulted in my rests being about 6 inches up from the bottom of the plywood.

I decided to use 3/8" dowels for my rests, though I could've just hammered in some nails. I used a 3/8" frostner bit to drill a hole so I could have a flat bottom and make a good glue joint, but I guess an ordinary bit would've worked as long as I didn't drill all the way through.

Sorry it's blurry. I'm a terrible photographer.

I used Titebond III to glue in the dowels, making sure they came straight out of the plywood at 90 degree angles and didn't lean to any side (because then they wouldn't be exactly 26" apart).

That's a walnut dowel I made myself, by the way. :-) It didn't cost me anything because my brother-in-law gave me a bunch of walnut flooring. There was a little play between the dowel and the hole, but with all that glue in there, it was easy to adjust the dowel to where it was 90 degrees from the plywood in all directions. Once I got that straightened out, I set it where the cats couldn't get to it and let it dry.

Meanwhile, I went to Discount Tires and said, "Do you have any of old wheel balancing weights that you're just going to throw away?" The guy I talked to told me they recycle them, but he let me have some anyway. I showed him my tuperware and explained that I just needed "about this much" which was actually more than I needed. He was happy to let me have some.

I don't know if you've noticed it yet or not, but so far this spine tester has cost me next to nothing. I guess you could say I spent a little money on gas to go get this stuff, I paid for the glue, and I paid for my Veritas Dowel Maker I used to make my dowel, and I paid for my pencil and yard stick.

Next, I made some macaroni and cheese.

Notice the bamboo spatula. I made that with some of my scrap bamboo from my bamboo backed bows. Resourceful of me, eh?

Then, I cut open a can of tuna (in water) and poured the contents into the macaroni and cheese.

Then, I ate the macaroni and cheese and rinsed out the tuna can.

I took the tuna can with the weights and a small hook screw to the grocery store, proceeded to the fruit section, and placed the tuna can in one of those scales.

Sorry about the glare. Again, bad photography on my part. Notice that it reads 1/16 lbs (or 1 oz). So I droped in the hook screw and added weights until it was 2 lbs and 1 oz.

***You ought to keep in mind that if you do this, you'll be putting lead weights on the same scale that people put their food in, and lead is poisonous. Think of the children! If you're going to do this, at least be careful to put the weights in the tuna can, and not directly on the scale. Of course there are alternatives to using a grocery store scale.***

It's important that the weight be exactly 2 lbs, and since I'm going to cut the tuna can away, I needed the weights plus the tuna can to weigh 2 lbs more than the tuna can by itself.

It's best to put the heavy weights in first until you're close, then put the smaller weights in last until you reach your target weight. By putting the smaller weights in last, you can tweek the weight and get it exactly right.

Once I got the weight right, I put the tuna can, hook screw, and weights in a zip lock bag, left the extra weights in the tuperware, and took it all home.

Once I got home, I put the tuna can on the stove, put the weights in it (left the hook screw out!!!), put it on the stove, and turned the fire on low.

Lead is poisonous, so I opened windows and turned on the fan above the stove. If you're concerned, you might want to do this outside on the grill or something. I once tried this with a candle, but the candle just didn't generate enough heat to melt the lead.

Having the stove fire on low was taking a long time, so I turned the fire up. It took about 15 minutes for the lead to completely melt. I kept a bamboo screwer handy to make sure the lead didn't fall over the side as it melted.

All the metal that was attached to the weights floated to the top, preventing me from getting a smooth surface. Anyway, I let that cool off for a couple of hours, then used some snips and needle nosed pliers to get the can off.

There's my hook screw at the top of the picture. Don't accidentally leave that in there when you melt the lead!

You don't even have to use a cooking spray because the tuna can has a non-stick surface already. :-) It's easy to get off.

Next, I spray painted the weight just so I wouldn't have to handle that lead all the time. It's poisonous, after all.

The last time I made one of these weights, I used fishing weights (which are more expensive than getting them for free at Discount Tires). The fishing weights didn't have metal things attached to them, so when I melted them, my weight came out a lot more smooth. Here's the front and back and both of my weights.

The old one is prettier because I used blue, red, and black. The new one has more personality because of the texture on one side. They both weigh exactly 2 lbs, and they'll both accomplish the same thing.

I tapped in a small finishing nail to make a pilot hole for the hook screw. Then I screwed in the hook screw.

Notice how I have the hook oriented to where the plane of the hook is 90 degrees from the plane of the weight. That'll help later on.

Okay, let's get back to the spine tester. I cut the top of the board about three inches from the arrow rests.

I guess I could've done that before putting the dowels in, but I didn't.

Next, I layed an arrow across the rests, then glued and screwed a 3 inch piece of plywood on the 13 inch verticle line, just above the arrow, so that the top of it protrudes above the baord.

And I drew a verticle line on it that lines up with the 13 inch line.

At this point, you can decide whether you want to attach it to the wall or make feet for it so that it's free-standing. You could attach it to a wall by nailing it, screwing it, or putting brackets on the back of it and hanging it like a picture. I went with feet because I already made a spine tester on my wall the other day (see video).

I just glued and screwed these pieces of plywood to the bottom. The advantage of a wall hanger is that it's always out of the way, and you're unlikely to lose it. The advantage of a free-stander is that it's mobile, and you can loan it to your friends.

The spine tester is essentially done. All you have to do now is put an arrow on it and clamp a calper to where the bottom of the caliper barely touches the arrow shaft.

If you used nails instead of dowels, your arrow might want to roll around, making it hard to get the caliper to sit right on top. It'll do that with dowels, too, just not as much. You want the caliper to sit right on top of the arrow, and not off to one side or the other.

This is easier if your 3 inch piece of wood you clamped the caliper to is thick. Using 1/4" plywood will make it difficult because the arrow will be sitting right against the wall. How will you hang the weight from it? I used a 3/4" piece of plywood. To make sure the arrow doesn't roll around while I'm getting the caliper zeroed in or hanging the weight from it, I cut a tiny groove on both dowels with a triangle file, then widened it a little with the rattail file.

You don't want to cut those grooves too deep because the crowns of those dowels are exactly 26 inches apart. If you widen that crown, the arrow will be sitting on rests slightly less than 26 inches apart. Another option is that instead of cutting grooves, you can just create a groove by wrapping tape around the dowel. This is especially a good idea if you're using nails. You can create a grove with the tape.

You can avoid this grooviness altogether by using hook screws instead of nails or dowels. In that case, you can screw the hook screws in or out to get the arrow to line up perfectly with the caliper.

How to use the spine tester

To use the spine tester, place your shaft on it with the tree rings verticle, not horizontal, because when you shoot your arrow, your nock should be lined up to where that's how the arrow will bend when it's shot.

Of course if you're using bamboo or some kind of rose shoot or whatever, ignore that. Just be sure you're spining correctly according to where you're going orient the nock to shoot it.

Next, zero in the caliper. That is, set the caliper to zero, lower it until it barely touches the arrow, then clamp it in place. Make sure the caliper is lined up with that 13 inch line on the plywood because you want to measure from the exact center between those two 26 inch rests.

Then put the two pound weight on the arrow as close to the caliper as you can, i.e. as close to the 13 inch line as you can.

Then open up the caliper until the little doohickie at the bottom barely touches the arrow.

Then read the caliper. It should be pretty straight forward if you're using a digital caliper. But if you're using a cheap one like mine, you'll get a fraction, like 15/32", and you have to convert it to a decimal since the spine chart uses decimals. You can find a spine chart by googling it, but here's the one I'm using.

15 divided by 32 is 0.46875. If you look at the spine chart, that's between "0.473 = 55#" and "0.465 = 56#," so your spine is between 55 and 56 lbs.

Where did they come up with this chart anyway? That might be handy to know in case you're ever in a situation where you don't have access to the internet. Easy:

26 deflection (in inches) = weight (in pounds)

It's a mystery to me why this works, but there it is.

You could make your own chart if you wanted to by replacing the decimals with fractions. I just printed this chart, calculated the decimals from 9/32 to 28/32, and wrote them on the spine chart.

You can tape or glue this spine chart to your spine tester for ease of reference.

Notice that the fractions are closer together on the lower spines on the left and farther apart on the higher spines on the right. That means the higher your spine, the less precise your measurement will be. My caliper reads in 1/32" intervals.

But you can get 1/64" measurements because if the arrow is on a line, it's a 32nd, but if the arrow is between two lines, then it's a 64th.

That will give you a little more precision in those higher spines. Another option is to use a 4 pound weight for higher spines instead of a 2 pound weight, but then you'd need a different chart. I read somewhere that you'd just need to double all of your inches. For example, on the AMO chart above, 0.289" of deflection equates to 90# of spine when using a 2 lbs weight. If you use a 4 lbs weight, 0.578" of deflection will equate to 90# of spine. I don't know if that's accurate or not, but that's what I read.

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