Black Frog

To me, straightness is fairly important, but spine matching is even more important. I have a very accurate spine tester, and when it comes to fixed blade broadheads I'd rather have .005" runout and consistent spine across my arrows vs. .002" straightness and big variances in spine.

And not only spine differences from shaft to shaft, but spine differences WITHIN the arrow itself- like having stiff or weak sides of the shaft.

As the energy of the bow is being transfered to the arrow shaft on the shot, spine variances will affect how that arrow leaves the bow- slight changes in how it bends and how it reacts to the energy put to it. The differences in how that shaft reacts from spine variances equate to erratic (or poorer) broadhead flight.

Going through different manufacturers arrows can be eye-opening as far as spine consistency.

Ever notice they advertise specifications for straightness and/or weight, but not for spine tolerances?

brucelanthier

As far as spine consistancy: Is there any correlation between spine consistancy and arrow straightness that you have seen? Are good quality arrows that are as straight as the manufac.'s specs usually consistant in spine as well? I don't have a spine tester but was wondering if I could take for granted that a high quality arrow would be consistant in spine if it was consistant in straightness.

Black Frog

Not especially. Maybe, maybe not. I dug up this post that I had made with spine results from a while ago. This was for a dozen of Easton Epic C2's and a dozen of ACC's. I didn't record the straightness specs, but I think they were within what Easton had stated:

My friend wanted to stick with all-carbon arrows and bought a dozen of the Easton C2 Carbon Epic 400's. I prefer ACC's and bought a dozen of the 3-49's for this upcoming elk and deer season.

First the C2's. They advertise +/-2.0grains across a dozen. Following weights are RAW shafts with NO nocks in them.

Spine: Weight:
.402 262.0
.401 261.8
.408 258.6
.390 259.6
.407 258.6
.396 259.5
.388 260.7
.397 260.1
.388 261.2
.390 260.1
.403 261.2
.400 262.7

Spine span of .020" and a weight span of 4.1grains. One shaft had a noticeable spine wobble of over .007" as it was turned with the weight on it. That was the worst shaft, some better than others.

No onto my ACC's.

They advertise +/-0.5 grains across a dozen- Following weights are shafts with ACC nockbushings installed from the factory.

Spine: Weight:
.375 291.0
.374 291.4
.373 290.9
.376 291.1
.373 291.1
.371 291.6
.373 291.6
.371 291.8
.374 291.2
.375 290.9
.372 291.5
.376 291.0

Spine span of .005" (pretty nice!) and a weight span of 0.9 grains. This kind of shows why the ACC's are consistently such an accurate arrow. I did notice one shaft had awobble of almost .005" in spine as I rotated it on the tester. That shaft is getting weeded out immediately....

So you can see that both sets of shafts were decent as far as "straightness" specs, but which do you think will yield more consistent fixed-blade BH flight?

I've always liked ACC's. They are a damn good, consistentarrow.

I haven't played with the new gen of Easton's alum/carbon hunting arrows so I can't comment on them yet.

davepjr71

Black Frog

That's a good post. May I ask what manufacturer and model of spin tester you have?

I'm just using a pine ridge. It's easy to use and you can quickly see if there is a problem with your arrow.

SwampCollie

How exactly is spine measured anyway? I understand what it is...but is there an industry standard for the amount and places of pressure applied to an arrow?

brucelanthier

Black Frog - Thank you. May I ask what spine tester you use? Could a homemade spine tester (I've seen plans around the 'net) be good enough quality?

TFOX

Pretty much same results I have had with my ACC's and I checked mine after a couple years use and afew deer and all but 1were still that close.(I believe that was the one that hit a tree)[]


I spin tested mine and most didn't even hardly move the needle of my indicator.




I agree spine is the most important but nock alignment and broadhead alignment run a close second.The shaft itself isn't as important if the front and back are in alignment with each other.


I also believe arrows are much more important than the bow used to launch them.Spend extra on arrows and save a few bucks on the bow if money is an issue.

Black Frog

Swamp-
The industry standard for measuring spine is 880grams hung from exactly a 28" span.

My spine tester is one I made myself. I posted this when I first built it couple of years ago:

JeffB had asked me a while ago if I ever got around to making a spine testing jig that I had mentioned I was going to build, and if I had any pics. We all know how busy we get and things get put on the back burner. For a year (or more).

But I finally got around to making what I think is a damn nice spine tester, and extremely accurate as well. I'm sure it can't compare to the ultra precise laser type spine testers that Easton uses, but for a jig with a dial indicator I haven't seen any better.

Couple of things to think about when making a spine tester. I had seen several home-made versions similar to this one:

http://www.jamesmhill.com/Spine_Tester.html

Have a look at the pics in the link and note the way the dial indicator is mounted BELOW the weight. That is a key factor in this design. After looking at several designs like this I thought it would be easy enough to make with some scrap aluminum I had laying around at work. I got to thinking about the standard weight of 880grams and how I was going to get to that amount for the weight.

I had a 1" travel dial indicator that I could use and I started tinkering with that and then the light bulb went on. When you press the plunger of the indicator and get near the 1" of travel, what brings it back to its starting point? A SPRING inside! That spring has a force, and in the designs where the indicator is below the weight- it is OPPOSING your downward weight. And the spring force isn't linear. The force increases as you stretch that internal "return spring". So I wanted to know how much force I was dealing with. It surprised me when I got near the 1" of travel it was approaching 100grams!

So in the designs where the dial indicator is mounted below the weight, you have a variance of your final downward weight depending on the deflection you're measuring. That 880gram weight could be off by more than 12% (880 down, 100 up) if the deflection is around an inch (at least with my indicator)- I would assume that others indicators are similar in construction. I suppose it would be ok for arrow-to-arrow comparisons, but if you're trying to actually measure the spine deflection amount it could be outta whack by quite bit I would imagine. The weaker the spine of the arrow your trying to meausure, the more the return spring is stretched as the deflection amount is greater and the more it would "help" the spine and it would seem stiffer than it really was. [:@]

Wow, that was surprising to me. So what to do? If you remove the back cover of the dial indicator, you can simply unhook the return spring. So I need to mount it above the shaft. There is enough "dead weight" in the plunger that the indicator works just fine without the spring if mounted above the shaft. The dead weight of the plunger just rides on top of the shaft ADDING to our hanging weight. So I simply measure the dead weight of the plunger on a accurate scale and it came out to just about 29.0 grams. Cool- now I knew my final hanging weight should be 851.0 grams to equal the correct 880 for the industry standard.

I have seen some weight "hooks" that hang on the shaft. I didn't want a hook that could possibly scratch into an expensive carbon arrow shaft, so I made a symmetrical hanging device that uses bearings on each side of the indicator. That way I know my indicator is measuring the lowest part of the deflection. If the hanging apparatus was off to one side of the indicator plunger (like only using two bearings) I could not be sure of getting the indicator in the correct spot for the full deflection amount. Thousandths of an inch make a difference. Having bearings also allows very accurate rotation of the shaft while the weight is haning in place. Fairly easy to find "spine runout" or a stiff side of a shaft this way.

The outside supports also use a couple of ball bearings. Currently at 28" span at the inside edges of the bearings, but I'll mill some more holes in one inch increments for spans of 26", 24", etc.... I have some 27.5" arrows with fletching on them, and while I couldn't measure the industry standard 28" span on them anymore, I could at least do arrow-to-arrow comparisons at 24". I would estimate that I'm withing .010" of the span as I had a couple of thousandths of play in the bolt holes on each end bracket. Close enough for me!

Now I can exactly see how consistent my arrows are from advertised spine and from arrow to arrow. Thanks for prodding me along Jeff!

Straightarrow

I'm another who agrees with Black Frog. I'd go even a little further in my estimation of importance of spine. I've found that when I spine test my carbon arrows, I end up with much better shooting arrows than if I don't. I trim from both end when possible and cull the arrows with spine ranges that are too far out from the average. Then, I test for the stiffest side and align the cock feather with that mark. Last, I make sure the broadheads spin straight. Combine all this with helical fletching and I've been able to get some really crappy carbons to fly amazingly well. I don't even test straightness, other than spinning the broadheads.

The difference from doing this and not doing this, has been night and day for my broadhead flight. I'm really quite satified shooting the middle grade of Gold Tips, that I normally wouldn't even touch without doing this procedure.

As a side note, Black Frog's spine tester is amazing. Mine has the indicator mounted underneath the arrow, so the spring does affect the reading. I've realized that I don't really care, because when I'm testing a dozen, I'm looking for consistancy and it does measure that. I don't care if the actual numbers are .309 or .306. I just care that I can compare one arrow to another. Now, if you're making arrows for others and someone orders a spine of .342, you don't want to send them .375. The only place I know where this might happen, are guys making traditional wood arrows for others.

If you decide to make one, I definitely recommend that you follow his design and hange the weight below the shaft and mount the indicator above.

davepjr71

Black Frog,

That's a great post. Thank you for the information. The one on your link is more in line with what I could build witohut machining tools.

Straightarrow,

Good additional info as well.