I've heard several people state the "Spine charts are just a starting point; from there you have to experiment" to tune. In a sense that is true, however if you know how various changes affect how an arrow reacts and by how much, those figures can be included into your calculations and eliminate most if not all of those experiments.

To be able to do that you have to know how spine is determined and what it means.

Spine is nothing more than calculating how much a shaft will bend under specific conditions, which is done on a spine tester or (static bend meter). The AMO standard spine tester consists of uprights spread 26" apart and a 2# weighty suspended from the shaft half way between the uprights with the edge grain of the shaft pointing up. You then measure how much the shaft bends in thousands of an inch. This gives you a deflection figure (Some old Easton aluminum charts include that) when aluminum shafts were spined the same as wood. Deflection can be confusing if you don't know how to use it.

If you use that deflection figure, lets say .510 and divide it into 26 it will give you #'s of bow weight=50.9# in this case. That will be the spine for a 28" arrow. Since the decimal point figure is above .5 increase the spine 1# to 51. For every inch the length varies, the spine will increase (if shorter) or decrease (if longer) by 5#. I call this the rule of 5, as wood shafts are usually grouped in 5# groups per dozen. In actually 50/55 is 6#.

This gives you the spine of an arrow that will shoot from a bow factory marked 50@28" and drawn to 28", if that bows shelf is cut 1/8" less than center and has a Dacron string and point weights are 125-145grs. If the bow is drawn more or less than 28" bow weight will differ from marked. I usually figure that at 3#/inch in most bows. that must be included into the calculations.

So, what other changes affect your spine need? 1. Depth of shelf; I usually figure for every 1/16" deeper than AMO standard (1/8") increase one spine range (5#) and for every 1/16" less decrease one spine range. 2. Bow string material; The newer low stretch materials add one spine range.

Let me give you an example of how I would select a spine: Our bow is a custom-made recurve factory marked 47#@28", it has a XXXl low stretch string and the shelf is cut to center. The bow is XX" long. Your draw is 26" and you want the arrows cut to 27" BOP (back of point) and are using a 145gr. point. So, what spine should I select?

Lets start with bow weight: Most bows will gain or loose 3#/inch, at 26" you will be drawing 41#, If you select a 35/40# AMO standard it will spine 40/45@27". The bow is cut to center that's 2/16" deeper so add one spine range (45/50) with the low stretch string add one spine range (50/54)

This shaft made into a arrow 27" long measured from the bottom of the nock groove to the BOP (back of point) should shoot extremely well with no further adjustment if the brace height and nock set location are correct for you. Let me add this; most bows will shoot a range of spines and the deeper the shelf is cut the wider the usable range of spines will be. With my Hill bows once I find the lowest spine it can use I can usually go 10- 15# above that, with recurves add another 10-12#

Bob

Very well written and easy to understand even for this thick headed old Marine. Thank you, your method is what I use for woods as well. Can’t recall who turned me on to this method but it was when I was trying to figure out why the wood arrows I made for my BW 80# limbs went sideways towards the target. Thanks agian

2 thumbs up.

Good post Bob.

Very well explained

Very well explained

Very nice!

Thanks for the very useful information!

Good stuff.

Thanks.

I love that, and have saved for reference. Thanks very much. Almost, that info eliminates need for bare shafting. I mean heeding your advice would put a shooter so close to correct arrow flight, he would have to make only the smallest of changes. Certainly it would make arrow shaft selection close enough that no different spine shafts would have to be purchased. Again, thanks.

You did good, Bob, and thanks.

Good explanation, thanks. Just one comment. Your 3#/inch change in bow weight rule probably works for the bows you and many shoot, i.e., bows that are 55# or heavier. But for lighter bows it's much less. A 30# bow might gain or lose less than 1.5"#/inch.

Another thing is, bows are often not the weigh they are marked at. I've had new bows that were 2# off (or more) from a very accurate calibrated scale that I use, using AMO methods. Some bowyers use different methods or don't have calibrated scales. For vintage bows the difference can be even greater. Up to 5#s off from the scripted weight, in some cases. It really helps to use a scale.

I am going to save this, well done!

However, This is stuff arrow merchants and proshops would need to know. My approach is that having made my own spine tester with an interchangeable dial (the part the pointer points to), I just mark my own arrows on it, and keep a chart for each bow that is weird enough to require it.

The industry way is ass backwards, but meaningful when trying to solve the problem of what arrows will fit the bow of a walk-in. A very difficult question with a lot of ifs in it. But I know what arrows shoot well in my bow, and all I need to do is walk that information back to the spine tester, and I can go directly to proven results.

Excellent and extremely resourceful for my archery undertakings- thanks for posting this!

Sandorweiss: There is nothing perfect in this world, for the most part that method works.

Bowlim: What do you mean "The industry way is ass backwards, but meaningful when trying to solve the problem of what arrows will fit the bow of a walk-in. A very difficult question with a lot of ifs in it. But I know what arrows shoot well in my bow, and all I need to do is walk that information back to the spine tester, and I can go directly to proven results." you see there really is not a lot of "IF'S", spine is not as exacting as some try to make it, and my method is proven with over fifty years of arrow making experience with almost 30 of those commercially.

Bob

Spine wasn't much of a mystery back in the 60's as it is today. We tend to make things difficult in concept, then offer an interactive chart so folks don't have to think. You can see that in all walks of life nowadays; instant answers via the internet, charts to circumvent the need to be analytical, and forums to fit your every need. Don't know? No need to learn, just ask on the internet....someone will know.

Bob, great post. I surely agree on the 10-15# variance with ASL's, Hill style. At my 26" draw my ASL pulls 45# and with the arrows cut to 27 BOP I can shoot from 45-60#. After that I see sighns of to much spine, arrows flying nock right. That being said, I found a few of my nock tapers to be off and that changes a lot. I personally believe that this is where a lot of people have flight problems with wood arrows. Anyhow, thanks again Bob your wisdom is greatly appreciated.

Good write-up aromakr. I know it has been beaten to death, but is it a good idea to bare shaft a wood arrow or do you just get the spine in the ballpark and make sure you have good arrow flight? I don't have any hair left from going through bare shafting carbons. For some reason my aluminums were a piece of cake. I ask because I have some shafts coming from sherwood and am going down the wood shaft road.

Live2hunt: As they say proof is in the pudding! The best way to double check your calculations, is to shoot them at a target with a vertical line on it like a piece of 1" wide piece of masking tape. Make sure that your eye is directly above the arrow shaft and place the arrow point on that line. A properly spined arrow will impact somewhere close to that line. For a right hand shooter arrow that impact to the left consistently are too stiff and impacts too the right are weak. Of course you must be acutely aware of draw length, anchor and release during this process.

I will add this comment, if you allowed Sherwood to select the spine for you, it should be fine. Steve & Bob are excellent arrow makers and know how to select the proper shaft!!! I've known the both more years than I can count.

Bob

The theory of spine makes sense to me. The theory that if the bow and arrow are tuned as such that a bare shaft comes out of that bow perfect, the fletching has little to do except correct errors in the shot, to a point. I agree that it was not looked at as much back when, but we are at that point now to where it is critical for some of us who strive for perfection in our setups. Once it is found that the bow and arrows are tuned, then we know our form/shot needs fixin if we are off. For me it comes down to the thought that if that arrow is coming off the bow odd, the broadhead out front is going to catch air and steer it where it wants to go. In my mind I cannot have that.

aromaker

These are two comments from your write-up, will you clarify for me.

I have already printed, scoured, admired and am getting ready to hang this on the wall above my spine tester.

but I am not sure of your intent on this one part.

1. Depth of shelf; I usually figure for every 1/16" deeper than AMO standard (1/8") increase one spine range (5#) and for every 1/16" less decreases one spine range.

The bow is cut to center that's 2/16" deeper so add one spine range (45/50).

In your example bow, should you have increased two spine ranges?

thanks for the write-up and the clarification.

oz

OZ:

Thanks for catching that, I stand corrected. It should be two spine ranges.

Thanks Bob

Good that is what I changed it to for my final print for the wall.

thanks for doing that.

oz

Have always been curious when the various Vendors sell the shafts in ‘alleged’ 5- pound ranges, Where as you correctly pointed out, 50 to 55 would actually be 6 pounds.

When I weigh and group my arrow shafts (or roundballs for my muzzleloading) I will group them, for example, from 50 to 54, and then 55 to 59, and likewise for the other weight ranges. In this way I only have to worry about the 1st number for whatever the range is, say a’5’, for them to be ‘lower or higher’ within that 10- pound band/range. Works for me!

.... errr, I actually sort my muzzleloading roundballs for competition much tighter than that, but you get the picture.

Thanks for the writeup aromakr. Appreciate how you laid it out as quite easy to follow. I am savings this and glad you took the time to put it here for us.

Thanks Bob, Great explaination

Deno

Heck yea George. It’s technology. I love it. I’ve fixed a lot of things from YouTube. I’ve even learned some grad archery on this site.

I did have a friend that could bend an arrow and tell what spine it was. I just used a spine tester

Pretty much the way I always choose a wood shaft. And it always works. Been doing that long before internet.

Spine test kit with arrows all finished the same really helped me figure out quickly what worked well for my bows.

Well written very nice.

This is very helpful, Bob. Thank you.

Hi Bob

This has been very helpful. I was wondering though can different point weight be also put into the calculation, for example for 20 grain differences? In your example you used a 145 grain point on a 27" arrow, how would the spine be effected by a 125 or 100 grain point or even the other way going up in point weight to 200 grains?

Thanks, Joe

Joe: Point weight depends on where the arrows spine falls within your usable spine range. Example if your bow is cut to center and is able to use lets say arrows spined between 45-60# and the shafts your using spine 55-60 you can probably go up to 160's or a little higher without a problem, but if the arrows your using spine 45-50 then you will be maxed out at 145's to use 160's increase at least one spine range.

The spine formula assumes your using a 125 gr. point and your bow is cut 1/8" less than center, with a dacron string, so you can usually use points of 100-145 grs. Using heavier points will depend on how deep the shelf is cut, the string material used and where the shafts your now using fall with your usable range. that requires some experimentation. The deeper the shelf is cut the wider the usable spine range is, conversely the further away from center the shelf is the usable spine range gets narrower.

I hope this is not too confusing!!! Bob

Good info. Thanks

Great job I love information yeahlike this especially since I’m currently shooting with arrows...thank you very much.

I don't see this working with a selfbow where the riser at the inlaid arrow pass (no shelf) is just over 1" wide, so let's call that 3/8" less deep (or '6' of the 1/16ths less deep/cut) than the nominal 1/8" out from center for the shelf in the info as provided.

The bow is actual 40#s at 27" draw, shooting 28" BOP wood shafts, and uses a low-stretch string.

From the above logic, I see one choosing a shaft of 40/45 to start, but then substracting 6 spine ranges due to it not being center cut. That takes it down to a 10/15 pound spine range, but now we add back in 1 spine range for the string choice.

... would ANY of us really attempt to shoot a 15/20 spined wood 'kids' arrow out of a bow pulling 40-pounds????????

And to the above specs ... Stu Miller's Dynamic Arrow Spine Calculator says that particular bow would need an arrow of 22.7 dynamic spine.

Well that would be in the 20 to 24lb. spine range so it seems to have worked pretty well.

Since as mentioned earlier all bows, and it doesn't matter what kind they are will tune with more than one spine range.

But it could be all wrong. Bob's only building wood arrows for nearly 60 years, for all kinds of bows, professionally.

I’m not questioning Bob’s years ... just trying to ‘learn’ ...

I’m not questioning Bob’s years ... just trying to ‘learn’ ...

Here's a handy chart I reference when playing in the woods. LOL ;)

"I don't see this working with a selfbow where the riser at the inlaid arrow pass (no shelf) is just over 1" wide, so let's call that 3/8" less deep (or '6' of the 1/16ths less deep/cut) than the nominal 1/8" out from center for the shelf in the info as provided. The bow is actual 40#s at 27" draw, shooting 28" BOP wood shafts, and uses a low- stretch string.

From the above logic, I see one choosing a shaft of 40/45 to start, but then substracting 6 spine ranges due to it not being center cut. That takes it down to a 10/15 pound spine range, but now we add back in 1 spine range for the string choice.

... would ANY of us really attempt to shoot a 15/20 spined wood 'kids' arrow out of a bow pulling 40-pounds????????"

I think you made a mistake there.

#40@ 27" - 40-45 spine, add #5 for 28" arrow you are at 45-50 Now cut 6/16 40-45/35-40/30-35/25-30/20-25/15-20. Add #5 for FF and you are in 20-25 spine

Stu calculator gave you 22.7 spine. I don't see where his formula is wrong

Draven - Except less than any arrow below 35#s spine shot out of this bow shows as 'weak spine' by any bare shaft or other arrow tuning test.

When I told the well known bowyer of this info here, who has been making selfbows for many, many years and wrote many of the the article for the TBB series, he had quite the laugh! He too said, and I quote, "For that bow I made, you'll need a shaft of 35/40 or 40/45, full stop".

... and not a 20/25 # rated shaft. Again, please don't beat me because I'm trying to learn ...

No, I don’t, I really just think there was an error there. What’s the bh or fielpoint weight?

Lefty: What you say might very well be true, as I have not made a lot of arrows for self bows, but let me relate something to you in that regard. Paul Brunner has been a customer and friend for many years. some 35-40 years ago he was planning a Cape Buffalo hunt in Africa. He was going to use a self-bow of his own make, it would be in the mid 70#@26" range and wanted to use WOOD arrow with a 135 gr. Magnus broadhead and wanted the arrow to weigh 1000grs. @ 26 1/4" BOP. That is a pretty hard order to fill. The key was the bow! he built it so when strung the string did not track down the center of the limb. It actually tracked toward the strike plate making it 3/16" past center. That made my job a lot easier. I made breast tapered solid Dymondwood arrows for him that weighed some 15-20 grains over 1000 grs. They spined 108#@28" cut to 26 1/4". According to Paul they shot like darts, as they say.

I guess what I'm trying to say is "There is more than one way to skin a cat" The formula I've related may not work in every scenario but it does in ways more than not!

Oh; by the way the Magnus 135gr head penetrated half way through the OFF SIDE RIB. Cutting the entrance side rib in half, a rib that was 4" wide and from 5/8-3/4" thick. Sorry I got off subject. Bob

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