Author Topic: Spine Tester Design  (Read 15742 times)

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Offline gfugal

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Spine Tester Design
« on: November 16, 2016, 01:21:19 pm »
I actually made one just recently from a 1x2" board, a small piece of plywood, and some screws and stuff. Super cheep. Took me a couple of days, but worth it. I cut the 1x2" into several pieces: a 27.5" for the main base, 3 sets of two around 9" tall for the posts, then remaining wood was cut into spacers.

The posts have to be tall enough to suspend a weight from it, I didn't have a convenient 2 lb weight and I didn't really feel like making one out of lead so I just used two 1.25 lb weights for a total of 2.5 lbs. My brother is an engineer and we figured out an equation to convert a 2.5 lb bend into a 2 lb bend, and since it has slightly more weight it magnifies any differences a little better making it a little more accurate than if I just did 2 lbs. The Deflection equals the two pounds divided by the heavier weight times the measured deflection with the heavier weight.    x1 = (2 lb/2.5 lb)•x2.    x1 = theoretical deflection with 2 lbs, x2 = actual deflection with 2.5 lbs or whatever weight you use.

In order to measure the deflection, I used a lever method. All you have to know is the distance from the fulcrum to the contact point, and the angle. I figured out the angle by printing off a protractor on several sheets of paper and a long lever arm to magnify it for a more accurate reading. With the angle as well as the distance from the fulcrum to contact point (the Hypotenuse), you can use simple trigonometry to figure out the deflection. The equation is   X = Sin(Θ)•H.    X = deflection, Θ = angle measured, H = hypotenuse or distance from fulcrum to contact point.   

As an addition step, you have to correct that value since the contact point is always going to be lower than the fulcrum so the angle will never start out as zero. In order to "zero" the scale and account for any bends or imperfections in the arrow you need to measure the distance from the fulcrum to the resting contact point and subtract that from the value obtained earlier. It's rather simple, you just do the same equation by getting the angle without a weight.

Lastly, I should mention where to place the fulcrum and how long to make the hypotenuse. All the fulcrum has to be is higher than the arrow with enough room for its diameter. Secondly, it needs to be far enough away, that the contact point sits close to the deflection spot (half way from 26"). The way I calculated the length for the hypotenuse I needed, was to put it exactly on the deflection point at a 40 lb spine deflection (0.65"). That means when it isn't deflected that it will be a little to the right of the actual center point where you hang the weight.

I will included photos and will walk through how I found this shaft's spine weight. First I measured the non zeroed deflection with the 2.5 lb weight (xnonzero=SinΘ•H → xnonzero =sin(39°)•1.34" → xnonzero = 0.84"). Then I measured the correction heigt without the weight (xcorrection=SinΘ•H → xcorrection=sin(10°)•1.34" → xcorrection = 0.23". By subtracting the nonzero deflection from the correction height I got the actual deflection from the 2.5 lb weight (x2 = xnonzero-xcorrection = 0.84"-0.23" → x2 = 0.61". Now in order to get the theoretical deflection at 2 lbs I put it into the equation that I first mentioned (X1 = (2 lb/2.5 lb)•x2 = 0.8•0.61" = 0.49"). therefore the spine deflection is 0.49 inches for this shaft. If I want to know the spine weight you plug it into this equation (Spine weight lbs = 26/x1 = 26/0.49 = 53 lbs). The spine weight for this shaft is 53 lbs.

This is my design. Hope you like.
Greg,
No risk, no gain. Expand the mold and try new things.

Offline gfugal

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Re: Spine Tester Design
« Reply #1 on: November 16, 2016, 01:40:02 pm »
Here's The pictures. I also forgot to mention that I balanced the lever so it was slightly heavier on the right side then the left. That way it would point the left side up and make contact with the arrow. I put a nail in to make the hypotenuse (or distance from fulcrum to contact point). That way it is free to move on its own.
Greg,
No risk, no gain. Expand the mold and try new things.

Offline JoJoDapyro

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Re: Spine Tester Design
« Reply #2 on: November 16, 2016, 02:07:33 pm »
The only issue I see is that the fulcrum isn't in the center.
If you always do what you always did you'll always get what you always got.
27 inch draw, right handed. Bow building and Knapping.

Offline gfugal

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Re: Spine Tester Design
« Reply #3 on: November 16, 2016, 02:32:04 pm »
That's what I thought at first too. But then I realized that it doesn't matter at all where the fulcrum is so long as the contact point is as close to center as you can have it. In this exaggerated photo I drew, you can see the opposite side measures the height from the contact point to the fulcrum, not the height from the fulcrum point to the arrow directly below it. If you inverse the triangle (blue) you see the opposite side is the direct height of deflection from the contact point.
Greg,
No risk, no gain. Expand the mold and try new things.

Offline JoJoDapyro

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Re: Spine Tester Design
« Reply #4 on: November 16, 2016, 02:37:27 pm »
So, do you hang the weight at the fulcrum?
If you always do what you always did you'll always get what you always got.
27 inch draw, right handed. Bow building and Knapping.

Offline gfugal

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Re: Spine Tester Design
« Reply #5 on: November 16, 2016, 02:44:34 pm »
I hang the weight at the center. The posts you see near the center of my spine tester aren't actually centered. I put them about an inch to the left. The contact point is close to the center but not quite center in any of my pictures. It's offset slightly to the right and moves more to the left with greater deflection. Once it reaches about 0.65"/40 lbs it would be at the center point that I hang the weight on. If it deflects more than that then it will actually be moving away again, left of center this time instead of right. Hopefully that made sense, its a little confusing.
Greg,
No risk, no gain. Expand the mold and try new things.

Offline JoJoDapyro

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Re: Spine Tester Design
« Reply #6 on: November 16, 2016, 02:58:34 pm »
Nope, that made sense. I wonder if the measurement is off by a bit since the weights are not centered with the fulcrum. Just curiosity questions. It is a way more advanced setup than my digital caliper and weight.
If you always do what you always did you'll always get what you always got.
27 inch draw, right handed. Bow building and Knapping.

Offline gfugal

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Re: Spine Tester Design
« Reply #7 on: November 16, 2016, 03:09:36 pm »
It wouldn't be off because the weights aren't centered with the fulcrum but there are other ways it can be off, Such as my accuracy in measuring the distance from the fulcrum, whether the nail and hole drilled in lever are exactly centered, the accuracy of my protractor (since its just paper), if I actually hang the weight on the center, and the distance the contact point is from the center. All these things should be pretty negligible and shouldn't cause significant error more than 5 lb spine weight. For example, it would seem that the distance the contact point is from the weighted center point would be significant when actually it only causes an error of a couple hundredths of an inch.
Greg,
No risk, no gain. Expand the mold and try new things.

Offline gfugal

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Re: Spine Tester Design
« Reply #8 on: November 16, 2016, 03:12:32 pm »
Sorry couple thousands of an inch (0.001-0.009 vs 0.01-0.03). A couple hundredths of an inch would be a big difference if we were calculating spine.
Greg,
No risk, no gain. Expand the mold and try new things.

Offline bow101

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Re: Spine Tester Design
« Reply #9 on: November 17, 2016, 11:56:36 pm »
"Spine Tester Design"  Mmmmmm let me see, bend it before it breaks, then shoot it, if it searches for wall flys then break it. If the arrow hits where I'm looking it must be a good flyer.  In a nutshell thats what I think about spine testers.
"The privilege of a lifetime is being who you are."  Joseph Campbell

Offline gfugal

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Re: Spine Tester Design
« Reply #10 on: November 18, 2016, 01:48:33 am »
Some people like something measurable. I understand some people can do this stuff with feel and i respect that but no need to disregard something as pointless just cause its not what you do, or you don't understand it. A measurable value is good even in your situation cause when we do those by feel tests, you can then determine after the fact the spine of that arrow. When you know what type of weight your bow shoots best you can alter those "bad flyers" and salvage them instead of breaking them. Plus you don't have to test each arrow you make several times to see whether it will work with your bow. It's really not that complicated. I put it in an Excell sheet and it does all the math for me.
« Last Edit: November 18, 2016, 10:35:15 am by gfugal »
Greg,
No risk, no gain. Expand the mold and try new things.

Offline bow101

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Re: Spine Tester Design
« Reply #11 on: November 18, 2016, 08:10:40 pm »
Some people like something measurable. I understand some people can do this stuff with feel and i respect that but no need to disregard something as pointless just cause its not what you do, or you don't understand it. A measurable value is good even in your situation cause when we do those by feel tests, you can then determine after the fact the spine of that arrow. When you know what type of weight your bow shoots best you can alter those "bad flyers" and salvage them instead of breaking them. Plus you don't have to test each arrow you make several times to see whether it will work with your bow. It's really not that complicated. I put it in an Excell sheet and it does all the math for me.

OOooooopss...... Maybe I'm off topic a bit......... ::)

I'm not down playing testers, in my experience in the last few years of testing and tuning I find that a few pounds plus or minus really makes no huge difference in accuracy.
A lot of it comes down to proper fletching, arrow weight, shaft dynamics, like straightness and grain orientation for wood shafts anyway.

Good Luck.
« Last Edit: November 18, 2016, 08:22:28 pm by bow101 »
"The privilege of a lifetime is being who you are."  Joseph Campbell

Offline loon

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Re: Spine Tester Design
« Reply #12 on: November 18, 2016, 10:53:34 pm »
Using calipers seems a bit simpler, so I think I'll go with that. Very nice design though. I might end up resting the arrows like that rather than like the one here
I think, once I make a jig, I'll make the parts holding the arrow like the one here.
Quote
http://poorfolkbows.com/images/spine39.jpg
I can barely tell the difference between 40 and 70 spine by feel so I think I need a jig.. Also very hard to tell if arrows flying wonky are because of my release or the spine. Or something else. Sigh

Thanks for that deflection measuring thing
« Last Edit: November 19, 2016, 02:07:32 am by loon »

Offline willie

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Re: Spine Tester Design
« Reply #13 on: November 19, 2016, 12:39:26 am »
nice project G,
One thing about testing that many do not appreciate, is that being overly precise is not such a bad thing when accumulating data. you can always work more by feel, ........once you know where you are going ;)

Offline bjrogg

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Re: Spine Tester Design
« Reply #14 on: November 19, 2016, 08:08:50 am »
I use to "do it by feel" and I made some good shooters and some not so good shooters. I really like my spine tester now. I also am a real believer in bareshaft testing. Once you have arrow that bare shafts good all you really need is something to measure that deflection. It doesn't really have to be same number as everyone else comes up with but it is nice. The important thing is it allows you to make more shafts with the same deflection. I believe proper spine and properly tuned arrow totally trumps fletching. If you have proper tuned arrow you can shoot it accurately without any fletching. If your shooting parallel store bought pre spined shafts probably not as big of deal. But for shoot shafts made for hunting I changed from "feel it" to test it and duplicate it. It is more time consuming but I don't have the bad fliers so I'm time ahead in the long run. I also think it is very important for optimum penetration for arrow to enter animal square straight on. If you don't have arrow tuned your fletching has to square up that arrow and that could take several yards. There are many other tricks like your weight forward of center. Properly halvting your broadhead so they spin test properly. Keeping weights close to the same,arrow length.  Just my feelings on the subject. I think what you got there should work for you gfugal.
PS for tapered shafts I put fat end forward and I need to increase my measured spine by about 8- 10 lbs. over parallel shafts.
Bjrogg
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