Primitive Archer
Main Discussion Area => Arrows => Topic started by: gfugal on November 16, 2016, 01:21:19 pm
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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.
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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.
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The only issue I see is that the fulcrum isn't in the center.
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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.
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So, do you hang the weight at the fulcrum?
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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.
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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.
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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.
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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.
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"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.
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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.
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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.
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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.
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
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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 ;)
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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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I've never heard of bareshaft testing but it sounds like a great idea. How do you normally tell if an arrow is too strong or too week like that? It seems that main problem is arrows with too strong of spine.
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Bare shafting is just what it sounds like. You take your arrow with point on and no fletching shot it at target from about 6 yards. If it flies straight it golden. If it flies sideways with tail end to the left (for right hand shooter) spine is to weak. To make dynamic spine heavier you can shorten length of arrow, or use lighter point. If arrow flies sideways with tail to the right spine is to heavy. To make dynamic spine lighter you can file or sand shaft diameter or use a heavier point. For hunting arrows you bare shaft with same weight target points as your broadheads, then switch points when done broad shafting. One other tip mark arrow to know which way you have arrow on bow. Sometimes arrow will shoot lousy one way and you flip it over it shoots great. You want to know which way so you fletch it right. Also it's usually best to have stiff side towards bow. It's also important to have constant release and anchor. I hope I explained that so you can understand it. It sounds complicated but once you figure it out it's really simple
Bjrogg
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I had light arrows with a 200gr point (plenty of foc, maybe spined 35#-40#), out of a 50# bow. They were still hitting the target at an angle at 20 yards. Maybe it was also my release, but spine definitely matters
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If spine isn't right for arrow to match bow. They will fly sideways with out fletching. They will if there to weak (tail to the left) and they will if there to stiff (tail to the right). A longer arrow with a heavier point will make actual dynamic spine of arrow weaker. If you cut a inch off same arrow, or put a lighter point on it. This same arrow will have a stiffer actual dynamic spine. The real nice thing about spine tester is once you find the magic combination for your bow you can much more accurately duplicate another arrow. If I didn't explain this so you can understand it just try shooting next arrow you make without fletching ( I never fletch mine without shooting them first anymore). Then flip arrow over and shoot it with other side against bow, or add tip weight, maybe shorten arrow .Once you see what's happening the light bulb might come on. The bow can make a huge difference to. A bow shooting off the handle will be fussier about arrow spine then a bow with arrow shelf cut close to center shot. Hope that helps I'm trying to keep it simple it actually really is.
Bjrogg
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If spine isn't right for arrow to match bow. They will fly sideways with out fletching. They will if there to weak (tail to the left) and they will if there to stiff (tail to the right). A longer arrow with a heavier point will make actual dynamic spine of arrow weaker. If you cut a inch off same arrow, or put a lighter point on it. This same arrow will have a stiffer actual dynamic spine. The real nice thing about spine tester is once you find the magic combination for your bow you can much more accurately duplicate another arrow. If I didn't explain this so you can understand it just try shooting next arrow you make without fletching ( I never fletch mine without shooting them first anymore). Then flip arrow over and shoot it with other side against bow, or add tip weight, maybe shorten arrow .Once you see what's happening the light bulb might come on. The bow can make a huge difference to. A bow shooting off the handle will be fussier about arrow spine then a bow with arrow shelf cut close to center shot. Hope that helps I'm trying to keep it simple it actually really is.
Bjrogg
Yabut. What is a Yabut.? Kind of like a Rabbit.? Nope.! Just a discrepancy in arguments or opinions.
Just finished 1/2 dozen arrows they all fly ok but one. Can you really get any better odds after doing paper tuning or bare shaft tuning.
I have read countless articles about the Masters like Fred Bear. The wall flyers were broken and discarded. Why keep a bad arrow.
Agreed...........?...... :) :D ;D >:( ;) :D ;D :) >:D
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Let's just agreed to disagree 101.
Bjrogg
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Let's just agreed to disagree 101.
Bjrogg
Yup your opinion, what makes you so right.? You disagree with the likes of Fred Bear, and you are an expert....... >:D >:D >:D >
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Happy Thanksgiving 101
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Gfugal I hope this offers you some help. I'm not claiming to be an "expert" but I have learned a few things from people much smarter than me and this is how I use bareshaft test and spine tester and scales to reproduce my arrows as close to the same as I can. I especially like it for my stone point hunting arrows that I want to replicate but really don't want to shoot a lot. Those I make to the numbers that work for my target points and knap point to that weight. I know there are some other good post on bareshaft testing and spine testers. Also a lot of you tube videos out there too.
Good luck try it if you like I didn't invent it but I do use it.
Happy Thanksgiving
Bjrogg
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I'm with ya BJ... ;):)
DBar
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Thanks Bill, you would definitely be one of the people I learned from. Thanks so much for taking the time to show me a few of the tricks of the trade.
Bjrogg
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Happy Thanksgiving 101
I'm on Canada time...
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I noticed that 101, I do hope you had a good one just the same. I really do mean that 101 and many happy days besides
Bjrogg
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Seriously bjrogg...: do you really get 80-85% accuracy from wood arrows...?
Second question how about 30 yard shots. ? Its a tough call especially from a wood bow.
Thirdly, would I attempt a kill at 30 yards. ? not with my shooting ability. Others have though using traditional bows... ;D
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I'm putting this here mainly for my own reference so I can find it again years down the line. Someone asked how to determine spine for an arrow shorter than 26" so you can compare short arrows bendability to longer arrows.
You can figure this based off the equation to the far right in figure 1. The little squiggly thing in the equation in both images represents deflection. The second image shows the simplified equation solved for deflection. Using this equation you can solve for the theoretical deflection at 26 inches.
For example, If you measured a deflection of 0.35 inches at 24 inches then to get the deflection at 26 inches you plug it in.
D1 = D2•(L1^3/L2^3) = 0.35•(26^3/24^3) = 0.35*(17576/13824) = 0.35*(1.27) = 0.44
Therefore the deflection at 26 inches is 0.44 inches or 59 lbs.
This is only helpful if you want to compare to 26" arrow shafts. If you don't care about that then just get a deflection value and use that to compare to other 24" or whatever length shafts you happened to measure.
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I feel my IQ just dropped several points {cant afford that} on all that math to just find out weather your arrow is spinned correctly for your bow. I do like the bareshafting therory thou..
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I'm putting this here mainly for my own reference so I can find it again years down the line. Someone asked how to determine spine for an arrow shorter than 26" so you can compare short arrows bendability to longer arrows.
You can figure this based off the equation to the far right in figure 1. The little squiggly thing in the equation in both images represents deflection. The second image shows the simplified equation solved for deflection. Using this equation you can solve for the theoretical deflection at 26 inches.
For example, If you measured a deflection of 0.35 inches at 24 inches then to get the deflection at 26 inches you plug it in.
D1 = D2•(L1^3/L2^3) = 0.35•(26^3/24^3) = 0.35*(17576/13824) = 0.35*(1.27) = 0.44
Therefore the deflection at 26 inches is 0.44 inches or 59 lbs.
This is only helpful if you want to compare to 26" arrow shafts. If you don't care about that then just get a deflection value and use that to compare to other 24" or whatever length shafts you happened to measure.
sadly it's a bad idea to put stuff for your own reference for years down the line uploaded as images to the PA forum.
[attachment deleted by admin] <3
[img]https://i.imgur.com/EKnZO3D.png[/img]
[img]https://i.imgur.com/HCsYrYF.png[/img]
(http://i1174.photobucket.com/albums/r605/pcoutin/beam%20deflection/9041331396904342415-account_id1_zpssayiq947.png)
(http://i1174.photobucket.com/albums/r605/pcoutin/beam%20deflection/Maximum%20and%20center%20deflection_zps0jwsbpfr.png)
I feel my IQ just dropped several points {cant afford that} on all that math to just find out weather your arrow is spinned correctly for your bow. I do like the bareshafting therory thou..
you describe every day at this university for me
This is only useful for arrows shorter than 26-27" right? Any advantage to using it with, say 33" arrows vs just measuring the 26" closer to the nock? Different if the arrow isn't consistently stiff?
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sadly it's a bad idea to put stuff for your own reference for years down the line uploaded as images to the PA forum.
So that's why people use imgur links on here so much. That makes sense. I know it's been frustrating when I've been reading old posts and the pictures were deleted. It kind of makes some topics obsolete.
This is only useful for arrows shorter than 26-27" right? Any advantage to using it with, say 33" arrows vs just measuring the 26" closer to the nock? Different if the arrow isn't consistently stiff?
You can use the same equation for longer arrows. however, you can also measure a longer arrow at 26" simply enough unless for some reason your rig doesn't have the capability to have the rests that narrow. So really in my mind it's obsolete except for the rare situations. Measuring spine is just a way of figuring how flexible the wood is. Its flexibility is based off the material really, so it shouldn't matter the length. If you measure it at 33" then convert it to a 26" deflection, it would be the same as if you measured it at 26", so why the extra step. The same can be said with measuring at shorter distances. Unless you specifically want to compare the flexibility of a short shaft to a longer one you might as well just use the numbers you get at 24" to use as a reference.
I feel my IQ just dropped several points {cant afford that} on all that math to just find out whether your arrow is spined correctly for your bow. I do like the bareshafting theory thou..
I understand that this can be confusing. I just thought I would put it out there for those who like to do such calculations. To me it's not an inconviencance and I don't mind doing it (I actually rather enjoy it but I'm weird). If your the type of person that it would be more trouble than its worth then don't worry about it.
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Bare shafting is just what it sounds like. You take your arrow with point on and no fletching shot it at target from about 6 yards. If it flies straight it golden. If it flies sideways with tail end to the left (for right hand shooter) spine is to weak. To make dynamic spine heavier you can shorten length of arrow, or use lighter point. If arrow flies sideways with tail to the right spine is to heavy. To make dynamic spine lighter you can file or sand shaft diameter or use a heavier point. For hunting arrows you bare shaft with same weight target points as your broadheads, then switch points when done broad shafting. One other tip mark arrow to know which way you have arrow on bow. Sometimes arrow will shoot lousy one way and you flip it over it shoots great. You want to know which way so you fletch it right. Also it's usually best to have stiff side towards bow. It's also important to have constant release and anchor. I hope I explained that so you can understand it. It sounds complicated but once you figure it out it's really simple
Bjrogg
agree totally, only thing i might add, if i may- is to start your initial bareshafting super close to the target- i never used to- and used to get more breakages- and my thought was the opposite- to get further from the target so the shaft had slowed down somewhat, but all that happened is that the arrows incorrect spine really exaggerated the shafts flight- then they were sure to break.
watch this video- very well explained bareshafting- dont ignore the starting close methodology.
https://www.youtube.com/watch?v=IGOPiriLbcM&app=desktop
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it's kind of ridiculous how i always get such angles, bare shafting with thumb draw.. maybe i'm just too far off in spine
>still not spine tuned after 3 years of shooting
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Using these formulas, I get that the Easton and "old AMO" spine measurements don't correspond. Yet I somehow assumed that they did...
deflectionweight1 = deflectionweight2 * weight1/weight2
deflection28in_2lbs = deflection28in_1.94lbs * 2lbs/1.94lbs
d_26 = d_28 * (2/1.94) * (26^3 / 28^3)
d_26(AMO) ~= d_28(Easton) * .825
edit: I happen to have a 2.5 lbs weight and when I built my spine tester it was about 27" apart, so the formulas are very useful..
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I'm not sure i follow. What were the two deflections you thought were the same? From what i understand, AMO tests at 26" and Easton at 28"?
A .5" deflection at AMO would equal roughly a 0.62" deflection at Easton. D2 = 0.5×(28^3/26^3) = 0.62. So AMO would be about 81% less for the same spine at that specific deflection. The % difference will actually change depending on the deflection since it isn't a linear relationship.
I think your getting thrown off on what the D1 snd D2 mean. They stand for deflection not length. Or maybe your doing something else that i'm not following.
Out of curiosity how do AMO and Easton measure spine? Do they measure it in inches deflected, centameters, in lbs or some other unit?
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AMO has a formula to convert deflection into lbs. Easton just uses deflection, an Easton 500 arrow deflects .5", a 600 deflects .6".