Spine Tester Design

[---GUTSHOT--->]

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.. 

[loon]

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

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[img]https://i.imgur.com/EKnZO3D.png[/img]
[img]https://i.imgur.com/HCsYrYF.png[/img]


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?

[gfugal]

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.

[TSA]

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

[loon]

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

[loon]

Using these formulas, I get that the Easton and "old AMO" spine measurements don't correspond. Yet I somehow assumed that they did...

deflection_weight1 = deflection_weight2 * weight1/weight2

deflection_{28in_2lbs} = deflection_{28in_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..

[gfugal]

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?

[DC]

AMO has a formula to convert deflection into lbs. Easton just uses deflection, an Easton 500 arrow deflects .5", a 600 deflects .6".