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.
