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spine tester

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Swamp Bow:
I just picked up a cheap dial indicator from harbor freight for $10, so I am going to throw together a cheap tester.  After reading up on them, the formulas for all of the testers seem to be based on the personal draw length that builder (I'm guessing), or the "standard" draw length of the "first" person to build and share the plans/concept.  I have a 29-32+" draw depending on bow/style I am shooting that day.  Is there any reason not to set up my tester and corresponding spine chart for 29"?  I don't see why it should not work as long as the ratio of draw length/deflection = spine is still used.  Just checking to be sure I'm not having one of my "duh" moments.

Swamp

boo:
You can make it for what ever length you want. You may have to Make a new scale with known spine arrows though? But Thats no big deal.
When i make a tester i just make the thing to my specs and just use known spine arrows to mark my scale.

scattershot:
The only problem I can see is if you want to spine some shafts for someone else, you would have to convert your non standard scale to a standard scale. If it's just for you, it doesn't matter.

Swamp Bow:
Boo:
I'm using the formula  X/deflection = Spine, where X = distance between arrow supports, 29" in my case.  The ratio should stay the same regardless of what "X" is? Now I'm starting wonder if I need to change the mass of the weight from 2# to adjust for the change in length?  Any MEs out there?

Scattershot:
Yep, I'd have to adjust for other arrows lengths or build a second tester.

I guess I should point out I'm using tonkin. The reason I want a longer tester, is because I've noticed two shafts can have the same over all stiffness but may bend in different arcs depending on the natural taper.  I have shafts that have different tapers and bend more in different areas, but have the same deflection per length.  So if I can measure closer to actual arrow length I think I'll be better off because the shafts are inconsistent in where they bend.  Since the shafts don't bend the same, I could be off if I just measured the same smaller section for different shafts of the same length.

Swamp

CraigMBeckett:
Swamp Bow,

I understand that the British GNAS used to have its own system for wooden bows, elb's I believe, they used 1.5 pound weights and suspended the arrow just behind the point and in just front of the nock, so it varied for each arrow and used the true length of the shaft, not an arbitrary 26 inches like the American AMO.
The GNAS spine scale was:

Bow weight       Deflection in 0.01 inches
Lbs                   GNAS Units
30                   81
40                 61
50                   49
60                   40
70                   35
100                   25
150                  16.5

I have tried to find out more on the GNAS spine scale but have failed and only found a mention of it in a couple of books/papers. Have plotted the scale and they form a curve the formula of which , according to excel, is y=2300.5x-0.9849 where x is the weight and y is the GNAS unit.

Craig.

***edited to show 1 GNAS unit = 0.01 inch not 0.1 as originally shown, my bad ***

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