spine tester

[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 ***

[Swamp Bow]

Craig:
  I'm not understanding something I think.  More specifically, the second column is labeled as being deflection in tenths of an inch and as GNAS units.  I understand deflection as a measurement, but not as GNAS units, or are they the same thing?  I'm probably just missing something simple, wouldn't be the first time. Thanks

Swamp

[boo]

I guess with a 29-32 in draw, what you are going to have to do is, make your arrow rest on each end adjustable. Then i would make a scale that never changes and record my known spine arrows at each setting, 28/29/30/31/32/. You dont have to change the weight unless you want the scale to read for all lengths.

[CraigMBeckett]

Sorry Swamp Bow,

Was trying to give too much info at once without an explanation.

1 GNAS unit is 1/100 of an inch that is 80 GNAS units is 0.8 inches, 100 GNAS units is 1.0 inch etc.

Craig


***edited to correct unit to 1/100 inch had written 1/10, my bad ****

[Swamp Bow]

I guess with a 29-32 in draw, what you are going to have to do is, make your arrow rest on each end adjustable. Then i would make a scale that never changes and record my known spine arrows at each setting, 28/29/30/31/32/. You dont have to change the weight unless you want the scale to read for all lengths.

A variable length tester is next, this one (which is now usable but not complete) is set at 29" which is my minimum draw.  For the next tester I had thought to make a deflection chart(scale) that had separate columns that showed deflection for each arrow length for a given spine.  For example:

Spine    29"       30"      31"       32"
29#     1.000"   1.035"  1.069"  1.103"
30#...


Just to be sure I understand, if I want the deflection to stay constant regardless of arrow length, only then would I need to change my weight to test different arrow lengths?  For example, if I want a deflection of say 1" to read the same spine for a 29" arrow as well as a 32" arrow, then I would have to change weights for the different arrow lengths.

Swamp

[Swamp Bow]

Sorry Swamp Bow,

Was trying to give too much info at once without an explanation.

1 GNAS unit is 1/10 of an inch that is 80 GNAS units is 0.8 inches, 100 GNAS units is 1.0 inch etc.

Craig

Okay that makes sense.

In that case, where does the arrow length variable fit into the equation?

Swamp

[boo]

Sounds about to me. Im sure there is a way you can do it where you use either the same weight with an adjustable scale or the same scale with a different weight. I just think it would be easier to use the same weight and scale with adjustable arrow legs and different marks on the scale per length. The simplest spine tester ive made is two nails in the wall and a weight with marks marked on the wall for different spine deflection. Not as accurate, but works better than doing it by hand.

[Swamp Bow]

One weight system = less stuff to loose.    I might make a simple tester like yours, just to get in the ball park when sorting shafts.

Swamp

[CraigMBeckett]

Swamp Bow.

I would use 1 weight, one indicator scale and make up a table for different lengths if different lengths is the way you want to go. The beauty of the GNAS system is its simplicity, single table fits all, all you are interested in is weight of bow and arrow spine at your draw length. The US AMO system was designed for arrow manufacturers and is used primarily to compare shafts with each other.

Craig.

[Swamp Bow]

Thanks Craig.

Swamp

[Pat B]

Google James Hill spine tester. It will show you how to make a spine tester and he has the conversion table there too.