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Static Spine Measurement Thoughts

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Del the cat:
Static spine in the usual sense is pretty meaningless for flight arrows, but it's obviously useful for comparing shafts.
I think that with our heavily barrelled or tapered shafts the standard measurement between say 28" centres is no good.
I'd suggest the shaft should be supported at a distance equal to 3/4 of the draw length, this will stop the extreme ends which are small diameter giving too low a reading.
So a 28" flight arrow would be supported between 21" centres. The scale on the tester could be left the same... it doesn't really matter as long as it's just being used for comparison.
My spine tester happens to have 24" centres and has been calibrated for that. Moving the centres in to 21" (for my 28" arrows) obviously gives a higher reading as there is less unsupported shaft but I'll do some measurements to see if the readings differ when using the centre distance divided by deflection.
Out of interest, just using the existing scale the reading went up from about 35 to about 50.
I'll do some measurements and get back!
Meanwhile, does the team think this has any merit as an idea?

Just done the measurements and arithmetic, and with the 21" centres it read 52 spine. So it's measuring the static spine of the stiffer centre section, which probably has more merit than measuring including the tips.
Del

Badger:
  I agree with you Del, taking weight off the front is the equivalent to raising the spine. Rule of thumb is usually 25 grains point weight = 5# spine. These little 200 grain flight arrows have very little mass, with no point up front we pick up 25# spine right off the bat. I usually figure about 20# spine for a 26" flight arrow measured at 24" will fly pretty good out of a 50# bow. 

JNystrom:
28" spinetest is done with supports 26" apart.
24" spinetest is done with supports 22" apart.
I regularly use the 24" spinetest. This deflection i then compare to the normal deflection chart. It gives me a "wrong" spine, but its just a reference point.
Even if i shot longer arrows than lets say 26", i would still use the 24" spinetest, since this would give me a reference point to my other flight arrows.
My flight arrow for the short osage bow was 46# spine at 23", shot out of 84# pound bow.

Spine test effectively measures only the 22" span. What about the rest? If you have really skinny tip on your arrow and the arrow would be much longer than 22", the skinny more bending part of arrow would be left out of the spine test. Wouldn't this be a problem, or just a little part of the equation?

Length of the arrow makes a difference in spine, which is also a point i struggle to adapt in my flight arrow making. How much does an inch in arrow length lower the spine rating? I don't think its exactly 5 pounds...

I like spine testing my arrows, it still gives us some proper data. In the future i will try the buckling method that Willie here previously suggested. It takes into account the arrow as a total, arrow length being indifferent.
http://basketmakeratlatl.com/?page_id=534

Del the cat:
@ JN :)
Leaving the skinny bit out of the test is the whole point of using a shorter distance!
The last couple of inches of an arrow aren't going to flex at all (same as the very tips of a bow) and most of the bend is going to at the centre.
My view is that supporting it 1" from each tip is unrealistic.
The tips are fee to move during loose... E.g The point isn't held rigid and the nock end is also allowed to move because the string isn't constrained from sideways movement.
It's just my feeling that supporting at 3/4 arrow length gives a more realistic figure for a flight arrow based on my observations.
Del

avcase:
To get an equivalent spine deflection at different spans, you need to scale it by the ratio of the cubed span lengths.

For example, to get the equivalent deflection on 22” span as 0.5” deflection across a 26” span, the deflection on the 22” span would be as follows:

Defl(@22”) = (0.500)* (22^3)/(26^3) = (0.500”) * (0.6058) = 0.303”

Alan

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