Static Spine Measurement Thoughts

[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

[Del the cat]

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

Yeah, but I'm not sure I want an "equivalent deflection"!
What I want is a static test that is more relevant to the dynamic behaviour of the arrow.
It's the method and set up of the test that concerns me... the actual resulting number is only relevant for comparison.

Let's condense this to the bare minimum:-
My assertion is that we shouldn't test over the entire length of the arrow as a fair bit at either end is irrelevant in the case of barrelled arrows.
Do we agree with the assertion? (We can argue about the detail later  )
Del

[JNystrom]

@ 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  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

I thought the 24" spine test is used just because we want to spine shorter arrows than 26". This way we stay on track on arrow stiffness in a finished arrow.
Whatever you do, the last 1" before the supports, wont be bending much. As in a bow. So in my mind taking that account should be useless, you would be just making this stiff part 2" long then.
I'm not sure... Just spine them and shoot some more.  Maybe something will happen.

[Badger]

  desired spine is based heavily on how much weight ahead of dynamic is being accelerated and how fast it is being accelerated. really has little to do with draw weight which acts as a rule of thumb more than anything else. A flight arrow might have 100 grains total in front of center while a target or hunting arrow might have 350 grains.  The hunting arrow also has the weight right at the tip while a flight arrow has the weight several inches back from the tip. Very little can be compared between the two. I like dels idea of mving the front of the arrow back about 25% of the length and measuring spine at maybe 20" spread. I honestly quit measureing spine, I just push it with my finger but that does little good when you are attempting to pass along info.

[willie]

What I want is a static test that is more relevant to the dynamic behaviour of the arrow.

That would be nice. A static beam type bend test with the load applied laterally, is useful for determining the stiffness of the materiel. The MOE is only one factor to be considered when a load is applied axially to a column. Barreling of an arrow or tapering the ends of a column is primarily about saving materiel and/or weight. Proportionately larger savings compared to small changes of the bend properties caused by the design change. The atlatl dart test tries to account for both shape and MOE.  But finding the buckling load with the axial dart test is still a static test. An arrow under acceleration experiences loads that are hard to duplicate with a static test, so it may be a improvement, but not the full story. Perhaps a more relevant dynamic test can be designed by observing the deceleration of an arrow? Maybe a bounce test of some sort?

Steve has made some good points about complexity of acceleration loads  while I was writing. These things can be calculated, but I honestly think that real life test shooting is more practical. Understand principles may help us to interpret results easier.

[Badger]

  Here is a simple test that might be pertinent to what we are doing. Take a 200 grain flight arrow say 26" long. Now that the center of mass on the front 1/2 of the arrow and divide it by 2 say you have 50 grains. 50 grains accelerated to 250 fps only carries about 6 ft# of KE, not sure how many g's it would be under but If we figured out the g forces I think we could take a weight roughly equal to the g forces and slip it over the top of the arrow while the arrow was standing on end. Maybe slip it down about 1/4 of the distance from the 1/2 mark on the arrow to the tip. Now just see how much it bends when you rock it back and forth a little.