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Static Spine Measurement Thoughts

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Del the cat:

--- Quote from: avcase on May 08, 2018, 02:31:47 pm ---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

--- End quote ---
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:

--- Quote from: Del the cat on May 08, 2018, 10:28:53 am ---@ 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

--- End quote ---
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.  ;D 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:

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

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.

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