Spine tester-maybe I'm just wishful thinking.

[Flinttim]

I made a tester a few years back using a dial indicator . Now the wishful question.Why could I not remake it to be adjustable  ? ie: make the supports for the shaft adjustable so that say I have a shaft that spines high, I can adjust the supports outward till the indicator reads the spine I need, mark the length and make the shaft that long ? I know increasing the length of any shaft weakens the spine so would this not be a good way to nail down that length ? Would this be true dynamic spine ?
 Also would both supports need to be adjustable so as to keep the dial indicator in the middle or would only one adjustable support work ?

[Jim now in Kentucky]

If it  worked,  my guess  would be  that you would need to move both supports. But it's also  my guess that the effort would not  be useful. Just checking  the spine with supports at 28 instead  of 26 would  give a different reading, but all  shafts  would  read weaker than they really were . ?????  IMCO

[Flinttim]

Do you think it would just be better and as easy to use the "5 # rule " for shafts longer or shorter than 28" ?. I now cut my shafts at 28" which is the "standard". If I had a shaft that spined 60#, add 2" (30 " shaft ) to get 50# spine ?

[DesertDude]

CupCake, a sponser on here. Has solved all this out. The Spine-O-Meter uses one tester with a 26" span. The indacator gives the results of woods, aluminium, and carbon. Alot of thought has gone into this Tester. You can change the indacator and adjust the needle to give the results of higher spine shafts. Now when I test a wood shaft, I can look and see what Aluminum and carbon shaft is equal in spine. I know he'll be by soon to go into the details. All I know is when he first showed us it, all I could say was "You made this?" I ask him if he could make a Indacator that would give you the spine of all 3 shafts? He did all the math and converted the metal & carbon to the 26" span and put the all on the same indacator.... Thanks again Kevin for a great "Spine-O-Meter"

[Fletcher]

I have heard of doing this before, except for making shorter arrows.  Adding an inch per five pounds is the easy way, but measuring it would probably be more accurate, esp if you started going even longer/stiffer.  I believe you would need to move both supports to keep the weight and max deflection in the middle of the shaft.  I keep the supports on my old meter at 26", but they are set up to be adjustable.

Give it a try and let us know what you find out.

[Cupcake]

I did a little calculation that shows me you might be on to something.  The deflection of a shaft in a spine tester would vary as a a function of the span to the third power, L^3.  Technically, you would have to vary both supports to keep the weight in the center of the span; however, applying the weight at small distances from the center won't have too much affect, try it if you can.  I can calculate how much it is affected, but not tonight.

The calculation I did that leads me to think that you may have something is as follows:

K2*L1^3 = K1*L2^3

Were K1 and K2 are the respective stiffnesses of two shafts, and L1 and L2 are the two spans.

Suppose we have a 50# spine at 26 inches and want to find the span for a 60# spineto get the same deflection, we have:

60*(26^3) = 50*L2^3

L2^3 = (60/50)*(26^3)

L2=27.63 inches after doing the math.

This is pretty close to "add 5 pounds per inch" which would give us the answer as 28 inches.

In the end it is probably more complicated than this but this is pretty close to years of established methods.

This is not dynamic spine, I believe.  To me dynamic spine has to do with the natural frequency of the arrow.  Changing length, and point weight changes this natural frequency.

I have to stop here and get to bed, and we can leave natural frequency for another time.
Kevin

[DesertDude]

LOL Kevin.....

[tecum-tha]

Another problem is,that the AMO does not specify the original measurement set-up,just the calculation (26/deflection).Since everything was developed back in the 1940ies?
This calculation shows a more accurate range from 40-60 pounds spine than for 60# spine and above. Here it gets more and more inaccurate. That means in heavier spines,measurement errors have more influence than with weaker spine weights.
It is right that one can compare the static  wood spine to the static aluminum and carbon spines using CUPCAKES formula. Dynamic spine is a whole different story, because the mass is distributed differently on wood and aluminum tubes. Carbon is a whole different story again, because it is a composite material in comparison to aluminum.
Naturally,wood is a composite,too,but the developers of the spine charts had no reason to compare.

[John Nail]

Tim, I believe I'd just sand the crap out of the stiff one.