Spine testing - old question I can't find answer to

[stinkbelly]

I, as well as lots of other guys, have asked this question, but I can't seem to find the answer after 45 minutes of searching the internet, so I am going to ask it again. Maybe I will remember the answer this time.

I am trying to determine the spine of some old cedar shafts. I made a setup like I saw on the internet where the supports are set at 28" and a 2 pound weight is applied to the center of the shaft. I measured the displacement both parallel and perpendicular to the grain, but my question is what does this displacement mean?

I got 0.520 -0.596 inches of displacement. What does that tell me?

[Mike Javi Cooper]

First..... wood is measured at 26" centers for a 28" arrow.. the 2lb weight is correct.

[bountyhunter]

Not sure which plans you used for your spine tester. Do want to point out that the spine of wood arrows are measured at 26 inches and not 28. To get the spine of a shaft you divide 26 by the amount of deflection of the shaft in inches. In this case if your deflection is .52 you would divide 26 by .52 and your spine is 50 pounds. If your deflection is .596 then your spine is 44 pounds. Hope this helps.

[stinkbelly]

Thanks. That is exactly what I was looking for. I guess I read it wrong and did 28". I will redo it at 26"

Thanks again.

[stinkbelly]

Changing from 28 to 26 inches decreased the deflection up to 20%.

[Mike Javi Cooper]

Changing from 28 to 26 inches decreased the deflection up to 20%.

Yes..

[Selfbowman]

Well I'll say...

[JEFFRO]

Well I'll say...

spine.....

[twistedlimb]

One more thing to look at is the spine difference between the tightest grain and the loosest grain in the shaft. Look at the shaft and turn it so the grain lines that are closest are on top and then chech the spine, then rotate it 90 degrees - this should be the loosest grain- the difference may be minor but regardless go with the lighter of the two for determining what you have. When you fletch- make sure to allign the cock feather on the tightest grain.

[meangene1969]

Good thread