Spine 340 vs 400

[gordydog]

What inch 400 do I need to spine the same as a 30" 340?

[Orion]

Hmmmm.  Interesting question.  You're talking dynamic spine, right?  Static spine doesn't change with arrow length.

Regardless, a .400 spine equates to 78-79# static spine, whereas the 340 is about 92-94#. Assuming a commensurate difference in dynamic spine, you would have to shorten the .400 enough to achieve a 14-15# gain in dynamic spine.  Was a thread a while back in which folks provide anecdotal findings for the I spine changes in 1/4-inch increments, but I can't find it and can't remember the results. If I had to take a WAG, I'd say about an inch, but that is a WAG.  Perhaps someone who's done it will let us know.

Might also consider reducing point weight on the .400. 

[Wudstix]

 

[McDave]

This would not be difficult to determine on your own, and would be more accurate than someone else’s experience, since the effect of a spine change is somewhat personal to the person making the change, at least in my experience.  You presumably already have a 30” .340 spine shaft.  Buy a .400 shaft and cut it to 29.75”, and put the same point on it you are already using.  Shoot them side by side, and continue to cut in 1/4” increments, if necessary, until the bare shafts fly the same.  If you seem to be getting close, you might want to change to 1/8” increments for the final adjustments.

[Wheels2]

I plugged in my numbers n the newest Stu  Miller calculator using GT Traditionals.  I can shoot a .400, 29", 145 tip.
To get the same dynamic spine using the .340 spine version, I had to go to 32" shaft in order to get matching dynamic spine.