With the long inserts available for carbon arrows it seems that shaft length for spining should be back of insert, not BOP. In other words, a 30" shaft with 2" of insert would increase spine similar to a 28" shaft?
Also with the options of new long insert/outsert adapters  the BOP can be 1.5" beyond the carbon shaft,thus acting with more leverage to decrease spine?
Anyone with a spine tester address these questions?

This will be interesting to see some of the responses. I bought arrows with brass inserts,and had a hard time tuning them. I've had way the best luck with standard inserts,and if anything footing them.

I'll be checking out what people say too.

Clearly, beyond a certain distance, adding internal insert weight to the front of the shaft does not change spine much, if at all, because the shaft length is effectively reduced by the length of the insert. In fact, removing the end section of this point resulted in the shaft testing slightly weaker, not stiffer as you would expect.  From now on, my own rule of thumb will be that if I want a weaker shaft, I will extend the shaft length by an amount equal to the length of any added internal weight that extends more than about 1 1/2" into the shaft.

But doesn't the weight of those brass inserts cancel out whatever added spine they may add? Only reason I use them, besides getting a bunch gifted to me is to raise the shaft weight without having to buy god awful long screw in points.

If the screw in adaptor/point threads and insert is 2 inches or so, I can't see it having much, if any, effect on the static spine.  And it's effect on dynamic spine might be close to a wash.  An increase in spine caused by more stiffness in the front would be counterbalanced, though perhaps not perfectly, by the additional weight of the adaptor and insert. 

Since we know that increasing the up front weight decreases the dynamic spine, it would seem that the actual weight, rather than the length over which the weight is distributed, is the more important factor. 

What I know from my own experience is that if you change from one insert to an insert of the same length, but heavier weight, you will decrease the dynamic spine.  If you add additional weight behind the original insert, without increasing shaft length, then the results are ambiguous, meaning that dynamic spine could either increase or decrease.  The effect of adding additional weight behind the original insert is offset, although not perfectly, by reducing the working length of the arrow.

McD, if I understand what you're saying, it's that if one makes the insert/weight long enough, for example, more than 2 inches, the greater length might have a greater effect on dynamic spine than the increased weight.  Certainly possible. 

I still have to believe that unless your preventing the arrow shaft from bending in the "middle" then it's the weight, not the length of the weight that matters.   

Quote from: BAK on September 05, 2021, 07:26:56 PM
I still have to believe that unless your preventing the arrow shaft from bending in the "middle" then it's the weight, not the length of the weight that matters.   

If you define the "middle" as being the middle of the flexible part of the shaft, then it's further toward the back of the arrow if there is a long weight in the front of the shaft than it is if all the increased weight is forward of the insert.

Anyway, this isn't very difficult to test.  Compare the difference in spine (if any) with a 200 grain point vs a 150 grain point and a 50 grain suplemental weight behind the insert, and let us know what you find.

I've stopped using those deep seated long inserts and now have an easier time tuning arrows. Keep it simple. If I want more weight I'll do it with a heavier head.