Doug Fir and Cedar not the Same?

[Bjorn]

I have been shooting 75-80# Cedar and 75-80# Doug Fir out of the same bow, same arrow length, same nock, same fletching, same point weight, same same same. Cedar flies weaker-more than 5#. Yet even on the spine tester they show equal. Anyone else experience this?

[aromakr]

Are both the shafts the same diameter? are they both parallel or tapered? What do you mean when you say the cedar shoot more than 5# weaker? What is it doing.
Bob

[Dr. Ed Ashby]

Bjorn,

A spine tester measures static spine. Static spine only measures the relative stiffness of the shaft. Dynamic spine (the spine the shaft shows when shot from an individual bow ... by an individual shooter) can be entirely different.

Different woods have different flexional qualities. They can show equal static spines and widely differing dynamic spines.  

The lesser a bow's degree of center shot, the more evident this becomes. A bow that is WAY FAR from being center shot requires a more precise dynamic arrow spine than does a bow that is near (or at) center shot.

Static spine gives you a starting place, but there’s no substitute for bare shaft tuning, when it comes to getting arrow spine perfect for both yourself, your bow and you arrow setup. That applies to all shaft materials. Check out Easton’s almost fifty pages of step-by-step instructions on how to see if you have the correct arrow spine AFTER you’ve started with the one their chart recommends for your bow type, draw weight, draw length, type of release, point weight and even the type of fletching!

Ed

[Bjorn]

Aromakr: The shafts are both tapered, and marked 11/32 although the fir is easily 23/64. When bareshafting one of the cedars broke on impact shooting at 45' from the bale. Bareshafting, the fir flies very nicely with 4" nock up at 60' while the cedar shaft is uncooperative; violent nock up and left even with different point weights. The bow is my new ACS 3 pc-49#. Dr. Ashby: Thanks for your observations about static vs dynamic spine and different wood types-the difference in this case was quite dramatic.

[aromakr]

Bjorn:
I believe the majority of your problem is the different diameters. If your bow is cut past center which I assume it is. The center of the 11/32 shaft could actually be past limb center. Try placing a thin strip of material like a couple layers of tape on the strike plate before shooting the 11/32 and if that cures the problem.
Bob

[Bjorn]

Thanks Bob, in fact the bow is cut past center I will give that suggestion a whirl tomorrow.
Bjorn

[Bjorn]

Bob, I tried out your suggestion this evening under the lights and Bingo! They now fly the same.
Thanks,
Bjorn

[aromakr]

Bjorn:
I'm glad it worked for you. I think the problem you experienced is why so  many have trouble tuning small dia. carbons to bows cut past center. They just don't shoot well when the center of the shaft is past center of the limb.
Bob