Arrow spine question

[LittleBen]

I was trying to tune a bow this past weekend and was having trouble, both with the tuning and coming to terms with my results.

The bow is a sinew backed eastern red cedar selfbow,  with a built up handle and cut away shelf.

It's 64" ntn, braced at about 7", I believe (not measured) it draws about 45-50# at 26" but I guess its possible its more. Shelf is cut roughly 3/16" from center (not quite centershot).

Anyway, I was shooting and trying to find a setup that shot well, I started with .500 spine carbons with tipe weight of 150grains. Tried everything from 220grains up fron to 100 grains up front.

The plain 30" .500 spine carbons with 100 grain tips seemed to shoot best, bareshaft seemed to confirm this. The arrow flight was laser straight.

Then I tried some wally world special .400 spine arrows with 125 grains up front (with vanes no less) and they also seemed to shoot really well.

The Stu Miller spine calculator have spine values of like 67lbs for the .500s and like 78# for the .400.

Is it possible that this is really the right setup or am I missing something?
Anyone had this experience?

On a side note, the CE Wolverine Hunter 5060 from walmart are cheap as anything, and with 125grains up front weight in at 495grains. Also the GoldTip insert weights will screw right in. I may have found a new go to arrow conidering the gold tips cost like 50% more. With those kinda grain weights its like they were made for trad, but without the specialty price tag.

[lpcjon2]

I think if you got a scale and figured the actual draw weight, you would save a ton of time and money. You can use a yardstick on a 2x4 and a bathroom scale if you dont have one. Its like your chasing a ghost. Find the true draw weight and work from there.

[gringol]

Carbon out of a sinew backed bow?  I think woodies might treat you better...

[LittleBen]

Luke, I dont know if the bow will tell the difference  but I sure do. I've got a set of Surewood Doug Fir Shafts I'm in the process of making. Need to order some points.
I was just trying to get a baseline for the spine I'd need to I can start at a reasonably point weight. I guess I need to just start shooting/cutting those woodies.

[Dimondback]

You only get accurate info out of the calculator if you put accurate info into it. I was getting some odd results and went back to "square one" by getting an exact measurement for all of the variables for example: I had estimated my Draw length at just under 28", actual is 26 3/4", Estimated my cut-to-center/strike plate at 1/8", actual was 1/16", roughly measured draw weight at 50#, actual was 44#. Ordered some 55/75 based on the estimated results and desired final weight and couldn't get them to fly...used the actual data, ordered arrows accordingly and they bare shafted perfectly within 1/8" BOP cut of the predicted length. I now have perfect flight, at the heavier arrow weight that I wanted and am a happy shooter as a result. Good luck! Hope this helps.

[Dimondback]

Also...to save some study time...the formula I used to get exact strike plate position is:

(A/2)-B = Strike plate position

"A" is the measurement of the limb width i.e. 1.5" for my T/D long bow.

"B" is the measurement from a flat surface to the thickest part of the strike plate when placed window down on the flat surface i.e. .675".

Then: (1.5/2) - .675 = Position
     .75 - .675 = Positon
     .125 or 1/8" = Position
Works for me!

[snag]

I like your formula Dimondback. I guess you could use calipers to measure "B" instead of placing on flat surface...?

[stujay]

Hard to argue with what your bow likes. If the arrows tune as you say then I'd say go with it.

[Dimondback]

Originally posted by snag:
I like your formula Dimondback. I guess you could use calipers to measure "B" instead of placing on flat surface...?

That would work if your bow is truly cut to center. I lose track while rasping on my sight window sometimes (always) and this gets me an exact number that includes the cut from center thickness and the strike plate. Experience may vary but it has made my tuning exact and easy. It is amazing how much better you shoot with a solid tuned set up....