Dynamic spine - bareshafting mysteries

[mrjsl]

Ove the past year, I've shot two different bows very similar in weight (57# @ 29" and 59# @ 28.5"), and 3 different CX heritage arrow setups that are almost identical in dynamic spine, but very different in flight. Here's what I've discovered.

Arrow 1: (46.4# Dynamic spine) CX250 cut to 29.75 with 50 gr insert and 250 gr point. 647 gr. Bare shafts fly to same point of impact as fletched, but don't actually fly very well, no matter how much tweaking is done. This arrow is not forgiving at all. Wet feathers really tell the tale. Must have perfect release.

Arrow 2: (45.9# Dynamic spine) CX150 cut to 29.25 with 50 gr insert and 175 gr point. 537 gr. This arrow flies great, bare shafts fly great. In fact I can and have shoot good 3d scores with bare shafts. They tend to stick in the target slightly nock right (stiff) but they always group with the fletched shafts. Occasionally I can see fletched shafts kick a little on a bad release.

Arrow 3: (46.1# Dynamic spine) CX150 cut to 30.88" with 50 gr insert and 125 gr point. 503 gr. This arrow flies the best fletched IMO. Bare shafts fly nock high and left a little and impact weak (right) a little. Fletched shafts always fly well, no matter what.

Currently I'm hunting with arrow 2 and 3, and got rid of all arrow 1. Can't decide between arrow 2 and 3, but leaning toward 3. May cut it a little bit - 30.75 or so and see how that works.

I'm posting this to illustrate the obvious - that there must be something else to dynamic spine  and good arrow flight that Stu's calculator doesn't consider, because these results are consistent, and the comparisons hold shooting through two different bows as well.

Incidentally, according to Stu's Calc. both bows require 75+ lbs dynamic spine, which is obviously off.

[Night Wing]

Just curious. Are both bow's shelves cut to center or past center and if past center, how much past center for each bow?

How about the strike plate thickness. How thick?

There are 14 boxes in Stu's calculator and if you input the wrong info in just one of those boxes, it's going to throw you off.

[mrjsl]

3/16ths past center on both bows, and both have the same strike plate - just velcro

[NBK]

Using Stu's calc. I have to go about 20+ lbs. lighter in spine than what it tells me.  I don't know if it's the point weight (300gr. up front) or what, but there's no denying how my bareshafts and broadheads fly with the lighter spined arrows.

[mrjsl]

I've long been aware that I need less dynamic spine than the calculator calls for, but my point is that all these arrows are about the same dynamic spine, yet have very different flight characteristics, and one of them is totally unacceptable.

Therefore, dynamic spine must not be the only factor in achieving good (and forgiving) arrow flight.

[Javi]

I think most people find a slightly weak spine to be more forgiving when shooting off the shelf with fingers.

On the arrows you have compared have you tuned for the stiff (seam) side of the shaft?

[ChrisM]

I had simular experience with beman bowhunters. One was 400's and the other wad 500's.  Same dynamic spine when done but a huge difference when shot.

[xtrema312]

Sounds like the weak one shoots the best bare shaft, but still a little stiff.  I would try to weaken it a little more and see what happens just for fun.  

I  have tried CX shafts and never got one to fly right.  250’s seemed too stiff and the 150 was way weak.

I also find I need to go about 20# weaker than what the calculator says when I use 100 gr. inserts.  With point weight only and alum inserts I get closer.  I find a big difference in where that weight is setting.  Moving the center of the mass acts about like the same in shaft length difference.

[xtrema312]

Originally posted by ChrisM:
I had simular experience with beman bowhunters. One was 400's and the other wad 500's.  Same dynamic spine when done but a huge difference when shot.

Same here.  I shoot mostly GT 5575’s, but also Beman bow hunters.  With both at the same dynamic spine per calculator the 500 is way weaker.  However, I find the 500 appears to be close to being what the calculator says I need where the 400 would be way to stiff.  Only issue is I run out of cutting room and arrow weight for my draw length and bow weight for the 500 so I never get tuned to see just how accurate the calculator is.

[metsastaja]

couple of things about the calculator


1.If you are using a specific bow from the data base you need to add the thickness of the strike plate. If using the generic you can just enter the number ie -3/16 + 1/8 = -1/16. Look at my signature you will see a link to bow center cuts.
This list was put together from members of TG, bowyers and manufactures.  

2. Asks for BOP not Cut length. you need to include the additional length of nock to the valley and extension of the insert. On my setups about 1/2 for nock and 1/8 for brass insert extension.

3. For any insert longer the .999 you need to use the footing box

If using 100g brass insert you need to check yes for footing and  .5 for length. no need to add weight since insert weight box takes care of it.

EFOC many of us have played with and experienced radical differences from the way an arrow shoots and the dynamic spine suggested by the calculator.  20lb spine differences are not uncommon.  I went so far as to buy a 35lb bow because the arrow I built, for my 54 rated the arrow for a 35. Yes I got great flight from both bows. Stu and many users of the calculator are aware of this yet no one can explain it.

[Night Wing]

The Center Cut/Strike Plate Position is where most errors occur when using the calculator. This is why if a bow is cut 3/16" past center (-3/16), you don't put that number in the box because the strike plate thickness has to be subtracted out.

My Blacktails are cut 3/16" past center (-3/16), but my velcro strike plate is 1/8" thick (+1/8) which is equivalent to (+2/16) so: (-3/16) minus +1/8 (+2/16) gives me a (-1/16) number for the Center Cut/Strike Plate Position box.

In essence, if you put (-3/16) in the Center Cut/Strike Plate Position box without subtracting the thickness of your strike plate, your arrow is going to be way off and you'll see it when you shoot it. I suspect this is the problem.