Really confused about spine now......

[Ranger Link]

So, there I was.........looking for correct spine for Easton Axis FMJ....

corrected my draw length to 30.5 on my Ally
have a QAD Ultra HD installed 2" back from forward most berger hole
poundage is 67.......so....

poundage 67
draw length 30.5
overdraw 2"
arrow length 28.5
point weight 100g (3 blade broadhead and field points)
blazer vanes

Easton Shaft Selector 2009 tells me 340s
OT2 tells me dynamic spine required is .2869...closer to 300s (this is on 2 Cam X-Hard). If I change OT2 to 2 Cam Medium, it matches the Easton program.

What the heck man? I am totally confused!

Please help.

Thanks,

[PSI]

Answered the same thread on AT...


Your 2008(?) Alligence
draw weight 67
draw length 30.5
overdraw 2"
arrow length 28.5

2008 Alligence specs
Ally w/speed mods 327-335
Ally w/smooth mods 317-325
letoff range 65-80 %

As pointed out above by previous posters, the Easton Charts (plural) use "complete arrow length", whereas the OT2 program uses "shaft material length"..defined as the "raw tube length" of the shaft..which does not include point insert, nock bushing, or nock length. When you use the Easton Charts, make sure you are using the correct one when making your selections. There are 2 charts...one for target and one for hunting applications. You didn't state your indended use, so just be aware that the results from each chart will differ slightly. The target chart will recommend "weaker" shafts given the same specs.

You'll note that I changed your OD setting from 2" to 0" for the computations. The OD setting provided in OT2 is for compatibility with the Easton charts. I recommend that people set OD to "0", because I haven't seen any studies or had any personal experiences that dictate changing rest position has any significant effect on a shaft's dynamic spine.

Assuming the following and your specs from above...

0" over draw
28" shaft material length
100 gr point
12 gr insert
3 x 5gr vanes
12 gr nock

Note..when selecting a bow from the database, OT2 converts the DB cam style to those the program uses based on the stated IBO velocity. When there's an IBO range in the DB, the lowest number is used when "apply"ing.

Selecting the Ally with Smooth mods and applying from the DB = "1Cam Hard" 317 IBO, 65% LO

Optimal "target" spine = 0.3504
Optimal "both" spine = 0.3419
Optimal "hunting" spine = 0.3335

Selecting the Ally with Smooth mods and applying from the DB = "1Cam Hard" 317 IBO. Changing to 80% LO

Optimal "target" spine = 0.3624
Optimal "both" spine = 0.3540
Optimal "hunting" spine = 0.3455

Selecting the Ally with Speed mods and applying from the DB = 2Cam X-Hard, 327 IBO, 65% LO

Optimal "target" spine = 0.3054
Optimal "both" spine = 0.2970
Optimal "hunting" spine = 0.2885

Selecting the Ally with Speed mods and applying from the DB = 2Cam X-Hard, 327 IBO. Changing to 80% LO

Optimal "target" spine = 0.3143
Optimal "both" spine = 0.3059
Optimal "hunting" spine = 0.2975


In between the 1Cam Hard and 2Cam X-Hard designations are 2Cam Hard and 1Cam X-Hard.

2Cam Hard 80% LO

Optimal "target" spine = 0.3443
Optimal "both" spine = 0.3359
Optimal "hunting" spine = 0.3274

2Cam Hard 65% LO

Optimal "target" spine = 0.3335
Optimal "both" spine = 0.3250
Optimal "hunting" spine = 0.3166

1Cam X-Hard 80% LO

Optimal "target" spine = 0.3291
Optimal "both" spine = 0.3207
Optimal "hunting" spine = 0.3123

1Cam X-Hard 65% LO

Optimal "target" spine = 0.3193
Optimal "both" spine = 0.3109
Optimal "hunting" spine = 0.3024


All four cam options will give slightly different results for "optimal spine". As you discovered, you can change the cam style to whatever you desire. This is part of OT2's flexibility. It makes a recomendation when you select the bow from the database, then gives you the option to "customize" selections to meet your experience or needs.


Mapping spine suggestionsacross the 4 cam styles using the "both" filter and 80% or 65% letoff you get the following...

0.3540 1Cam Hard 80%
0.3419 1Cam Hard 65%
0.3359 2Cam Hard 80%
0.3250 2Cam Hard 65%
0.3207 1Cam X-Hard 80%
0.3109 1Cam X-Hard 65%
0.3059 2Cam X-Hard 80%
0.2970 2Cam X-Hard 65%

0.057 spread