Dynamic Spine calculator. .NEW DOWNLOAD AVAILABLE

[pointystick]

Okay, I measured my Wesley incorrectly. I initially had it at +1/4" offset, but I ran across an illustration of how to measure, remeasured it, and it turns out to be between +1/8 and +3/16, right around +5/32. SInce this isn't an option on the pulldown for offset, should I use +1/8" or +3/16" for my offset number?

Hmmm...maybe I should just figure the numbers for both +1/8 and +3/16 offset and split the difference between the final numbers?

[pointystick]

On another note, speaking of discrepancies between the calculator and actual experience, allow me to add my own. According to the calc, my Wesley needs about a 46# spine (measured between +1/8 and +3/16 offset).  I just finished putting together a dozen lam birch arrows that I've had stashed for a while, because I thought they might be too stiff. They're 5/16" diameter, 50-55#.  I put my usual 3 5" RW shield cuts @ 120 degrees and 125 gr. field points on them and started shooting at 20 yds.  I started full length and they were noticeably weak. When I got down to 28" on the nose (nock saddle to bop) they flew like lasers.  I don't have a spine tester, so I'm trusting the vendor's numbers (Allegheny Mtn), but assuming they all truly fall into the 50-55 range, such arrows would give a spine between 55.3# and 60.8#, according to the calculator. That's 9.3 to 14.8# over the 46# number for the bow. That fits in well with what I've read so far about people using arrows 10# or so overspined for HH bows, but misses the calculator's "ideal" 2# variation between arrow and bow by a wide margin.

The only shortcoming with these arrows is that FOC is only going to be around 10%, even if I used 140 gr. HH broadheads (the heaviest 5/16" heads I know of). So...for hunting shafts, I'm thinking of some Surewood firs, with 100 gr. Woody Weights and either 150gr. Woodsman or 160 gr. Magnus I, which will give me about the same overall weight as the lam birches (about 14 gr. per lb), but an FOC of around 20%. The thing is, I figure with all that weight up front I'll likely need 65-70# spined shafts to maintain the same spine range as the birches.

Ultimately, it looks like the calculator is good for figuring basic arrow and bow spine numbers, but the suggested 2# variation between those numbers seems way low to me, at least for Hill bows.    :confused:

[Rob DiStefano]

pointystick, i think that for the most part, stu's dynamic spine chart is at least  a spine base line that works very well for most folks, and way better than any static spine chart (i.e. - easton and others).  

we are all "experiments of one", and there can be valid differences in what works and what doesn't.  i'm one of those anomalies in that suggested spine weights prove way too heavy and weak spined shafts with heavy points works best for me.  

in the long run, as with most things in life, there's no substitute for personal testing and surely ymmv!  

[Fletcher]

Back on page 3, Wingnut asked for the formula for converting deflection to pounds.  It is 26/deflection in thousandths, ie: 26/.500 = 52 lbs.  This works for the 26" centers/2lb weight method.

I find this calculator very handy, but about 5 lbs light for me, shooting various wood arrows from various bows.

Many thanks to Stu Miller for making this available to us all.  I'm sure he has more than a rainy afternoon invested in it.

[pointystick]

Hey, guys. I hope I didn't come across as knocking Stu's work.  I was only adding my experience to reiterate the fact that not all bows will necessarily follow the suggested formula.  I plugged some of my cedars into the formula last night and they came out very close to the suggested weight (about 2-5# heavier than the calculated bow weight). Like I said, the claculator seems to do very well at figuring basic arrow and bow spines, but actual results can vary quite a bit. As Rob said, there's no substitute for personal testing.   :cool:

[metsastaja]

I just got a reply from Stu regarding the center cut discussion.
-Stu Miller

PS. Regarding Les' question above, yes the strike plate should be included in the centercut input. Building this out is a great way to tune for a slightly underspined arrow and the caculator can help identify how much is needed. On additional word on centercut measurements, since this is a critical input and somewhat difficult to measure accurately, I will share a simple method I use. Please refer to the attached diagram.

 

[jrchambers]

wing any word on the arow dynamics?

have tested a few of my buddies setups on here, only the ones with good flight were close or weak
high spine was the usual wobler

[daveycrockett]

My main bow likes arrows 10lbs. under what this calculator suggests. Funny my other bows are real close to what it says.

[wingnut]

Nope, still waiting to get them.

Mike

[hkmp5]

Hello All,
I appologize for any confusion caused by the use of this calculator.  There seems to be a lot of questions coming my way about static spine measurement methods, centercut measurements, more carbon shaft details, etc......so....in hopes of clearing some of this up I have added the Carbon Express Hertitage shaft data to the dropdown list and also a section the will convert both ASA/AMO and ASTM deflections into AMO spine poundage.  I agree with pointystick's point above that this will not work for some bows out there.  It was designed around modern R/D longbows and recures and will get you very close for the majority of traditional bows out there.  For the others you can tune in and when you find a good physical setup then run the numbers through the calculator and find out what the delta is between the bow and arrow numbers.  For that particular bow you can then simply use this diffence which will remain constant when you try different arrow setups.  
-Stu

[southpawshooter]

No apology is necessary Stu.  The calculator is obviously a labor of love and I, for one,  appreciate your sharing all your hard work.  The calculator has been extremely useful for me.  Thanks again!

I will be looking forward to the updated version when it is released.

[wingnut]

Stu,

How do I get a copy of the new REV?

Mike

[metsastaja]

Stu thanks for the continued hard work. I have tuned 2 bows in using the calculator and am in the process of doing 2 more that I am changing over from b-50 to fast flight strings and setting up for a week of hog hunting.

Where is the new revision available. I am very interested since I use CE heritage.

[wingnut]

Here is the latest edition of the calculator

Allows custom input of deflection and conversion to spine weight.  Also has some more carbons listed.

 http://briefcase.yahoo.com:80/bc/[email protected]/vwp2?.tok=bcCz6kcB3cFO.9ke&.dir=/Public&.dnm=Dynamic+Spine+Calculator+Rev3-20-09.xls&.src=bc


Mike

[NW Jamie]

Originally posted by wingnut:
Here's a dynamic spine calculator that lets you set up your arrow with the FOC you want and calculate that spine the bowyer was talking about.

   http://briefcase.yahoo.com/bc/[email protected]/vwp2?.tok=bceRJecBj15UbCko&.dir=/Public&.dnm=Dynamic+Spine+Calculator+Rev1.xls&.src=bc    


Mike

Wingnut,
Thanks for the calculator and this thread, it's great! I used the calculator and had a couple of different sets of arrows made up and with out going into details, the set up that worked on the calculator fly like darts. Best results that I have had, without going through extensive trial and error. I realize that calculator is just a tool and can not be taken as absolute, but it sure got me to a great starting point. Thanks.

[Van/TX]

Stu, is the man.  His calculator is the best thing since sliced bread    ...Van

[metsastaja]

Love it. What a time saver.. This version works on my Mac so now I am mobile..

[NorthernCaliforniaHunter]

I can't open it. I get an error message...

[wingnut]

You need too download the file to your computer and use it with Excel.

Mike

[metsastaja]

Make sure you get rev 3-20-09 I think it is the only one on the link now.