Stu's calc prediction vs fact ???

[Don_G]

I checked the dynamic spine predicted by Stu's calculator for the actual arrow tuned for the bow.  According to the calculator, the required spine for the bow is 73#.  The arrow comes out at 50# according to the calc, yet it flies great. Why the big diff?

The bow is a 52#@28" ACS TD CX that is in the data base. I put .100 for the strike plate thickness, and my measured draw length of 29.5"

The arrow is a 30.25" to BOH, Axis ST 300 with a 2.5" footer weighing 25 grains. I entered 1.6" for the footer per the directions.  The arrow was checked bareshaft and fletched by the guys at Rocky Mtn Specialty Gear, and trimmed to the final length. The head is 265 grains, the adapter 111 grains. The measured weight and FOC agree closely with the calculator's predictions: 750 gr /26%, but the spines differ greatly.

Any help understanding this big difference would be greatly appreciated.  Anybody have experience with heavy heads and/or EFOC arrows and Stu's calc?

[Kentucky Jeff]

A 300 spined shaft is a very stiff shaft.  But all that weight up front makes it act like a much weaker shaft.   You also have a fairly long shaft which makes it more flexible than a shorter shaft.   So the dynamic spine on your arrow works out to be roughly matched to your bow.  Which is the point.

[Don_G]

Well, I think it's fairly closely matched, but Stu's calc says it is 23# to 25# underspined, which is a huge difference. Hence my question.  

[saumensch]

When it comes to FOC higher then say 15% Stus Calculator gets to its limit. At least thats what i found.

[metsastaja]

It happens with Extreme FOC and Stu can not explain it.  Go and find a bow that matches up with your predictied spine..I imagine the bow will be in the 25-35 lb range.  In my case though slow and aim was higher then normal the arrow flew beautifully.

[Piratkey]

my set up have allways 4# to 6# more in dynamic spine than indicate the spine calculator.
This is also depend the release form,i think.

[wingnut]

A lot of the differences is in the riser measurements.  Actual past center and strike plate thickness.  A little more to center will effectively strengthen a weak shaft and allow it to fly well.

It's real interesting to experiment with a riser with an adjustable centershot like a Dalaa.  You find out real fast how it makes a big difference.

Mike

[Don_G]

Thanks, guys. Having seen this I strongly suspect that the arrows I built according to Stu's calc and sent to a friend in Tanzania will be severely overspined. I did leave them full length, but I'm afraid they'll still be too stiff.

Thanks again.

[xtrema312]

From what I have gathered wood and aluminum shooters look to get real close, but carbon shooters vary a lot. I get much closer with 500 spine shafts, but I am way off with 400 shafts  and high FOC.  

[jacobsladder]

I agree...I think the calculator is great for woodies and aluminums......but I think carbons are a different animal.....u can't just add 5 # of spine for each inch taken off...carbons stiffen real quick when shortening.....and the difference between one inch shorter and two inches shorter can be huge

[Caughtandhobble]

Personally I've have had the best of luck with Stu's Calculator. I just ran your setup and it shows 175-200gr to be the best. Did you actually bare shaft tune, I was a little confused? Good Luck!!!

[calgarychef]

Don I think the difference lies in the fact that you're shooting a compound and can adjust the centershot.  We trad guys have the arrow bend around the riser and that's where the bareshafting comes in so nicely.  If the arrow is overspined it shoots left of where we aim.  But with compounds and an elevated/drop away rest there is nothing to bounce the arrow off of.  So it might actually be a lot more forgiving of shaft spine....I dunno because I dont' have experience with that stuff.  If I were you I'd go to paper tuning instead of bareshaft and then report back with your findings.  

Hopefully this thread doesn't get pulled because I used the "C" word.  Your efforts in broadhead design might be of great use in the trad community.

[tradbower]

On another site Stu says he is coming out with a new updated version of the Caluclator. This might make a difference in the calculations

[Don_G]

chef,

This arrow is for my new ACS trad bow described in the first post. Stu's calc will not work for the other bow types at all.

The situation is that I did bareshaft tune an arrow to the bow with the help of experts at RMSG.  I then came home and tried the arrow on Stu's calc. My hope was that I could use the known good arrow to find my "personal Form Factor" on Stu's calc, to help predict proper arrows in future.

Both the bow and the arrow are in Stu's data base, but the difference persists when I enter all the info by hand as well.

I started this thread looking for an explanation for the relatively huge difference between the observed results and those predicted by Stu's calc, but I think we are now past the point of diminishing returns.  

Maybe the calc will work better for "more normal" point weights and FOC.  It's too bad, because I like to use heavy points and heavy arrows.

[kenn1320]

"The arrow was checked bareshaft and fletched by the guys at Rocky Mtn Specialty Gear, and trimmed to the final length."

So they were shooting it, or you were shooting it while they watched? At what distance were you comparing the bare versus fletched arrows? Were you looking at flight of bare shaft, or just grouping with the fletched shaft?

[Don_G]

I was the only one shooting. I mentioned RMSG because they are truly experts - so it's not just this noobie's notion.  We started with a bare shaft. They started off showing weak, as we cut they stiffened up nicely. When we stopped the bare shafts were flying perfectly as far as weak/stiff (no side-to-side.) They were flying and sticking just a bit tail high, which they said was desirable. We did this at 10 and 15 yards. We then fletched them and they flew like darts.

I re-tested with bare shafts and fletched shafts at 20 yards later. The arrows as cut appear in all respects proper for the bow and head weight. It's just that Stu's calc says I need a 23# stiffer arrow!

[Jake Diebolt]

I have the same thing - my arrows on Stu's show about 15 # weak with my setup, but my bare shafts group good out to 20 yards, and they fly straight with no left or right lean. I think a lot depends on individual shooters and form - my release is not the smoothest out there, and I may need a weaker arrow to compensate. For other people it could be a host of different reasons.

[calgarychef]

Sorry I read your initial post then came back a while later and made my response and totally forgot about the bow you're using....I'm feeling a tad stoopid!  

Have you looked at OL Adcocks site about bare shaft tuning.  It doesn't use knock up/down it compares the group of fletched and the group of non fletched shafts.

[Don_G]

chef,

It surely could be my form, but I can see in the arrow flight when I get a bad release.  

Within the limit of my skills the bare and fletched shafts group together.  

I have not shot a lot of broadheads yet - and only at 20 yards - but they seem to fly like darts, too. Again within my skill level they seem to group with the practice points.

[calgarychef]

Ok so you have some arrows that seem to be flying ok.  Get shooting until you get consisntent with the new bow, then go back to the tuning stuff.  Tuning gives me fits, but it's worth doing and re-doing until you've got good consistancy.  When your shooting is good and the equipment is nicely tuned 3" groups at 25 yards should be very common.