Weakening spine formula

Trad Bow 1979

Active Member
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649
So is there a rule of thumb as to how much a heavier broadhead, inserts or weights may change an arrows spine. For example: 300 spine arrow w/40 grain weights attached to inserts, and 100 grain tip. Arrow appears to be on the stiff side. Will a 125 tip improve it or do I need more weight?
 
I use this forget how much it cost per year.

I got it to print my sliding sight and you can input components and it's tells you if you are stiff or weak. Then you can adjust weight add to point or fetching to find correct.

If you have arrow info, components, length, lb and draw of what bow (brand and model). I can input it
 
Awesome thank you!!!

My bow is 29? at 60 pounds.

Arrows are 28 3/4? shaft only. (300 spine gold tip Kinetic). 40 grains of FACT weights attached to insert as well.
 
I shoot the same specs, 60@29. With 29" 300 arrows, 50gr inserts, 150gr heads, it worked perfectly. You'll be stiff with 125gr heads but tune it, it might work just fine.
 
LAST EDITED ON Nov-21-17 AT 05:42AM (MST)[p]>I shoot the same specs, 60@29.
>With 29" 300 arrows, 50gr
>inserts, 150gr heads, it worked
>perfectly. You'll be stiff with
>125gr heads but tune it,
>it might work just fine.
>


I'd like to stick to moving up to125 heads simply because going to 150 May limit my choices of broadheads. So maybe I should stick to the 125 head and add 1 more 20 or 30 grain weight inside? Or just go to the heavier Gold Tip insert which might add enough weight that I don't need to move up in Broadheads. Thoughts???
 
Hey we are old school. Stiff spine, heavy arrow, FMJ 340, 100gr. G5 Striker, 2 inch Blazers, 271/2 inch arrows total arrow weight 566gr. Total pass throughs on elk at 40yds. This shooting at 60# draw weight....we throw the book away and go heavy and still make those long shots its not always about speed.

))))???->
 
>Hey we are old school. Stiff
>spine, heavy arrow, FMJ 340,
>100gr. G5 Striker, 2 inch
>Blazers, 271/2 inch arrows total
>arrow weight 566gr. Total pass
>throughs on elk at 40yds.
>This shooting at 60# draw
>weight....we throw the book away
>and go heavy and still
>make those long shots its
>not always about speed.
>
>))))?�?�?�->


Who said anything about speed? I'm shooting 510 grain arrows at 60 pounds, not 70. I'm just trying to improve my broadhead flight is all.
 
>
>I'd like to stick to moving
>up to125 heads simply because
>going to 150 May limit
>my choices of broadheads. So
>maybe I should stick to
>the 125 head and add
>1 more 20 or 30
>grain weight inside? Or just
>go to the heavier Gold
>Tip insert which might add
>enough weight that I don't
>need to move up in
>Broadheads. Thoughts???

I would bare shaft tune it and see if the lighter set up works. If you can get straight arrow flight to 30 yards at least, it should work perfect. If not, try adding weight, sometimes 25grs is all it takes, insert or head weight it doesn't matter, as long as its up front it will weaken the spine.

Just for comparison, I currently shoot 28.75" 340's with a 16gr insert and a 145gr head, weighs 470grs. One bow is a Bowtech Binary and it loves the setup. The other is a dual cam that will also shoot a 125gr head really well. My head of choice is a Steelforce Phathead in 145grs, after going clean through a deer this yr length wise, I won't be changing heads any time soon.
 
Finally got to running your setup. You will still be on the stiff side with 125 gr tips.

If you want the slower harder hitting arrow go with 125 bh. As others state stiff is better than week.

I shoot 250 VAPs full length with 125 tips. Slow but should pass through on any broadside shoots with 60 to 70 yards.
 
60-70 is a little far for me. I prefer keeping shots within 45 or so. I haven't picked up any 125 tips yet but I did add an addition 40 grain of FACT weights to an arrow to try it out. Thank you for running the numbers for me. I'll let u know how it goes.
 
If it's grouping your liking and will walk back and broad head tune, you should have plenty of weight for what you want to do.

My setup is overkill for most game in US
 
So I added the weight and as soon as that arrow left the rest it looked like that arrow turned sideways LOL. So I think that weight weakened the spine. I ended up making a few more rest adjustments and it seems to be good to go out to 50 yards. I haven't tried 60 yet, but I don't take shots that long on animals anyway. I think the pursuit of getting my broadheads and fieldtips to hit the same is a bridge too far with my bow. I say my bow because I have tried arrows from 400, 340, and 300 spine as well as slick tricks and grim reapers heads all having the same result. They all hit left from where the fieldtips.
 
Moving the rest doesn't bring them closer together? Move rest as if moving broadheads towards field point (if BH are left move rest right) Both should move and gap should get smaller.

I do this at 30 then 80 for real fine tuning. I'm shooting fixed montecs 125gr and grim repeater Razortips. Both hit together 5 to 80. If you can get them together at 30 go out as far as you can group on a target and make extremely small adjustment. Shoot groups of 3 to 5 and take your time over a week or so.
 
Moving my rest in gives me better broadhead flight but it moves my field tips the same distance away. I've decided to leave my rest where it works best for broadheads and just move my sight to transition from one to another. I found that my fieldtips still do fine with the rest offset for the broadheads.
It's odd that my bow shoots broadheads the same (left) regardless if it's 60,70 pounds or 400,340, or 300 spine
 
Yea I've the timing checked on a regular basis, at least whenever I see a little fish tailing while arrows are in flight. i spin test my arrows after each shot when practicing with broadheads too. I'm always watching the level bubble too. I'd say I'm very meticulous in my process of practice.
I've thought about going to a more aggressive helical or even a fourth vane too. My concern would be it creating noise while in flight.
 
>So I added the weight and
>as soon as that arrow
>left the rest it looked
>like that arrow turned sideways
>LOL. So I think that
>weight weakened the spine.
>I ended up making a
>few more rest adjustments and
>it seems to be good
>to go out to 50
>yards. I haven't tried 60
>yet, but I don't take
>shots that long on animals
>anyway. I think the pursuit
>of getting my broadheads and
>fieldtips to hit the same
>is a bridge too far
>with my bow. I say
>my bow because I have
>tried arrows from 400, 340,
>and 300 spine as well
>as slick tricks and grim
>reapers heads all having the
>same result. They all hit
>left from where the fieldtips.
>

I'm curious, what make/model bow are you shooting?

Did you shoot a bare shaft? If so, if it turned sideways which way did the nock swing?
 
I think I ran the numbers for you before but we ran it at 70lbs. At 60lbs, with the 125gr heads and the extra weight on the insert, 340's should work but you need to find the center or at least close to it.

I have a neighbor that has a Hoyt, can't remember the model. Its set at 60lbs at 29" draw. He's been using 400's with 100gr Wacems but over the summer picked up some 340's. He's got the same problem as you do, broadhead will not hit with fieldpoints but the Wacems will so thats what he uses. I have a saw so I cut the arrows for em and glued in the tips and made sure they spun true. Ran the numbers and just to verify screwed in 175gr fieldpoints. He shot it so we could see where it was hitting, then I put on a big ole 175gr 2 blade and he shot that. Bingo, broadheads hit with fieldpoints. He went back to shooting his wacems, he has no interest in letting me tune the bow. lol
 
Actually I'm using 300s not 340s. But had same result with 340 as well. But I was thinking about changing heads too. I tried a grim reaper and did the same.
And so the arrows I use are the Gold Tip Kinetics. I spin test my broadheads before each shot too. So I think the only thing else to do would be to bare shaft tune.
 

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