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Thread: Spine Opinions

  1. #1
    Member icb12's Avatar
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    Default Spine Opinions

    I'd like some opinions.

    I'm considering changing broadhead weights. My current setup tunes great, shoots great, kills great. But I just put new strings and cables on the admiral, and figured after break-in would be a good time to make the change if I'm going to.

    Current set up: Bowtech Admiral, 70#, 28.5" Draw. Arrows are Easton FMJ 340s at 28 7/8", fobs, and 100gr montecs. Total Arrow weight right now is 427.

    I'd like to jump up to 125gr broadheads, possibly even 150s. Just to increase my FOC, get a little more weight, & a little more cutting dia. Looking at single bevels if that matters.
    125s I don't think would be any issue. I think with the 340s they would tune and shoot fine. Disagree?
    150s--? I'm guessing I would be underspined. Could lower the poundage I suppose... Thoughts?

    I'm not going to switch arrows. I can always just buy a 150 or two and give it a run and see what the paper says... but I thought I'd ask around first.


    Opinions?

  2. #2
    Member DanC's Avatar
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    I'm hardly qualified to respond as I am far from being an expert but I have just gone through an extensive tuning process for my main compound hunting bow and you may benefit from my experience. The whole process is similar to working up a set of handloaded ammunition for your favorite rifle and took a few weeks to accomplish. I made extensive notes of my observations during the process and here is what I did:

    In addition to the arrows I have been using for hunting, I purchased three examples of every arrow I thought might work for my needs. I did jump the gun and have them all cut to my normal draw length, instead of waiting to tune them to my bow, but that made everything easier when adding inserts and calculating weight and FOC. I also purchased a set of field points for the weight of each broadhead I was considering. I even purchased some weight tubes and brass inserts for adding weight to the front of the arrow.

    I first tuned and paper tuned my bow.

    Next, I determined all possible combinations of arrow weight, field tip weight, insert and weight tube addition and created a spread sheet.

    I then shot groups of three arrows of each permutation through a chronograph and into a target at 20 yards. This allowed me to calculate KE and M(omentum) for each arrow set. I also took note of where each set of three arrows grouped and determined which arrows my bow "liked" best.

    I was amazed to learn that Easton Axis FMJ 300 with 200 gr field tips shot best and grouped best through my bow. Calculated KE was 65 ft-lbs (88 Joules) and Momentum was .58 Slugs. Second best through my bow were my standard hunting arrows, Beman Camo Hunter 400, but with 5 gpi inserts and 200 gr field points added. KE and M calculations were about the same as the FMJ arrows. FOC of these arrows was around 15%. Total arrow weight was around 9 - 10 grains per pound of my bow's draw weight. Adding more weight to the arrows resulted in diminution of KE with this particular bow.

    I then had to order a couple of new sets of 200 gr broadheads because I did not expect my bow to like the 200 gr field tips quite so much. While waiting for the broadheads to arrive, I started experimenting with FOC. My experiences with extreme FOC were discouraging because it resulted in extreme arrow drop out of my bow. Since my next archery hunt is for Musk Ox on Banks Island, I wanted to be able to shoot from 35 - 45 yards. If I were hunting Brown Bear at 20 yards, I would probably experiment more with FOC.

    When my broadheads arrived, I experimented to see how they grouped compared to field-tipped arrows. I was pleased to learn that 200 gr two-bladed, single-bevel broadheads grouped the same as the FMJ field-tipped arrows through my paper-tuned bow. The same broadheads on the Bemans grouped about 4" away. Performance was about the same at 30 yards and I have not yet shot further than that.

    In conclusion, I can either purchase a new set of FMJ arrows, as that is what my bow likes best, or tune the bow to shoot the Eastons I have on hand. I haven't yet made a decision.

    As they say, your bow and your mileage may vary.

    By the way, my bow is a Hoyt Maxis and is set for 60 lbs.

    Good luck to you. The process sounds like a lot of work and did turn into a bit of expense, but the process was so much fun that I am working out best arrow combinations for my other bows.

  3. #3
    Member AKducks's Avatar
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    Ok so my understanding (it's very little in regards to spine) is that when you increase broadhead weight you get more bend in your arrow (a higher dynamic spine) I shoot a 330 (easton bloodlines) out of a hoyt 60lb with 29 inch arrows. I think you might already be on the high side for spine on your bow.

    That being said buy a 125 grain tip shoot it through paper and you can tell pretty quick if you need a lower spine. (I believe if you see the vanes pushed to the left or right that means you may be overspined) note I'm assuming your bow is tuned already. And I really don't know much about spines all I know is mine works right now.

  4. #4
    Member AKducks's Avatar
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    http://www.huntersfriend.com/carbon_..._chapter_3.htm

    here is a site that explains it better with Spine selction chart that brings in Tip weight

  5. #5
    Member icb12's Avatar
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    Default

    Thanks for the replies.
    I swapped a guy for a couple 125s. I'm just going to kill some paper and see what happens.

  6. #6

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    ducks,

    when you increase point weight, you decrease dynamic spine of the shaft (making the arrow act weaker). When you increase nock weight, you increase the dynamic spine of the shaft (making the shaft act stiffer). When you add overall weight to an arrow, say weed whacker line in an gold tip arrow shaft, you actually increase the overall dynamic spine (contrary to what you'd think, by increasing arrow mass, and not bow weight, the arrow is going to act stiffer).

    static spine is what an arrow reads on the meter....it is X pounds or X deflection

    Dynamic spine is how the arrow acts/reacts to your given set up.

    You're on the right track with the dynamic spine game though I think the way you presented it you're saying the right thing, and than going the wrong way lol. Or I'm reading it upside down and backwards which is highly possible. This is the one area where compounds really shine unlike sticks. With wheels you can just crank it up or down a pound or 3 and problem solved. Most don't have the need to understand it, they just fix it and drive on. Kudos to you! With sticks I have to reset up shoot arrow after arrow, try and narrow a group down to a few spines and than hope its not something dumb like a nock giving me a false positive/negative, or a poor shaft. Woodies are fun, but they're a hellacious amount of work to do it right. in 20 some years I really only feel like I had one bow dialed in to a lazer, the rest are close. That one bow when she hummed things died. I've never duplicated it since. I had a 2lb window on spine in that bow, everything else went to grouse'n arras. The bulk of of tuning in the trad world is tuning the person and I haven't been tuning myself these last few years , as much as it is tuning the gear. More like beating myself up with a hammer, or today an impact wrench. Don't get me wrong, I thoroughly enjoy/get frustrated with trad gear, back and forth, its a vicious and rewarding cycle!

    Icb...I like to tinker, though I don't do much of it anymore. After reading your first post I'd say you have your answer already. It shoots/tunes well and kills well. I'd leave the thing alone lol. If someone says it does OK and had a reason to back it up (say so so penetration), than I'd start breaking things down though I'd be looking at everything else before I took a good flying shaft and swapped her out. Personally I think more people have issues with flight killing penetration (and broadheads being second) than the shaft weight itself unless you're making 'big' moves. After reading Ashby's more recent material, unlike his natal study (which is an interesting read, I had an actual copy at one point, not the typical summary we all read all over the place), making a FOC move more than an oal weight move may help in penetrating...but now that takes us to a different topic all together. Fun how it all ties together.

  7. #7
    Member AKducks's Avatar
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    Thanks Tradbow, honestly a lot of this is still academic for me. But I prefer to know why I'm doing thing (I became a engineer for a reason). I've been looking at spine a lot trying to figure it out. I saw some charts that helped a lot and what it seemed to me was you need less spine (lower number) for heavier arrow/ bigger broadhead. I know I got some changes to do with arrows this year (I drew a archery moose and would like a heavier arrow for that hunt) so have been looking into spine a lot.

  8. #8

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    If you have a tuned well flying setup already as much as most people hate to admit it you'll likely be fine. There's more to penetration than arrow weight.

    That's exactly right. By adding weight to your arrow from end to end , the arrows dynamic spine is going to be stiffer (or the shaft will act stiffer due to it needing more energy to move it like it did before you added the weight, think flex/paradox) so you need to lower static spine (weaker arrow) or....crank the bow up a skosh at a time (weakening the dynamic spine or making the arrow act weaker by bumping bow poundage up in small increments).

    If you ONLY added weight up front the arrow will ACT weaker. And visa versa for the tail end.

    Ducks you may really enjoy a old book called....archery the technical side by klopsteg, nagel, and Hickman. It's some of the original 'why' studies from a loooong time ago. I have it in hand, you won't regret buying it!

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