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Arrow Building - Dynamic spine calculator. Total arrow weight, speed, kinetic energy

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lwe

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Discussion starter · #1 ·
I have a 50# recurve that is a quarf built on a Black Bear riser with Sage Samick limbs. 62" and my draw is 29"

I have been playing around with different arrow combinations in the 3 Rivers Dynamic Spine Calculator looking for the "optimal starting point" for a hunting arrow.

It is no secret that as weight increases then speed decreases, but I'm surprised to see kinetic energy remain roughly the same when you find the happy spot in the calculator. I am wondering how to determine where to stop and which variable is more important.

If I am delivering the same kinetic energy, then is the heavier arrow a waste, or do I want the extra weight to keep the arrow moving longer after it meets resistance (the target)? How do you determine where the tradeoffs between more mass vs arrow speed become equalized?

Four arrow builds:

#1 250 spine 682.4 grains, FOC 25.8, Projected velocity 179.2, and Kinetic energy 43.8
#2 300 spine 580.5 grains, FOC 24.3, Projected velocity 183.9, and kinetic energy 43.5
#3 400 spine 429.1 grains, FOC 20.7, Projected velocity 215.3, and kinetic energy 44.1
#4 500 spine 335.1 grains, FOC 16.8, Projected velocity 249.5, and kinetic energy 46.2
 
#2 ·
KE is only good for comparing potential energy between bows....its not good for estimating an arrows efficiency. MO is better...BUT....an arrow with perfect flight and an efficient BH is very effective at any weight.

KE will continue to go up as arrow weight increases [up to appx 1700gr]...and momentum will go up too of course. The reason; the heavier arrow absorbs more of your bows energy.

The tradeoffs are; trajectory, bow noise, arrow efficiency.

You can improve your arrows efficiency with a good 2 blade BH.....so now we are talking trajectory v bow noise. How long are your shots? What game you shooting at? A 50# ILF will cast a 580gr arrow pretty good....it really zips a 429gr arrow.

Forget #4, 1-3 is personal preference, depending on the factors above.

You should be able to get that bow almost silent with a 10GPP arrow.....silent is good......I'm shooting 12GPP....the extra weight for elk....but like that 10-11GPP range for just about everything, YMMV

If I were you...I would shoot between 2 and 3
 
#3 ·
This is just my two cents but I might suggest that you take a lot of these online calculators with a grain of salt. I have played around with the calculator mentioned and I have found it to be surprisingly inaccurate. What I don't like is that they don't show you the math, and when it comes to arrow formulas and metrics, there are a number of formulas used which can lead to varying (albeit minute) results. I was surprised to find that 3rivers calculator calculated my current arrow 50 gr over what it actually is. I can only imagine what sort of inaccuracies will be relayed to of the metrics it purports to convey.

While it can be a pain in the ass, I would suggest that you do the math yourself. I am currently working on a pyhon script to more accurately compute arrow stats (such as FOC, &c.) and what I have found in my research is that many such calculators can tend to be quite off.

As far as your question, I would say that you would want to find a good balance for both overall weight and FOC--while taking in consideration of spine stiffness; the more weight up front the more your arrow is going to want to paradox. All-in-all, I would say that most of the arrow builds look good. I personally like numbers 2-3. I would say that #4 is too weak-spined for the FOC and #1 is a bit off in the other direction. Personally I like #3, but that is only because it is close to what I shoot. In the end, it really depends on what you want to do. Are you target shooting, hoping to take a white tail, or maybe go after larger game? To this I would say that the balance being sought goes beyond the makeup of the arrow but rather extends to the shooter and his/her intent.
 
Discussion starter · #4 ·
Thank you both for the informative replies. This build will be for whitetails. I would definitely be looking to beef it up to the top end if I were chasing elk or hogs. I do realize the calculations are just "suggestions", particularly in my case. Since I am shooting a frankenbow, there's no way a calculator will produce accurate numbers. Just trying to find a starting point.
 
#5 ·
For whatever it's worth, with my 29" draw length and my 50# recurve I use .340 shafts (Easton ACC 3-60), 30.75" long with 250 grain points. That wouldn't necessarily be right for you or your bow, but for me .400s are too weak and I wouldn't even like to think about how much point weight I'd need to add to use a .300. Shafts of .500 spine are too weak for even my 45# recurve.

The thing with the calculators developed by Stu Miller is that they only work with very accurate input values. Even then they should be taken as starting points for serious tuning, which is covered in this link...http://www.acsbows.com/bareshaftplaning.html

In my opinion the calculator on the 3Rivers site or Stu Miller's latest work are a marvelous tool for archers but don't expect perfection. Used properly they will get you close, maybe even close enough but tuning should follow.

When you use the calculator it's not a bad idea to build in some tunability to your arrow recipe. Leave the shaft length at least an inch longer than you think is necessary, or even leave them full length. Length is a powerful tuning variable and if you start too short you loose it.

Use a medium weight point in your calculation, this lets you go up or down a bit during the tuning phase. If you use 200 grains with the calculator you have +/- 50 or more grains to tune with.
 
Discussion starter · #6 ·
One other thing about BH selection. I have seen many posts from people saying 2 blade BHs fly better. I was hoping to use the Magnus Stinger Buzzcuts I already own in either 125gr or 100gr with weighted inserts to achieve the higher weight. They aren't a true 2 blade but the bleeders are relatively small. Guess I'll see how they fly and go from there.

Great point about starting full length as a tuning aid. I will definitely do so.
 
#7 ·
I forgot to add, when it comes to kinetic energy values from the calculator, they will not be as accurate as if you actually measured the arrow speed and calculated it yourself.

As point weight goes up both kinetic energy and momentum will also go up, at least until arrow weight becomes unreasonable (upwards of 1500 grains). The calculators don't take into account that as arrow weight goes up, bow efficiency also goes up. In other words, as arrow weight increases, more of the potential energy stored in the limbs is converted to kinetic energy in the arrow at release.

The only real downside to heavy arrows is the trajectory penalty. Light arrows definitely shoot flatter and heavier arrows carry more momentum, everybody has their own idea of what is the optimal compromise. Fortunately for whitetail deer hunters just about any well tuned arrow tipped with a sharp broadhead and put in the right spot will work.
 
#8 ·
Just as the others said, the online spine calculators are really just a starting point to get you in the ballpark. They work well for some but not as well for others. My shooting buddy has found the calculators to be pretty close to what he ends up shooting,but I always seems to be a full spine off from what the calculators tell me. As far as arrow build I think right around the 10gpp will give you an optimal arrow for whitetail. The best thing to do is build a few arrows and shoot them through a chronograph to get your arrow speed, that will allow you to calculate your numbers more accurately. KE is not as important as momentum when it comes to archery. In my experience usually changing an arrows weight by 50gr will change the arrow speed 10fps. Going off that or what your readings are you can put different arrow weights and speeds into the calculator to see what arrow will be optimal. Once you get above that 10gpp you will also have to take arrow trajectory into account, those heavy arrows can shoot like a rainbow if they are too heavy. I’m not sure how you came up with your projected velocity but I doubt you will see those speeds with any of those arrow weights. Are you bareshaft tuning these arrows? That is really the only way to tell if you have the correct spine. One other useful note for tuning is if you are unsure of what shaft to use, Lancaster sells arrows shafts individually so you don’t have to buy 6 or 12 if your not sure of what to get. Buy a few different shafts and keep them bareshaft for tuning other bows you might have or buy in the future.
 
#9 ·
iwe -

Seriously, it's all bull excrement.
With a 50# bow, shot at close range (hunting), pick an arrow that's in the tuning window, tuned it, and go shoot.
Nothing else is going to make one bit of difference.

Viper1 out.
 
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#10 ·
What Viper said. 2% extra kinetic energy might result in your arrow sticking 1/8" further into the tree on the other side of the animal it just passed through.

I'm having a hard time believing the "projected velocity". 249 fps with 50# Sage limbs? Yeah, sure.
 
#11 ·
For most practical purposes the rough "rule of thumb" 10 grains per pound of draw weight has been taking every game animal to be found in N America since rocks were soft.

If you're contemplating doing something different, check your results yourself. You're looking to see what it does to your aiming within what you consider to be your effective hunting range. This is true whether you run a conscious aiming system or shoot instinctive. Does that heavier arrow make the trajectory too pronounced, such that it falls off early? Does that lighter arrow assist in smoothing out errors caused by incorrectly estimating different distances? What has been pointed out already is correct as to the physics of the ballistics. As arrow weight goes up bow efficiency KE AND momentum all go up. To say that either KE or momentum is somehow more important doesn't follow in that they are all inextricably tied together.

What happens to your sight picture and aiming is as arrow weight changes is probably going to have the largest impact. Regardless of how you do it, aiming, shot placement, woodsmanship, are all going to have a LOT more to do with your success at hunting.

Fortunately it seems more people are "getting" it about the calculators now than when they first came out. People were using them, building the arrow exactly as indicated by the calculator, and then wondering why their arrows flew like crap and they couldn't hit anything. I started to get a serious negative attitude about the calculators. But if you use the calculator, play with it, make your shaft selection, and then proceed to run a full tune process, they just might save you some $ in shaft selection AND you still come out with a good set up.
 
#12 ·
To me the arrow speed and performance ( accurate flight ) for whitetails boils down to bow noise. I've tested points of 100, 125, 150 and recently 200 grains all on Black Eagle 400. The jump to 200 grain heads made the most noticeable drop in "twang" on my (53@30 ) bow.
Arrow speeds on my chrono went from about 211-213 with 100 grn. to 189-193 with 200 heads. Out to 24 yds the heavier arrows fly better also, straighter with little noticeable drop over lighter heads.
Putting on 41lb limbs, the heavy arrows shoot good to 18 yds then drop like a rock at 24yds. Speeds were around low 171-173. Still plenty to kill a deer. Total arrow weight with 200 heads is 538 grains. Right on 10gpp.
Now I just need to figure out which 200 gr broadhead to pick.
 
#13 ·
Ignis-Fatuus said:
This is just my two cents but I might suggest that you take a lot of these online calculators with a grain of salt. I have played around with the calculator mentioned and I have found it to be surprisingly inaccurate. What I don't like is that they don't show you the math, and when it comes to arrow formulas and metrics, there are a number of formulas used which can lead to varying (albeit minute) results.
Click to expand...
I read an article on the creation. Stu Miller is an engineer. The calculator is parameterized using something like a linear regression approach. I am impressed with the model, but having constructed many scientific models myself, understand the deficiencies. And there is where the problem lies. Most folks expect to punch in numbers and get the right answer on the display. The expectations of the model far exceed its capability. If your specs land close to data in the parameterization data set then you could get a really good answer, and thus, have an artificially high view of how well the model performs. Let it be known, I am a real fan of what Stu Miller accomplished. I just have realistic expectations.
 
#14 ·
Hank D Thoreau said:
I read an article on the creation. Stu Miller is an engineer. The calculator is parameterized using something like a linear regression approach. I am impressed with the model, but having constructed many scientific models myself, understand the deficiencies. And there is where the problem lies. Most folks expect to punch in numbers and get the right answer on the display. The expectations of the model far exceed its capability. If your specs land close to data in the parameterization data set then you could get a really good answer, and thus, have an artificially high view of how well the model performs. Let it be known, I am a real fan of what Stu Miller accomplished. I just have realistic expectations.
Click to expand...
I would actually be very interested to read the article mentioned. I do not wish to disparage anyone who has clearly made an arduous effort to craft a model based on so many variables for the purpose of putting arrow setups within so many respective ballparks. As a graduate student who codes both for work and research I understand the limitations which can be afforded by such generalizations. I can only assume that said Mr. Miller's model is on par for what it attempts to do. I have found, however, that other basic errors have shown themselves in some of the more basic functions of the model, such as calculating overall arrow weight. I imagines that this has to do more with human error in product/variable definition than anything else. Regardless, I posit that that simple troublesome errors such as these lead to further miscalculations. But, of course, you are right in saying that the model does get one close to the physical reality of an arrow-bow setup. I will stress, however, that all such formulas should not be taken as dogma...and really that was only the point I hoped to make.
 
#15 ·
Yeah, I like that calculator...its been really close for me...but then I'm pretty good at following directions. /grin

50# ILF with Samick limbs....you will be right in there with Easykeepers arrow.

I have a 30" draw. My current go to Morrison 17" riser with WW Inno [carbon/foam] long limbs at 50# tunes perfect with a 30 3/4" Beman MFX 340, 50 gr insert and 150gr Buzzcut [the straight 2 blade not the one with bleeders, I don't like those] OR the VPA 150s in both 2 and 3 blade. Total arrow weight just under 600gr. When I bump up to 54# with that setup I have to chop a little off of those arrows to get it to tune.

For perspective, my tradtech riser with Dryad RC longbow limbs at the same weight 50# can't shoot the 340's above- too stiff. I have to drop down to 400's....[and drop a little point weight]

This^ just goes to show you the stored energy in those bows varies a lot.
 
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