Primitive Archer
Main Discussion Area => Arrows => Topic started by: upstatenybowyer on January 05, 2018, 06:46:58 pm
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Forgive me friends, but I'm really trying to wrap my head around this "matching arrows to bows" thing.
Where does the 10 gpp guideline come from?
I'm wondering cause while waiting for my grain scale to arrive, and having a brand new chronograph to play with, I decided to shoot an arrow whose weight I know for sure through the chrony. That arrow was an aluminum alloy arrow that weighs 320 grains (I know cause I looked it up on the maker's site).
I shot the arrow with a 60# hackberry static and the speed was a little over 180 every time. I was getting low 170s with the same bow when shooting a cedar shaft arrow that I know was heavier.
My question is why do we go with 10 gpp as a guideline if a lighter arrow goes faster? Is the guideline specific to wood arrows? Is it a hunting/penetration thing?
Thanks for baring with me. (W
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It's probably as simple as 10 is an easy number to remember :D I really doubt that there is a solid reason.
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It is an ideal set up for a hunting arrow. Target arrows would be lighter.
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This will be lengthy:
A lighter arrow travels much faster but the trade off is it has less kinetic energy transfer and less momentum. So while it will travel further with height drop over distance, it will also not deliver as much force upon impact.
A heavier arrow will have much more kinetic force and more momentum, but the trade off is less speed and more of an arrow drop.
See this example here (albeit using carbon arrows and a compound, but same concept). If I recall correctly this is using a 60 lb compound as the example:
Weight Speed Kinetic Energy Momentum
350 gr. 340 fps 89.75 ft. lbs. .527 Slug/ft. per. sec
480 gr. 290 fps 89.54 ft. lbs. .617 Slug/ft. per. sec
750 gr. 175 fps 50.95 ft. lbs. .582 Slug/ft. per. sec
Notice the 750gr arrow has a much lower speed than the 350gr and the 480gr, but it's still got a higher energy? That's due to the GPP in relation to the poundage. The 480gr arrow is roughly in line with the 10gpp range. Notice it's kinetic energy is actually LESS than the 350gr arrow, but because of it's increased mass it is substantially more powerful when momentum comes into play. Not only that it has a pretty hefty speed vs the 750gr and not much less speed than the 350gr.
So essentially what it boils down to is 10 GPP is from a physics standpoint where the energy transfer is balanced well between weight that the poundage of the bow used can propel without sacrificing stability of shot, structural integrity of the arrow, or making the arrow too fast with less than ideal kinetic energy and momentum.
TLDR: 10 GPP is the sweet spot for an arrow achieving ideal performance and not just a random number lol.
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That's way better than my answer :D
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That's way better than my answer :D
LMAO I obsessively learn about my hobbies to the point of insanity so.... Say, wanna know WHY steel reacts to heat violently at 2000 degrees sustained?! lmao
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TLDR: 10 GPP is the sweet spot for an arrow achieving ideal performance and not just a random number lol.
nidhoggr, isn't the sweet spot something to be worked out for each particular bow limb? its my understanding that a warbow compared to a compound, compared to a target bow might not all be the same?
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TLDR: 10 GPP is the sweet spot for an arrow achieving ideal performance and not just a random number lol.
nidhoggr, isn't the sweet spot something to be worked out for each particular bow limb? its my understanding that a warbow compared to a compound, compared to a target bow might not all be the same?
I more meant it's the "ideal" sweet spot for the arrow GPI vs bow poundage. My longbow prefers 13 gpi, while my recurve fires 10gpi like a ballistic missile and doesn't like much heavier. 10gpi is the industry standard and go to starting point.
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Basically: too heavy for the bow and you won't get enough speed or distance, too light and too much speed but not enough kinetic energy. 10 GPP falls right in line with the "best of both worlds" in most cases.
Sort of like Goldilocks "This arrow is too heavy, this arrow is too light, this arrow is juuuuuust right".
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GPI?
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I meant GPP. GPI is "grains per inch". Been making arrows today and calculating their GPI so i keep derping and typing it by mistake.
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So how does one find that sweet spot GPP for any particular poundage?
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These are great answers! I'm totally glued in. Keep um comin'! (-P
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So how does one find that sweet spot GPP for any particular poundage?
Testing. Rule of thumb though as stated is 10 GPP. Has a good amount of speed and power at that ratio. So a 60# bow would do fine with a 600 grain arrow, where as a 50# bow would do decent (12 GPP), and a 30# bow would probably do very poorly with it (20 gpp).
Keep in mind though: GPP is not the only concern. There's also arrow length compared to your draw length, the spine of the arrow, the straightness of the arrow. Every wood weighs different and will behave different. My oak 3/8D" 24L" are faster than my 3/8D" 24L" poplar arrows due to being lighter but I get much better performance out of the poplar due to it having a better spine and more GPI. Oak are less durable as well compared to poplar.
So there is no "magic formula" for arrows. I shoot at a 27" draw on my recurve but do a 24" on my long bow (don't know why) and I cannot interchange the arrows very effectively. My long bow HATES 5/16D" poplar @ 24L", but my recurve loves 29L" 5/16D" poplar and fires the 24L" fine as well.
My typical arrows with my 55# @ 28" recurve are 5/16D" poplar cut to 29L" with a 125gr field point and 4" shield cut feathers. My 50# @ 28" long bow uses 3/8D" poplar 25L" with 100gr field points and 4" parabolic cut feathers.
So.... yeah lol. Basically: 10gpp is the ideal zone to be in, but not the only factor. As people have said: you WANT target arrows to be lighter (6 - 9 gpp) because they are straighter shots on target. You want a hunting arrow to be heavier so it has more momentum and kinetic force (10 - 13gpp).
These are great answers! I'm totally glued in. Keep um comin'! (-P
Not sure if serious lol.
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I'm very serious Nidhoggr, you know a ton about this stuff and I know zilch, so I really appreciate your insight! :OK
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I'm very serious Nidhoggr, you know a ton about this stuff and I know zilch, so I really appreciate your insight! :OK
Oh! Well thank ya! And not a problem at all. I don't really have the tools to make bows effectively at the moment, so I have decided arrows are more my "thing" right now. Plus I plan on selling arrows at the renfaire this year. So I researched the bejeebus out of them to see what all I needed. Just had a guy commission me today to make him some arrows for his 120lb english warbow with his 30" draw. So that is my current big challenge.
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sounds like you are jumping into arrowmaking with both feet!
Consider posting pics as you go.
Sometimes the best "build along" threads are "learn alongs"
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If someone commission me to make some arrows for a 120# bow I sure wouldn't think of it as a "challenge."
I would think I could deliver a superior product with no defects. Good luck with your endvers.
DBar
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That 10gpp is calculated for a 28 inch draw. Go shorter on your draw and you will drop the gpp. I started a thread on ut years back, titled something like, 10gpp not effecient or something like that.
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https://www.tapatalk.com/groups/paleoplanet69529/10-grains-per-pound-not-effecient-t49349.html
Found it.
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thanks sleek
good perspective.
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If someone commission me to make some arrows for a 120# bow I sure wouldn't think of it as a "challenge."
I would think I could deliver a superior product with no defects. Good luck with your endvers.
DBar
To clarify: it's a friend of mine wanting them. We were discussing archery and I told him about the arrows I have been making. He explained he can't ever find warbow arrows for sale that aren't insanely priced so he asked if I would be willing to make him some. I told him that while I have not made warbow arrows I would happily give it a go and only charge him supplies. I refused any more than supplies as i cannot say for 100% certain they will be great or not. So it's not technically a "commission". I have a policy of not charging for more than supplies unless I would be willing to stamp my name and therefore reputation on it.
That 10gpp is calculated for a 28 inch draw. Go shorter on your draw and you will drop the gpp. I started a thread on ut years back, titled something like, 10gpp not effecient or something like that.
I will both agree and disagree. it is calculated at 28" because that's the standard (ie: 50# @ 28"). But if you subtract the 2.5lbs per inch not drawn you still can effectively pull off your 10gpp in accordance with your draw length. So a 50# @ 28" becomes a 45# @ 26" and you would want 450 grain vs 500 grain. While it's not perfect science as far as poundage loss per inch you will still be right in the realm of where you need to be.
sounds like you are jumping into arrowmaking with both feet!
Consider posting pics as you go.
Sometimes the best "build along" threads are "learn alongs"
I definitely will think on that. Super proud of my most recent arrow experiment and will hopefully be posting a picture later today.
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I draw my 40# primitive hunting bow at 23# and feel more comfortable with the momentum from a heavy 555gr arrow than a 10gpp 400gr arrow. Because i am mainly a hunter vs. flight or target, i use this 555gr arrow as a constant, on every bow i build. Perhaps this isnt the best way to judge effeciency, its just what ive been doing.
On my 70# compound bow i draw 27" and shoot a 365gr arrow(which is just over the 5gpp minimum that the mfg recommends) at 301fps, resulting in 73 KE, where as the selfbow goes 135fps with the 555gr arrow resulting in 22KE. The difference in what goes on the business end of each arrow is as different as night and day, of course.
Why even bring nasty wheels bows into the conversation, i just think its interesting to look at the huge difference in equipment, performance, and then revel at the fact that a selfbow kills em just as dead when used properly (SH)
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I draw my 40# primitive hunting bow at 23# and feel more comfortable with the momentum from a heavy 555gr arrow than a 10gpp 400gr arrow. Because i am mainly a hunter vs. flight or target, i use this 555gr arrow as a constant, on every bow i build. Perhaps this isnt the best way to judge effeciency, its just what ive been doing.
On my 70# compound bow i draw 27" and shoot a 365gr arrow(which is just over the 5gpp minimum that the mfg recommends) at 301fps, resulting in 73 KE, where as the selfbow goes 135fps with the 555gr arrow resulting in 22KE. The difference in what goes on the business end of each arrow is as different as night and day, of course.
Why even bring nasty wheels bows into the conversation, i just think its interesting to look at the huge difference in equipment, performance, and then revel at the fact that a selfbow kills em just as dead when used properly (SH)
If the wheelbow question is regarding my initial physics heavy post I was just using a handy chart I found to explain. And there is (hear me out) nothing wrong inherently with compound bows. I don't like them by any means, but plenty of people do. I find it to be like playing a game of golf with a handicap: Yes you are still golfing, but you are doing it with a ton of extra help. Or fishing in a over stocked pay lake: You are still fishing, but you are not doing it without severe aid. To each their own but I feel compound bows are a huge crutch. But at least it's not a damn crossbow lol. And even that has it's chance for a debate, too, as medieval crossbows are a vastly different beast than "KILLEMDEAD 2000! NOW WITH A 20X SCOPE! FIRES 4 BOLTS AT ONCE!!".
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That 10gpp is calculated for a 28 inch draw. Go shorter on your draw and you will drop the gpp. I started a thread on ut years back, titled something like, 10gpp not effecient or something like that.
I will both agree and disagree. it is calculated at 28" because that's the standard (ie: 50# @ 28"). But if you subtract the 2.5lbs per inch not drawn you still can effectively pull off your 10gpp in accordance with your draw length. So a 50# @ 28" becomes a 45# @ 26" and you would want 450 grain vs 500 grain. While it's not perfect science as far as poundage loss per inch you will still be right in the realm of where you need to be.
sounds like you are jumping into arrowmaking with both feet!
Consider posting pics as you go.
Sometimes the best "build along" threads are "learn alongs"
I definitely will think on that. Super proud of my most recent arrow experiment and will hopefully be posting a picture later today.
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You are looking at it wrong. You cant just figure a draw weight at a shorter draw and say thats what your arrow weight should be. It has only 50% to do with draw weight. The other 50% is draw length. A 45@26 as you mentioned is different than a 45@28. Both bows cannot compare equally shooting a 450 grain arrow. Its impossible. A shorter draw length is at a power storage disadvantage and needs a lighter weigjt arrow proportional to the bow of a longer draw. A 30" draw stores more at 45#than a 28 and a 26. Imagine the difference in performance of a 26 and 30 inch draw all other things being equal. The arrows must be of differnt weight.
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You are looking at it wrong. You cant just figure a draw weight at a shorter draw and say thats what your arrow weight should be. It has only 50% to do with draw weight. The other 50% is draw length. A 45@26 as you mentioned is different than a 45@28. Both bows cannot compare equally shooting a 450 grain arrow. Its impossible. A shorter draw length is at a power storage disadvantage and needs a lighter weigjt arrow proportional to the bow of a longer draw. A 30" draw stores more at 45#than a 28 and a 26. Imagine the difference in performance of a 26 and 30 inch draw all other things being equal. The arrows must be of differnt weight.
That is what I am saying as well.
Assuming a 28 inch draw and a 50#@28" bow:
29" arrow, 500 grain with 50lb bow = 10gpp. On a short draw of 3 inches (7.5 lbs loss for a total of 42.5lbs drawn) the arrow would be too long and too heavy, weighing in at about 12gpp.
Using that draw length of 26" you would want a 27" arrow weighing roughly 425 grains for 10gpp.
We are both saying the same thing it seems.
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And using a standard arrow weight relative to draw weight allows you to objectively compare the performance of different bows.
For Gf recurves 9 gpp is typically used, for compounds it’s 5 gpp