Author Topic: 10 grains per pound is not correct  (Read 7873 times)

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Offline sleek

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10 grains per pound is not correct
« on: April 22, 2018, 09:39:42 pm »
I have an issue with the 10 grains per pound and feel if this were fixed, all bows would compare equally in performance without the longer draw length people taking the advantage of shorter. We need to change the standard.

I wrote this back in 2012 and i still feel this way. If I can make Any contribution to archery, i want this to be it.

I would like to express an opinion based of things I understand to be true and things I have read that others believe to be true. I am asking only to raise more questions, not present facts.

I think that the idea of shooting a 10 grain per pound arrow is a bad one. I think it can be misleading and overload or underload a bow depending on the bows powerstroke ( draw length minus brace heigth ). My point is bassed off stored energy. To start off with an example, I will use the cross bow. A cross bow may have a 14 inch draw and a 200 lb draw weight. ( not listing numers from any specific bow, just numbers that could be. ) If this bow were to shoot a 10 gpp ( grain per pound ) arrow it would fire a 2000 grain arrow. That arrow would fly about as far as a brick.

What is the reason for this? I feel the first answer is so obvious you would wonder at my question, but the answer is more detailed than the obvious " the arrow is too heavy " answer one would be likely to spurt out at first thought. I would be more inclined to say that the poor flight of an arrow 2000 grains out of a 200 lb @ 14 inches bow could be blamed on the arrow not being correct to the ratio of power stroke to draw length. The distance a bow is pulled in combination with the weight it reaches ( see note below ) will determine how much energy the bow stores. In this argument assume a well built bow with low tip mass, set, ect... This energy storage is all the bow has to offer to the arrow.

If you always make the arrow 10gpp, the shorter power stroke bow will suffer because of the reduced energy presented to it compared to that of a longer powerstroke bow. I would venture so far as to say that a 50 lb @ 15 with 10 gpp would perform less in arrow flight than would a 30lb @ 30 inches with a 500 grain arrow from the 50 lb bow. ( I havent done the math, but I used the numbers to illustrate my point. )

There must be a ratio of energy storage of the bow to grains per pound of arrow mass. The idea of having a standard by which to measure a bows performance is good and needed, but I think I have shown why I think 10 gpp is a bad way to measure performance of a bow, just as bad and flawed in the same way as ( those who have read the TBB series will remember this one ) measuring all bows performance with a 500 grain arrow regardless to poundage or draw length. These two factors must be calculated into measuring a bows performance. And since it is draw weight and length the bow offers ( potential energy ) and weight of the arrow I think a ratio should be calculated and used to determine the effeciency of a bow based on draw length, weight and arrow weight.

As an additional thought to this topic, I think there is a maximun effecient draw length for every draw weight of bow and that will depend on the arrow weight chosen. Shorter draw lengths get lighter arrows, longer draws get heavier. But again, here I say there is a ratio.

( Ref. note above )
The end poundage of a bow does not matter as much to the kenetic energy of an arrow as does what the force draw ( FD ) curve looks like. High energy storage in a bows early stages of draw gives the arrow the benifit of the rest of the draw length to absorb that energy. The longer the arrow is inside the fat of of the FD curve, the happier it is, as an arrow can soak up energy as fast as it is given. The more energyin the FD curve that is ahead of the arrow ( read early draw weight ), the more it leaves the bow with. This was not exactly part of my original discussion but felt it played a significant part of the performance of a bow and needed to be understood by people who havent read about fd curves yet.

After presenting my argument, Dave, aka woodbear, came up with this simple formula and I believe its the one that should always be used.

 10gpp x draw/28

This little gem can normalize any draw length and put every bow on an even playing field by adjusting the arrow weight to the bkws draw weight AND draw length. I bring this up every chance i get. I feel like if this oportunity to adopt this formula as a standard is wasted, progress will NEVER be made.
« Last Edit: April 23, 2018, 05:59:06 pm by sleek »
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Offline StickMark

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Re: 10 grains per pound is not correct
« Reply #1 on: April 22, 2018, 10:42:37 pm »
Interesting, very.  I need to keep better notes.  doing my own penetration tests, I could see that the reality overtook the 10gpp rule.

Offline Stick Bender

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Re: 10 grains per pound is not correct
« Reply #2 on: April 23, 2018, 04:09:48 am »
Its been the standard for a lot of years I have a longer draw ,I always post chrono numbers with 11 gpp arrow and only post numbers after the bow has been shot in 300 arrows & strung for 1 hour and 12 arrows shot before chrono, I want to know what the bow really does in real world conditions if I would shoot some of the bows fresh I could say its doing 192 fps but after 300 arows and strung for 1 hour 12 arrows prior it ends up being a 178 fps bow ! My point is there is a certain amount of comon sense that goes into all published numbers !
If you fear failure you will never Try !

Offline Del the cat

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Re: 10 grains per pound is not correct
« Reply #3 on: April 23, 2018, 04:30:31 am »
"standard"
"Always"
For pities sake gents  ::)
Remember our roots... did the native Americans, neolithic man or the English Yeoman at Agincour* follow the "rules"?
Ok, 10 gpp is a reasonable guide for most normal situations...
But is this forum really about the normal or is it about the different, the exceptional and the practical realities ?
Get grip! ;)
Del
* actually there was some standardisation and mass production of bows and arrows for the 100yrs war, but I don't think the arrows conformed to 10gpp *  ;D
« Last Edit: April 23, 2018, 07:14:24 am by Del the cat »
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Offline Eric Garza

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Re: 10 grains per pound is not correct
« Reply #4 on: April 23, 2018, 05:31:48 am »
Sleek, I think the 10 gpp figure is often cited so that at least one variable can be held constant when comparing the performance of two different bows. That is all the 10 gpp figure is for. No one is saying everyone should always shoot with arrows that weigh 10 gpp.

The method you are suggesting would effectively invite us to compare the performance of two different bows by holding nothing constant. What is the point of making the comparison, then?

Offline JWMALONE

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Re: 10 grains per pound is not correct
« Reply #5 on: April 23, 2018, 06:47:29 am »
I'm with Del and Eric. On the other hand if you don't follow the rules set by the ole timers the earth will fly out of orbit and kill us all.
I find similar "rules" in all of the crafts and or skills I've dabbled in over the years. And almost always it goes back to what Erick said, it was originally a rule of thumb or basic guideline that eventually turned into a hard rule.
And I believe the reason these standards turn into unbreakable rules that must be adhered to is people loose sight of why they came about.
Red Oak its the gateway wood!

Offline Badger

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Re: 10 grains per pound is not correct
« Reply #6 on: April 23, 2018, 08:11:52 am »
       I have used Sleeks method for years when testing longer or shorter draw bows. I mainly use it to keep efficiency figures consistent. One of the main reasons I am against it in a flight shoot is that we are attempting to build a kit with a set of rules and protocol that can be passed from shoot to shoot anywhere in the country and they will all be stored in the same data file and are competing for the same records. 28" was chosen because it is the most common draw length bows are built to and 10 grains per pound is easy enough to compute and is also a well accepted number used by hunters.

      In scenario like this I believe the shorter draw bows would start having an advantage unique to self bows. In the later inches of draw is where we enter what I call the death zone where bows start to pick up set and hysteresis. It is a credit to the bowyer that he can get his bow out to the full 28" draw and still maintain the same high level of efficiency that he had at 24". The vast majority of bows start falling off once they go past 24".

    I would be all in favor of the chrono test being run Sleeks way except for one reason. I was hoping we could use the chrono test to compare speed against distances so that bow makers working and testing at home with hopes of breaking existing records would be able to use their chrono to find out if they were in that record range or not.

    I actually have several reasons that are not arbitrary as to why I chose the 10 grains at 28". I am very aware of Sleeks method and use it myself in testing but I just don't like it for this particular type of event. For regular flight shooting where light arrows are used of any length it is an entirely different matter.

Offline Pat B

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Re: 10 grains per pound is not correct
« Reply #7 on: April 23, 2018, 10:18:58 am »
I think the 10 gpp is a suggested arrow weight for hunting arrows
Make the most of all that comes and the least of all that goes!    Pat Brennan  Brevard, NC

Offline Jim Davis

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Re: 10 grains per pound is not correct
« Reply #8 on: April 23, 2018, 10:39:50 am »
In testing arrow speed, 10 gpp is a good thing. It means you can vary everything but the weight of the arrow draw length and the draw weight in your effort to get speed. The only measure is speed.

You can make shorter limbs, lower mass,  radical limb shape and whatever you want. The measure of your efforts is the speed of the arrow.

Doing otherwise is like handing out blue ribbons for particiation.
Jim Davis

Kentucky--formerly Maine

Offline PatM

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Re: 10 grains per pound is not correct
« Reply #9 on: April 23, 2018, 10:53:37 am »
"  In scenario like this I believe the shorter draw bows would start having an advantage unique to self bows. In the later inches of draw is where we enter what I call the death zone where bows start to pick up set and hysteresis. It is a credit to the bowyer that he can get his bow out to the full 28" draw and still maintain the same high level of efficiency that he had at 24". The vast majority of bows start falling off once they go past 24".


 Confirmation bias at work.

Offline Badger

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Re: 10 grains per pound is not correct
« Reply #10 on: April 23, 2018, 11:15:43 am »
  This is not confirmation bias at all, on the contrary it is an area I have spent considerable time testing out. We know for a fact now that set not only contributes to loss of performance due to loosing profile but also due to increased Hysteresis. It is well documented that bows are more prone to set the further we draw them. Your last comment was out of line and not qualified. Shorter drawn bows given a handicap based on stored energy would have a distinct advantage.   

Offline Eric Garza

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Re: 10 grains per pound is not correct
« Reply #11 on: April 23, 2018, 11:31:38 am »
Intriguing post Badger. Do you think that might be why the average Native bow was around 55 inches, and many Natives snap-shot with shorter draw lengths? Might they have gravitated towards those size bows and that style of shooting to minimize set, reduce hysteresis, and optimize efficiency?

Offline Badger

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Re: 10 grains per pound is not correct
« Reply #12 on: April 23, 2018, 12:03:49 pm »
Intriguing post Badger. Do you think that might be why the average Native bow was around 55 inches, and many Natives snap-shot with shorter draw lengths? Might they have gravitated towards those size bows and that style of shooting to minimize set, reduce hysteresis, and optimize efficiency?

   I am almost certain that's why they built them that way. When I get back I will give you a real life example done at an official test contest we used to have.

Offline Marc St Louis

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Re: 10 grains per pound is not correct
« Reply #13 on: April 23, 2018, 12:12:48 pm »
  This is not confirmation bias at all, on the contrary it is an area I have spent considerable time testing out. We know for a fact now that set not only contributes to loss of performance due to loosing profile but also due to increased Hysteresis. It is well documented that bows are more prone to set the further we draw them. Your last comment was out of line and not qualified. Shorter drawn bows given a handicap based on stored energy would have a distinct advantage.

The bow may have an advantage due to hysterisis but that is probably negated with the longer draw length adding to the power stroke

I think 10 GPP is a good common denominator.
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Offline Badger

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Re: 10 grains per pound is not correct
« Reply #14 on: April 23, 2018, 12:35:21 pm »
  I was going to give a real life example of a test that was done publicly. In 2008 we held a walk the talk event which was open to all kinds of bows both modern and primitive. I had a bow that was doing particularly well at home drawing 24" and I chose to leave it there to demonstrate short draw capabilities. At 24" draw and 10 grains per pound it was shooting at 175 fps. About the fastest I had ever gotten at 24". Using the Woodbear stored energy method shooting a 428 grain arrow it was hitting 186. When it came time for the formal test there were a lot of bows in line and it was too much trouble to readjust the shooting machine for 24" so I told them to just shoot the bow at 28" after a series of pulls to seat the bow in. At 28" and 10 grains per pound the bow was now shooting at 176 I believe. 9 fps below what the equivalent arrow weight was hitting at 24" draw.