I have an issue with the 10 grains per pound and feel if this were fixed, all draw length questions would be answered.
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.
