10 gpp?

[upstatenybowyer]

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. 

[DC]

It's probably as simple as 10 is an easy number to remember I really doubt that there is a solid reason.

[Pat B]

It is an ideal set up for a hunting arrow. Target arrows would be lighter.

[Nidhoggr]

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.

[DC]

That's way better than my answer

[Nidhoggr]

That's way better than my answer

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

[willie]

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?

[Nidhoggr]

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.

[Nidhoggr]

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".

[DC]

GPI?

[Nidhoggr]

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.

[willie]

So how does one find that sweet spot GPP for any particular poundage?

[upstatenybowyer]

These are great answers! I'm totally glued in. Keep um comin'! 

[Nidhoggr]

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'! 

Not sure if serious lol.

[upstatenybowyer]

I'm very serious Nidhoggr, you know a ton about this stuff and I know zilch, so I really appreciate your insight!