# This whole 5 grains per pound thing...



## BarneySlayer (Feb 28, 2009)

So I'm hearing that an arrow should have at least 5 grains of mass for every pound of peak draw weight.

This number nicely coincides with an IBO arrow of 350 grains for a 70 lb draw, which seems to be a standard 'heavy' available weight.

So, we all know that going with heavier arrows increases energy transfer, and delivery more kinetic energy at the destination. Heavier poundage allows you to get heavier arrows up to the same speed.

However, if this 5 grains per pound is a relative constant, that means in terms of FPS, if you're target shooting, since your arrow mass floor goes up with draw weight, there's no advantage then, theoretically, to a higher draw weight. Longer draw length will help you pick up speed, but I can't make my arms longer, and if i pull back farther than comfy, I shoot crappy.

How flexible is this 5 grains per pound guideline? I mean, if my draw length is 27 or 28", and the bow is fine with a 350 grain arrow at 70# and 30" DL, could I go even lighter? i would expect that the real issue we're dealing with when it comes to light arrows is that we don't want them going too fast, lest the limbs and cams get too much of that kinetic energy we hoped to put into the arrow. I once blew up a browning discovery (29lb peak draw) shooting my daughter's arrows, that worked fine for her. Only difference, draw length....

Got some wisdom for me?


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## SonnyThomas (Sep 10, 2006)

5 grs. of arrow weight per pound of draw is IBO and accepted by bow manufacturers, Hence IBO fps.

No matter what, drop below 5 grs and your warranty just went out the door.... And danger to equipment and/or shooter or bystander increases.

Years back some tech/math wiz figured out the grs per pound for those of us with shorter than 30" of draw. Yes, draw length could allow for less grs. per pound, but the difference is too little to consider. And doing the math?

Here's the deal/headache; 70 pounds of force over X distance of string travel. 70 pounds of force over X - 1.0" distance of string travel. Something on the order of 10 fps difference and center of force changed. So a arrow of 30 grs lighter than 350 grs to achieve the same velocity. BUT there are other things that effect this -soft cam thru hard cam, make up of arrow (alum, carbon, mix), weight of tip, weight distribution. And then all this must be figured for each arrow make up/brand/size.

High Country.... Is it still alive? HC has a arrow super light, but super spined that will hold up, but would your bow?


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## dwagoner (Sep 27, 2007)

BarneySlayer said:


> How flexible is this 5 grains per pound guideline? I mean, if my draw length is 27 or 28", and the bow is fine with a 350 grain arrow at 70# and 30" DL, could I go even lighter? i would expect that the real issue we're dealing with when it comes to light arrows is that we don't want them going too fast, lest the limbs and cams get too much of that kinetic energy we hoped to put into the arrow. I once blew up a browning discovery (29lb peak draw) shooting my daughter's arrows, that worked fine for her. Only difference, draw length....
> 
> Got some wisdom for me?


dont forget that with a 27" draw or 31" draw its still a 70# bow, its just lost a few inches of its power stroke, the initial force is still pretty relative, it just stops sooner on a shorter DL bow vice a long draw. I believe at 5gpp is light enough if thats where you want to be, heck some of the bows are shooting a 400g+ shaft well over 300fps, so depending on your bow/setup i dont know where your at but i do believe in the 5gpp minimum to lessen stress on your limbs.


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## SonnyThomas (Sep 10, 2006)

dwagoner said:


> the initial force is still pretty relative


DW has what I was trying to say. I guess the question would then be; What would you rather be hit by; A 3 pound hammer thrown from 10 feet or a 3 pound hammer thrown from 9 1/2 feet? Same principle applies to the force thrust upon the arrow.


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## Kstigall (Feb 24, 2004)

I've written about the IBO rating a few times. I'll be briefer than in previous posts.

Rest assured that if a 70 lb. bow can safely shoot a 350 grain arrow at 30 inches of draw that the same bow can safely shoot a much lighter arrow at a 27" draw. There are enough variables in bow design, materials and how the bow is configured that it's easy to see that "5 gpp rule" is only a "rule" for a 30" draw and 70 lbs. Even then "efficiency" of the bow isn't taken into consideration, not all bows are equal.


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