The lighter arrow from the heavier bow will be faster....which helps to compensate for the arrow weight with higher velocity building more KE. If you shoot a light arrow it needs to go fast, if you can't drive a light arrow fast enough, then you are better off using a heavier arrow for the increase in momentum. If you always stay around 7 to 8 grains per lb of pull weight. What this does, is generally speaking, you will get about the highest percentage of energy efficiency transferred to your arrow from your bow. If you build below that, you are trying to increase speed and flatten trajectory in trade for some loss in efficiency, if you build more than that, you are typically trying to build something for closer ranges that may punch through a bone or break it rather than stick in it. The lower poundage you use in bow weight, the higher up the scale you need to be in grains per lb of pull weight. Because , lighter bows, the light end of the spectrum, just makes for a really light arrow. 50 pound bow at 5 grains per lb, is a 250 grain arrow. Even though it will be pretty fast, the combination of weight and speed is still too low in KE and momentum to be useful. Where a 70lb bow at 5 grains per lb is a 350 grain arrow. While that's very light, it would generate enough speed and energy, Ke and momentum ,to be lethal on light to medium game, where the 250 grain would not. Actually building a few test arrows, and shooting them during sessions over the year, helps you understand exactly what your bow will do with a 5 or 6 grain per inch arrow or a 9 to 10 grain per inch arrow. Even though your everyday do it all arrow will probably fall in that 7 to 8 or so. Just playing around some with arrows built toward the opposite ends of the spectrum, helps to see in reality what happens. And then you know. Lighter poundage shooters will just average more weight in grains per inch of arrow and total weight....then higher poundage shooters in general....its just the way the math works and practical physics.

And draw length can play into this also. Because total bow energy is not just the pounds of pull but also how far you draw it back , so arrow selection can change some also, based on the draw length.

 

I must not have worded my question correctly. I understand all what you said and I understand the differences in bow weight and arrow weight.. What is still unclear is this...
A 400 gr arrow from a 50lb is 8gpp and is fine for Deer hunting.
That same arrow from a 70lb bow is frowned on as too light even though more energy is going in.

 

A 400gr arrow is fine regardless of you shooting 50# or 70#. (assuming the arrow is of the correct spine) There are folks who insist hunting with less than 600gr is "Unethical" but that isn't true.

I tell my customers to shoot the heaviest arrow that provides them with the trajectory they can live with!!! In most cases those arrows fall in the 7-8gpp. Some guys like more speed so they shoot 5-6gpp and their deer are just as dead.

Don't over think it. If you have an arrow set up you like and are confident with, go hunt!!!!!

 

A mid-range 400gr arrow from a lighter bow will much more likely absorb a greater amount of the generated energy of the "system". So a 400gr arrow from a 50# bow is going to be quieter and penetrate pretty well.....especially if coupled with a cut-on-contact type BH which maximized penetration. Younger hunters, older hunters and lady hunters often draw less bow poundage and can get good results from such a setup, but keeping in mind they are also limiting shot distance to maybe 25 and in.....out to maybe 30 yards.

Now take the exact same arrow from a bow with a draw weight 20# higher......it is NOT likely going to be absorbing the greater amount of generated energy. This results in an arrow that is faster, but a bow system that tends to be more noisy. The 70# shooter is often looking to shoot longer distances and very possibly looking at a mechanical BH because they can "blow through deer like butter" at that poundage. Thus you have the more conservative approach with the lighter bow weight and more radical mindset with the higher poundage setup. Many experienced hunters shoot higher poundage so they can up their arrow weight to get superior penetration, while still maintaining a respectable fps. That is the basics......

 

I hunted for years at 5gr/#. Never had an issue with it. However, my DW was higher too. Now coming off an injury with my bows cranked down, I am closer to the 7gr/# line on my hunting rig.

Here is the theory for me, and I am sure others will disagree. Understand, I am a shorter DL too which factors into my choice. At 70#, I can sacrifice a little weight and still get the energy/momentum needed to make a clean kill. I am going for that super flat trajectory knowing that the arrow will do its job at any range I am willing to shoot to. At 55-60#, I am trying to absorb as much of the bow's power into the arrow. I can still shoot to the same range, but now the arrow arcs more to get there.

The noise heard when firing is unused energy being dispersed through the bow rather than into the arrow. Way back when Beman arrows were being fired out of 4" overdraws by 85# bows (I may or may not have had such a rig in the 90's), you heard the shots like .22's going off in the woods during a 3D shoot. Many who have gone to heavier arrows with higher weights are understanding that subconsciously. They know they can milk more of that power into the arrows by using heavier arrows. Do they NEED it? Not really on most game, but it doesn't hurt. Especially given the more common mech heads in use now.

Also the common usage of laser range finders as changed the game a lot too. In long range rifle shooting, knowing your range and trajectory takes away any reason to use a lighter bullet. If I KNOW my target is at 650yds, I can use a 200gr at 2600fps bullet rather than a 150 at 3000fps- numbers guestimated for a 30-06 here- and get less drift (the only variable you have problems with in LRS) and more energy on target. Same for a bow. Back when only a few guys were walking around with the dial to focus, optical range finders, flat could make a huge difference in a hit, miss, or wound. Now, you can carry that heavier arrow with confidence that you will KNOW the deer is at 31yds not 35.

 

PS I forgot to add I have owned several bows that were prone to limb failures and none of mine ever had limb issues. I usually shoot around 7 - 8 GPP and that's a lot easier on the bow. I'm not saying you should have to but if you do choose to it's a lot easier on the setup than running arrows at 5 GPP.

 

Ranger, a 400g arrow is a killer…..but its only one factor. If that 400g arrow is wobbling its way to the target, the wobble robs it of a lot of its energy.

If you match a light bow and a light arrow to a large inefficient Brodhead, you are also robbing that arrow from its energy.

The problem with pushing a light arrow faster is that it increases the resistance at the animal. Yes, generally faster is better but in regards to arrow penetration, you get diminishing returns from more speed. A drastic example would be a guy shooting a 70# bow with a 350g arrow at 315fps….that guy would get better penetration with a heavier arrow. Search ‘Old Joes’ posts for the physics explanation of that.

Then we have folks trying to become celebrity status on Youtube with crazy heavy arrow and foc recommendations.

The best advice is along the lines of 1/2 Bubble’s comment above…find a trajectory you can live with, tune your bow for perfect arrow flight and if you do have a lighter setup match it to a very efficient BH design.
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I have used 420 grain arrows at 70 pounds for around 30 years, worked fine always got pass throughs and usually buried deep in the dirt, even hit roots and had to unscrew the broad head. I now shoot 55 pounds with a 550 grain arrow, works fine, get pass throughs (I do have a 31" draw length which helps). My advise is either one will work, but I have found that the noise issue is very important as well as really sharp broad heads. If you are shooting 50 pounds, stay away from mechanical heads.