Question about FPS and poundage

[blackhorn]

Just wondering how FPS relate to poundage.  If I had a bow set at say 60# and it shot 300fps and another bow that was set at 70# and shot at 300fps what would be the deference?  More kinetic energy? 

[Button Nubbs]

Given they are shooting the same arrow at the same speed everyting would be equal. The only difference would be that you are pulling 10 more pounds. Maybe I'm wrong but i don't see how anything would be different.

[Ray]

The energy which archers should be concerned with is when the arrow hits. An arrow weighing more than another arrow and both traveling at the same speed and hitting the target exactly the same... The heavier arrow will usually penetrate better. However that depends on if the arrows are of the same makeup for example...

[blackhorn]

That was my thinking too, but I was not sure.  Thanks!

[Hornseeker]

I can see where button nubs confusion came from... but you are assuming the 70# bow is shooting a heavier arrow than the 60... like Ray said, if that is so... then the 70 will give you the benefit...

Or am I confused?

[blackhorn]

I am thinking that a 70# bow would allow you to shoot a heavier weighted arrow giving more force on delivery.  Kind of like a semi will do more damage than a vw bug going at the same speed.  Now I am wondering if the trajectory would be the same?  Is the heavier arrow going to drop sooner or later than a lighter one?

[yajsab]

You can make the light arrow and heavy arrow hit the same point at certain distance.  But you'll have to aim higher on the heavy arrow.  So to answer your first question, no, they won't have the same trajectory angle.  However, if the bow can produce excessive fps on the heavier arrow, then they can have the same trajectory angle.

For the 2nd question, the heavier arrow will have more drop due to g-force.

 

[MIKEXRAY]

The more I thought about this , the more I confused myself. At first I thought you could have a 60# bow & a 70# bow both shooting 300fps & shooting the same arrow and I was sure the kinetic energy would be the same since the arrow is the same weight ( didn't see a different arrow weight mentioned) . BUT the more I though about it I realized that one arrow could maintain the 300fps for a longer time ( not measured ) thus allowing more kinetic energy down range ?
I know all bows have an efficiency percentage. It is the energy required to pull the bow versus the energy the arrow is delivered with. You could have two different brands of bows, one set at 60# and the other at 70# and both could shoot the same arrow with all the same stats. Simply because the 60# bow ( what ever model ) is more efficient. Just because you are shooting 70# does not mean you are way out performing your buddy at 60#. Will think about it more while sitting in traffic today. Mike

[Jason]

you'll always get more Kintic Energy with a 70lb bow, simply because you will always shoot a heavier arrow faster from a 70lb than a 60lb bow, and your not going to get a 60lb and 70lb bow to shoot the same weight arrow at 300fps   
a 60lb bow uses a 400 arrow
a 70lb bow uses a 340 arrow

you could shoot a 340 arrow out of a 60lb bow but its not going to be near the speeds of a 70 lb bow

you could set both bows up with a 340 arrow.
so the reality is your never going to get a 60lb bow to push the same weight arrow at the same speeds as a 70lb bow will and it wont be anywhere near the same kinetic energy
my 70lb 28" Allegiance will shoot a 440gr total weight arrow at 293fps = 83.9lbs KE
if you could get a 60lb Alley to push the same arrow you might get 240-250fps then you would get 240fps=56.29  250fps=61.08lbs KE

[high country]

don't forget about differences in bow efficincies.....my 70lb longbow has nothing on my 70lb compound in speed of arrow, but whips it clean in the time it takes to loose an arrow on target.

[xXx Archery]

60lbs with a arrow at 6 gr per lbs
Total Arrow Weight: 360
Bow IBO Speed: 340
Adjustment for Draw Length: -10
Adjustment for Draw Weight: -20
Adjustment for Arrow Weight: -3
Adjustment for Weight on String: -6
Calculated Speed: 301 fps
 
Kinetic Energy: 72.44

now 70lbs 6 gr per lbs
Total Arrow Weight: 420
Bow IBO Speed: 340
Adjustment for Draw Length: -10
Adjustment for Draw Weight: 0
Adjustment for Arrow Weight: -23
Adjustment for Weight on String: -6
Calculated Speed: 301 fps
 
Kinetic Energy: 84.52

[blackhorn]

XXX that is something I can wrap my head around.  Now for the dumb question, which one will have the furthest arrow flight?  I would assume that it would be the heavier one because it has more kinetic energy behind it, but I am not sure. 

[Tony 270]

I took the question of having a newer, more effecient bow at 60 pounds shooting the same weight arrow as an older, or less effecient bow at 70 pounds. Original post didn't say anything about arrow weight difference, only draw weight. 

[Jellymon]

If the arrows are the same I would bump that 60 pounder up to 70 and be shooting 320!

[lokidog]

For the 2nd question, the heavier arrow will have more drop due to g-force.

 

Gravity has the same effect on objects regardless of their weight, a feather and a cannon ball in a vacuum will both hit the ground at the same time if dropped at the same time.

Since we don't live in a vacuum, the arrow that has the largest cross section will slow down faster due to air drag (like different ballistic coeficients of bullets).  For practical purposes, i.e. bow ranges, both arrows will have the same trajectory if they leave the bow at the same speed.  The heavier arrow will have more momentum, meaning it will penetrate farther with the same broadhead as the lighter arrow.  Of course, if it has a larger diameter, that could slow it down more going through bone......  Oh my head hurts