Stored energy

[Jjpso]

Hello there.

If all the rest is the equal, wich bow has more stored energy: a 50 pounds at 26 inches or a 50 pounds at 28 inches?
And between a 50 pounds at 26 inches and a 44 pounds at 28 inches?

Thanks in advance.

Joćo Pedro

[Badger]

  The longer draw will store more energy but the 50#@26 will store more than the 44# even at the longer draw.

[mwosborn]

My physics is a little rusty but for gravitational potential energy:  E = mass x force (gravity) x height

I think the same principle would apply to a drawn bow.

50 @ 28 would have more stored energy.

[Badger]

  Mitch it is similar to the gravity but with gravity the force is constant. When measuring a bow you simply add up the force applied at intervals usually 1". A 50# weight dropped 24" would have about 100# of stored energy where a 50# bow drawn 24" ( plus a 6") brace height would likely store a little over 50# energy depending on the force draw curve.

[bradsmith2010]

wow

[mwosborn]

Badger - you are correct, the force on the arrow could (would) vary throughout the "stroke" of the string being released.

[Chadwick]

Graph it! Draw weight at each inch. Draw weight on x axis, draw length on y-axis. You get a force-draw curve. A computer, regular addition, (or exhaustive calculus) can tell you the amount of area under the comparative curves, which will equal the stored energy. Longer draws are usually extra inches of near-maximum energy storage, depending on the shape of the curve (More energy!). For instance, a 25" draw 40# compound bow will store more energy than a 28" draw 50# longbow, usually, because of the relative curve shapes and area under their curves. Efficiency is how well stored energy becomes moving-arrow energy. You can detect inefficient bows because they are loud. Increase efficiency by increasing arrow weight --- you'll notice that bows get quieter with heavier arrows.
Although I've studied it, these ideas are not mine- all are explained in TBB volumes.

[KS51]

Potential Energy (PE) = draw x final poundage /2   (approximately)

however, the draw length is the effective draw length = draw length - brace height

a 28" draw with an 8" brace has the same effective draw length as a 26" draw with a 6" brace height.

Ken

I say approximately because the actual Potential Energy available is the area under the force-draw curve on the return, not the draw.  But we can never truly know this curve.

[Badger]

I say approximately because the actual Potential Energy available is the area under the force-draw curve on the return, not the draw.  But we can never truly know this curve.
[/quote]

  Ken, there actually is a method you can use to track this curve, by establishing the virtual mass of a bow. If no hysterisis is present the virtual mass will stay consistent regardless of arrow weight sshot form the bow. The way wood reacts when shot hysterisis will become more pronounced as the limb speed increases with lighter arrows. I started a thread on this the other day with regards to set and how it affects the shot. The concept is simple but I am not good at trying to explain it. Part of this concept you can see with the naked eye. Say you unstring a bow, if you could neasure the set like within 1/10 of a second you might have 2" set. 1 second later it may be 1 3/4",  10 seconds later 1 1/2"  1 min later 1 1/4" and 10 min later 1".  You see a pattern develop that shows the closer you get to unbraced profile the slower the limbs return, with a super high speed camera you might be able to see this same phenomina continuing as the bow is shot possibly stretching into the bow 6" before brace height. It is happening fast but not as fast as the arrow leaves the bow so we are loosing power.

[Jjpso]

Thanks for the answers guys.

Joćo Pedro

[Pappy]

I'm with ya Brad. WOW. got some very smart guys on this board. Don't know anything about the numbers but I know from experience that a 50@28 will shoot quite a bit harder and sink the same arrow in the target much deeper than a 50@26 will.So I would have guessed it stored more energy.
   Pappy

[Jjpso]

I'm with ya Brad. WOW. got some very smart guys on this board. Don't know anything about the numbers but I know from experience that a 50@28 will shoot quite a bit harder and sink the same arrow in  target much deeper than a 50@26 will.So I would have guessed it stored more energy.
   Pappy

I expected the same, but not from my own experience...
The only problem is that my draw length is only 24 inches... 

[Pappy]

Mine was only 25 , I worked on getting it a little longer just for that reason,now I am at about 26,every little bit helps in that respect.
 Pappy

[Blaflair2]

Ok,  so I just read all this... My head hurts. I just make sticks that bend. I don't get all that mumbo jumbo. Physics class was in the morning, I was in a fraternity, u see where I'm going...

[Badger]

  Blafair, there really is no need to worry about all that stuff unless you are flight shooting anyway. Its fun to try and figure out why one bow is faster than another but as long as you are using good wood and good demensions and doing a good job tillering past that you are just splitting hairs. You don't need any math to know that if a bow took too much set it was because it was either wet, underbuilt or poorly tillered. No need to know much more than that unless you just enjoy crunching numbers.