Percent of energy storage to draw weight ratio

[gfugal]

So the old method of adding the numbers and dividing by 12, does that give me higher or lower numbers than I actually have?
And, what percent is it off by in average?

What numbers are you adding together? The draw weights at each measurement? The final draw length times the final draw weight ÷ 2 then 12? I'm not sure what the old method is.

[gfugal]

Adding up squares on a graph paper then dividing by 12? That will get you in the ball park but not be the most accurate.

[willie]

Anything over 100% is excellent like mentioned.

Greg, could you give the short version of how getting results over 100% is possible?

thanks

[Hrothgar]

I'm late getting in on this discussion, but when you are talking about desiring a high percentage stored energy, like the 95% mentioned for a flight bow, doesn't that come with a shorter life expense?

[gfugal]

Anything over 100% is excellent like mentioned.

Greg, could you give the short version of how getting results over 100% is possible?

thanks

Sorry, I realize this topic is super old, but I just rediscovered the "see new replies to your posts" button and didn't realize willie asked me a question specifically.

So, this percentage is a ratio: Stored energy (in ft-lbs) divided by draw weight. 100% doesn't mean maximal energy stored. It just means the stored energy value happens to match the draw weight value. Modern compound bows are well over 100%, like 200% and even more. It's very much possible to get over 100%. 100% would be a straight line on a force curve like this:


I just picked Simson's bow #100 to illustrate that it's very possible to get over 100% with a "primitive bow". ( I chose his because he's one of the few that actually posts force curve data). Here's a picture of it to recall your memory (hopefully Simson doesn't mind)


And here is his force curve. I didn't know his arrow speed, but I calculated that at 65% efficiency and 10gpp it would be 183 fps, which seems very reasonable if not slow for a bow of this caliber. But that's not the point. Notice the box that says stored energy per draw weight. This bow is at about 115% SE/DW.


Now compare that to a compound bow I have which has a SE/DW of 233%


As well as too this bow I made a while ago at a SE/DW ratio of 55% (which isn't anything to brag about). However, it has an extreme overdraw at 25" (28" shown in picture) despite it's length of 43" with a non-working handle. It stacks badly!

[willie]

thanks, Greg. that was better than the short version.

[Badger]

Anything over 100% is excellent like mentioned.

Greg, could you give the short version of how getting results over 100% is possible?

thanks

Sorry, I realize this topic is super old, but I just rediscovered the "see new replies to your posts" button and didn't realize willie asked me a question specifically.

So, this percentage is a ratio: Stored energy (in ft-lbs) divided by draw weight. 100% doesn't mean maximal energy stored. It just means the stored energy value happens to match the draw weight value. Modern compound bows are well over 100%, like 200% and even more. It's very much possible to get over 100%. 100% would be a straight line on a force curve like this:


I just picked Simson's bow #100 to illustrate that it's very possible to get over 100% with a "primitive bow". ( I chose his because he's one of the few that actually posts force curve data). Here's a picture of it to recall your memory (hopefully Simson doesn't mind)


And here is his force curve. I didn't know his arrow speed, but I calculated that at 65% efficiency and 10gpp it would be 183 fps, which seems very reasonable if not slow for a bow of this caliber. But that's not the point. Notice the box that says stored energy per draw weight. This bow is at about 115% SE/DW.


Now compare that to a compound bow I have which has a SE/DW of 233%


As well as too this bow I made a while ago at a SE/DW ratio of 55% (which isn't anything to brag about). However, it has an extreme overdraw at 25" (28" shown in picture) despite it's length of 43" with a non-working handle. It stacks badly!

   I didn't see your old posts, I plan to download your calculator. On bows where I have gotten 120% with a 28" draw it usually has involved massive recurves and lots of reflex. 100% can be had with minor reflex and gently flipped stiff outer limbs. Stored energy is a very poor predictor of performance as most everything we do to store more energy has a negative effect on efficiency. Overall high energy storing bows do usually tend to be faster but not by as much as you might expect.

[gfugal]

   I didn't see your old posts, I plan to download your calculator. On bows where I have gotten 120% with a 28" draw it usually has involved massive recurves and lots of reflex. 100% can be had with minor reflex and gently flipped stiff outer limbs. Stored energy is a very poor predictor of performance as most everything we do to store more energy has a negative effect on efficiency. Overall high energy storing bows do usually tend to be faster but not by as much as you might expect.

It's definitely not the only thing that matters for sure. You can see the compound bow I posted had a SE/DW of over 200% yet could only spit out an arrow the same grains as 10 times it's max (not final wright) at 160 fps. Even estimating it's speed at 10 gpp of 196 fps, it isn't that far out of reach for the top wood bows half that ratio. The compound is actually a really old bow, probably from the late 70s or 80s. As you can see it's efficiency is pretty low.  I don't have some good wood bows to test out, but I'm guessing they are more efficient than it by a lot. That is the other factor: efficiency.

Those two things are what determines the speed of a bow. You have to have both. If you you had a horrible force curve like my bow, it doesn't matter if it had 100% efficiency. If it's a stacky mess it's going to be slow regardless. On the otherhand, if it has loads of stored energy but is highly inefficient, it will be like my compound.

It's interesting that you bring up the point that the things that make bows store more energy also tend to make it less efficient. So it becomes a balancing game where you try not to optimise one at the expense of the other. There are things you can do to increase efficiency: lighter tips, non-stretch string, minimizing set, etc. I wan't to point out, like you said, by designing large hooks which give you good energy storage, also causes higher stress on the working limbs increasing the chance of set as well likely increasing tip weight since the recurves/siyahs are static. These are just a couple of examples

[simson]

Of course I don't mind!
I foloow this thread with great interest, have to reread the most. I haven't understood everything, math and physics in English isn't my thing.
But I'm hooked!!

[mullet]

You guys make my head hurt.

[willie]

You can see the compound bow I posted had a SE/DW of over 200% yet could only spit out an arrow the same grains as 10 times it's max (not final wright) at 160 fps. Even estimating it's speed at 10 gpp of 196 fps, it isn't that far out of reach for the top wood bows half that ratio.

SE/DW
if the compound max draw is 48#, is not that what you should  used in the SE/DW calculation? do folks in the compound world actually use this metric? 233% seems to be about the dropoff more than anything else.

It's interesting that you bring up the point that the things that make bows store more energy also tend to make it less efficient. So it becomes a balancing game where you try not to optimise one at the expense of the other.

is the compound more efficient?

is high early draw weight more efficient?

[Badger]

Willie, high early draw weight bows are usually fast because they store a lot of energy. How efficient they are depends on several other things. How much hysteresis was  created in the limbs do to cell damage in the wood and how much distortion is the limb experiencing prior to releasing the arrow. High energy storing designs will typically have lower string angles which are much better at slowing the limbs and sucking out the energy near the end of the power stroke but if they have too much working limb available to distort it will often loose some energy by flexing in the inner limbs. If the recurves on a bow are bulky and heavy reducing working limb area is the only way to control the distortion.

[willie]

so if there is any efficiency advantage to a compound, it is because of the low hysteresis of the limb rather than any particular shape in the force draw curve?

I have always wondered if having so much energy available later on in the powerstroke makes a difference?

[Badger]

  The newer compounds are pretty efficient, I think the older ones lost a lot more through their pulley systems than the newer ones do. I really don't know squalt about compounds but I know they have very little limb movement.

[PatM]

Kooi's papers address the energy storage versus efficiency battle.  The gains are obviously pretty marginal but considered worthwhile when you are going for absolute max performance.
   A lot of people still trumpet the F/D curve as telling the whole story of a bows potential.