Percent of energy storage to draw weight ratio

[willie]

  The newer compounds are pretty efficient.......... I know they have very little limb movement.

the smaller movement would show a gain in efficiency due to a smaller hysteresis loss, but we often cite hysteresis losses as being around 10%.

even if a compound was 0 %, would that explain all the difference a compound has? efficiency wise?

[Springbuck]

With compounds, they have come a long way.  All the stuff that made them quieter makes them very efficient.  Parallel limbs, for one.

[gfugal]

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

Well the ratio is based on final draw weight, which is usually the highest in traditional bows, but yes compounds are the exception. I suppose if you based it off that it wouldn't be quite so ridiculously high. If I use the max draw weight instead if final I get a SE/DW of 105% which is still good, but nothing fantastic. Compou d people (not that I am one, or know of any) probably don't use this value because they aren't into building compounds. I think only the engineers who make the bows for the company would care, and even then it's a less useful tool for them than it is for us.

is the compound more efficient?

is high early draw weight more efficient?

Compounds aren't inherently more efficient because of high early draw weight. The force curve and efficiency are two very different things. Just because you have one doean't necessarily mean you have the other. Sure they may frequently show up together, good energy storage and good efficiencg, in those cases you have a great shooter on your hands. Jowever, it's also possible to have one or the other as I gave example of before (although I would add that poor force curves will tend to lead to poor efficiency as well because if you have a bow that stacks bad it most likely is overbending to get those bad string angles. Therefore extreme set is likely which induces more hysterias).

So yes, modern compounds may be more efficient (remember my old one from the 80s wasn't efficient). But they may not necessarily be more efficient because of their good force curve and early draw weight. I'm guessing it has more to do with their materials and design that reduces weight, and minimilizes the distance the limbs have to bend.

[Badger]

   I took an fdc on one this morning  77#@28". I plan to retiller down to 70# and flip the tips just a bit and am curious how much it will affect the FDC.  Starting at 7" and ending at 28" the fdc reads like this.    6-11-14-17-20-24-27-29-32-35-39-43-47-51-54-57-60.5-64-67-70-73.5-77. I figured it using the method of just adding up the numbers for inch pounds and then dividing by 12 for foot pounds. I cam up with 76.5 I believe. Curious what the actual number is. Going to retiller and flip tips now.

[willie]

interesting experiment , standing by for the news at 6

[Badger]

  I finished the heat treat and recurve process, it picked up a lot of weight. I had about 1 1/4" reflex at that time and now I have about 3 1/2, I hope to keep about 2 3/4. Try to get it tillered back down tomorrow.

[Badger]

  I finished her up this morning 70#@28"  67" N/N.  I picked up almost 4 percentage points on the FDC with the recurves and I suspect I will pick up a little efficiency as well because it is lowering the string angle some. The bow maintained 2 3/4" reflex so I lost 3/4" which I am happy with. No change after sitting for an hour which is a very good sign and usually indicates very low hysterisis. Best case scenario I think the bow could hit 184 to 185 with 10 grains. Anything over 180 I would be tickled pink with. I need to rebuild my shooting machine as I can't draw 70# anymore. So once I test it out I will report back with some photos. I am really curious as to how efficient it will be with much lighter 450 grain arrows I have to shoot in the broadhead event.

[gfugal]

Could you post the new force curve numbers. I'm going to calculate your old one's but it would be nice to see the difference. Keep us updated on the speed.

[Badger]

Could you post the new force curve numbers. I'm going to calculate your old one's but it would be nice to see the difference. Keep us updated on the speed.

Zero is 6"      0-5-10-14.5-19-22.5-26-28.5-31-33.5-36-39-42-44.5-47-49.5-52-55-58-61.5-65-67.5-70

[gfugal]

Alright here's what I calculated

Before re-tiller


And after re-tiller


So on your first one I calculated a stored energy a little less than what you get when you add all the numbers and divide by 12: 73.3 vs 76.5 Ft-Lbs. that's only a difference of 4.4% so I think it's safe to say that it gets a good estimate, however, that method won't work if you have measurements anything other than every inch (I like to do it every 2 inches for example, but some people may like to uses centimeters or something too).

If you have an efficiency of 65% I'm estimating a velocity of around 170 fps. however, I'm guessing that you'll have an efficiency better than that, since it seems to be your speciality. If you get in the 180 fps you'll be more than 72% efficiency. I'm eager to see what you get. keep us posted.

[willie]

Greg

I see that your new spreadsheet also has calcs for virtual mass and efficiency. There have been a few questions posted recently (at least about virtual mass), would be willing to explain how you actually make those calcs in the example above?

[Badger]

  Greg, the virtual mass on a primitive wood bow can be tricky as the hysteresis is time sensitive. In other words we have more hysteresis with lighter faster arrows than we do with slower heavier arrows so it is very difficult to extrapolate speeds of various weight arrows using the virtual mass method. It is possible to isolate and identify a good portion of the hysteresis from other more typical vibration losses but it is a lot of work. When the integrity of the wood has been only minimally compromised during the tillering process the hysteresis can be very low and when it has been more typically compromised it can easily exceed 10% power losses.

   I have found several ways to increase efficiency. Some of the obvious ways are to build shorter bows, efficiency goes up but stored energy goes down and the bow is at higher risk of taking set increasing hysteresis. By building a bow using a no set tillering method where the integrity of the wood is constantly being monitored you get an instant jump of up to 10% this is huge. The other method is to time the unfolding of the limbs so that the inner limbs hit home first and it rolls out toward the tips. This puts much less load on the arrow to try and slow down the limb tips at the end of the power stroke and eliminates a lot of the opportunity for the limb to vibrate or distort. You can pick up as much as 10% here also and even with a longer 68 to 70" bow still realize up to 80% efficiency if everything goes just right. I am thinking about 75% with a 10 grain arrow on the bow shown above but won't know until I test it.