### Re: WTT3 bow photos and FD curves (images on page 3 added May 9)

Posted:

**Sun May 24, 2009 1:16 pm**Kirk, 450240 as far as I know is just a constant that works with grains of arrow weight. It seems to be pretty accurate.

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Posted: **Sun May 24, 2009 1:16 pm**

Kirk, 450240 as far as I know is just a constant that works with grains of arrow weight. It seems to be pretty accurate.

Posted: **Sun May 24, 2009 2:03 pm**

This is one of the reasons why some testers never use the same weight arrow but a variety of weights and then extrapolate out the speeds for specific weights, it's actualy very accurate that way. By trying one of the bows with a very heavy arrow maybe filled with dirt or something and then shooting a somewhat lighter arrow maybe like 7 grains per pound I bet we could narrow down where there is a problem or if there is even a problem. 98% efficiency throws out red flags in a big way. I suspect that stored energy may in fact be a tad higher than we are calculating. The more measurements you take the more accurate it normaly is. Most of us consider measures at increments of 1" close enough but measurements at 1/4" would even be closer. I think that is a bit too much trouble to be realistic though and feel 1" should be close enough. It may change the final answer by 2# or so at the most I think for stored energy and this might just be what is giving us the high efficiency rating. If I ever get my fdc machine working it will take ten measures per inch, all done in one single stroke. Steve

Posted: **Fri Jun 26, 2009 8:16 am**

in order for efficiency to be 100%, the energy output has to equal the stored energy or another way to look at it is that when the bow string is released, all available energy is transferred into the arrow. This is physically impossible as there is always energy disperssed into the string, limb vibration, ect. Thats why heavier arrows always increase the efficiency of your bow but i have yet to see any bow come close to 100%. Low 90s maybe.

efficiency is found by using the potential energy you get from the force draw curve and the speed you get by shooting through your chrono.

as far as force draw curves go, the best way of calculating your potential energy is using integral calculus. The other "easier" ways of getting this are basically just estimations and won't be as accurate. I used to plot these by hand adn do all the calculus on paper before I got a software program to do it for me. gotta love technology

efficiency is found by using the potential energy you get from the force draw curve and the speed you get by shooting through your chrono.

as far as force draw curves go, the best way of calculating your potential energy is using integral calculus. The other "easier" ways of getting this are basically just estimations and won't be as accurate. I used to plot these by hand adn do all the calculus on paper before I got a software program to do it for me. gotta love technology