Force/Draw Curve

[Wooden Spring]

OK, so I was going through my TBB vol. 1 last night and reading about Force/Draw curves, and I decided to plot my most recent bow, documented below...  Here's the problem - I DON'T KNOW WHAT THE HECK THE CURVE IS TRYING TO TELL ME?!?!?

My first thought is "get a better scale..."    Any thoughts?

[Del the cat]

There is always a fair bit of experimental error in plotting a F/D curve.
Yours looks good.
ELBs are remarkably linear, what you want is it to bow upwards to give more area under the curve (that is the stored energy)
Yours rises steeply bulging upwards which is good. The kink as it approaches full draw could well just be measurement error.
You need to pull it quickly to 6" look at the weight, let it down.
Pull it quickly to 12", look at the weight, let it down... etc

Clarence N Hickman build an automatic F/D curve plotter which plotted the curve as you drew the bow.
I made one, but it didn't work very well as it was mostly of wood and the sliding bearings would bind up... I plan to make a better one some time.
Del

[Wooden Spring]

Clarence N Hickman build an automatic F/D curve plotter which plotted the curve as you drew the bow.

OK, that sounds neat! Was that from "Archery The Technical Side?"

[Del the cat]

There's a picture of him using the machine in
Clarence N Hickman the Father of Scientific Archery.
https://en.wikipedia.org/wiki/Clarence_N._Hickman
I sort of worked it out from that.
I think the book is out of print.
He was a great engineer, he did a lot of work on rocketry and invented the Bazooka. He had an rocketry accident whick lost a couple of his fingers, so he designed a more efficient bow, which is effectively the start of the modern target recurve.
Del

[Aries]

I haven't read that chapter in a long time but here is I walked away from it thinking.

The more area under the curve represents greater total energy stored and an efficient bow.

Short bows have less area under the curve compared side by side with a recurve of similar poundage and draw length which Is reflected in the recurve transferring more energy to the arrow and having more fps at the same poundage and draw length.

[bradsmith2010]

with out anything to compare it too,, it is a little myserious
but if you do a curve on some other bows you will be able to see where that bow is as far as storing energy or how efficient it might be,, some of the more scientific guys will chime in and explain better I am sure,, 

[DC]

What can you do to a bow to make the curve hump up?  A while back I did an F/D curve on a recurve I made. You could definitely see the hump where the string lifted off. I understand how that works but I can't see how anything but a recurve can hump up the F/D. Is there any design other than a string lengthening recurve that can produce a fat F/D?

[Badger]

      For the most part a well tillered long bow will usually store about 95% of its draw weight at 28", anything over 100% is real good. The way it is figured is by adding up all the draw weights from the first inch of draw ( usually at about 7") all the way until the last inch. The total of all these numbers will give you inch pounds of stored energy, divide them by 12 to get foot pounds of stored energy.

    Non recurves can have a fat FDC by adding reflex to the bow, the more weight it has up front the smaller increments it will have to build up to draw weight so the curve will be flatter. A bow that stacks, usually a shorter bow or whip tillered bow will show a sharp rise in the pounds per inch it draws toward the ened of its stroke.

    Lots of things that make a force draw curve fatter are also energy robbers so FDC aint everything, just one aspect to be measured.

[Wooden Spring]

      For the most part a well tillered long bow will usually store about 95% of its draw weight at 28"....   

Awesome! I love math... 
So, by looking at that figuring, my F/D curve reveals 582.44 square inches, that's 48.53 foot pounds. Since the bow was 52# @ 28", that makes the bow 93% efficient. It is slightly inefficient, but it's a very accurate bow, so, I'll take it! 

[Badger]

  93 % is pretty good for a long bow, I just through out a round number. Efficiency is a different matter. Once you have your stored energy established you can establish your efficiency by shooting the bow through a chrono and finding out how much of your stored energy is going into the arrow. About 70% efficiency is a common number.
48.5X.70=33.95 footpounds kinetic energy.  This would give you a speed with a 520 grain arrow of about 170 fps which is very good for a self bow.

  The formula for kinetic energy is vXvXm/450240=ke               170 X 170 X 520 / 450240= 33.3 ft KE