force draw curve graph question

[don s]

can someone explain to me how to interpret a f/d curve graph? i know how to set the graph up and transfer the information from your bow to points on the graph. when your finished and the dots are connected, how do you read the line? what exactly does this represent? i always mean to ask but end up forgetting. now i see one posted and figured i'd ask while i'm thinking about it. don

[hova]

it shows the force curve. if your line is almost straight , then it shows an even , smooth stacking draw. if you get to 20" then your weight jumps from 20# to 28# , thats a big stack.


thats how i interperet it. im not knowledged enough in th scale to know if a r/d should have a different looking line , but from what i understand , a good thing to aim for is an even line or arc. i would imagine the more arc , the more stack.


someone else will know more about it
-hov

[don s]

thanks hova, so far that makes sense. don

[K30bowfisher]

Like Hova said, it can show you where your bow begins to stack. Also, the area under the curve is a measurement of the energy being stored by your bow. It can give you some useful information if you are looking to empirically compare several bows to one another.

[don s]

k30, how do you read the area under the curve? don

[4est trekker]

Stacking is often times (always?) caused by a string angle near the tips that approaches or exceeds 90 degrees, whether due to tiller profile, overdrawing, the use or non-use of siyahs/recurve/reflex, etc.  Stacking is the manifestation of poor mechanical leverage.  However, EVERY bow, at some point or another, will begin to stack.  Some just won't begin to display it within the reasonable range of draw length, but if you draw it too far then you will inventively force the bow to the point of poor mechanical leverage, and thus stacking.  

Here's a little experiment to demonstrate, but it only works on straight and deflex/reflex profiles, NOT recurves because it can easily twist a limb.  Get a piece of cord (like those used in stringers) about 1.5 times the length of your bow and tie a small loop on each end.  Place a loop in each nock, put your feet shoulder width apart, and center the string under your feet.  Gently pull up on the bow at its center until the limbs are bent to about brace height.  Now, spread your feet farther apart and try to pull to the bow the same distance.  You'll notice that it's substantially harder to do the farther you spread your feet apart.  In effect, the bow is demonstrating stacking because of the poor mechanical leverage (i.e. increased string angle) being imparted at the tips.  

Hope that helps.

Curt

[don s]

thanks 4est. i understand stack. what i'm trying to learn is how to interpret the area under the line on a f/d curve graph, as k30bowfisher states. don

[Swissbow]

Hi Don, look at the graph, it shows the draw weight curve of a reflex/deflex bow ( blue ) and of a straight longbow ( red ).

 

If you look at the red curve you can see that the draw weight starts slowly at the beginning of the draw. The bow draws very smooth and soft. On the other hand the d/r bow draws pretty firm from the beginning and the weight increases steady until the end. The area under the curves represent the energy that is stored while drawing the bow. TheDue to the reflex the preload of the limbs is much bigger and therefore the d/r bow stores more energy.

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Andy

[4est trekker]

Oops...sorry, Don.  Swissbow nailed your question, though.  

[don s]

thank you swissbow. i do believe i've got it. don

[hova]

excellent. answered my questions as well!

[Aznboi3644]

4est sorry but stacking is not a horizontal line.

On a graph the line would be rising up to be stacking...meaning a sharp increase of draw weight per inch.

A horizontal line near full draw is a smooth stackless draw with more energy storage.

The area under the graph can be interpreted in inch-pounds of force.

[BenBow]

Here's a link to a great free force draw curve program. Easy to use and gives you lots of good info.   DrawCurve Calculator

[4est trekker]

Aznboi: Thank you for point that out.  I just realized that, unlike the standard orientation of draw length on the x-axis and draw weight on the y-axis (as in the graph above), I orient my oppositely.  Also, thanks for correcting my error regarding a sharp INCREASE in draw weight.  The truth be told, I very rarely use an f-d curve when studying a bow, and my interpretations of stack in a bow generally lead me to a study of poor mechanical leverage. I should have spoken on that subject alone and not on f-d curves.  

Regards,

Curt

[monterey]

Originally posted by don s:
can someone explain to me how to interpret a f/d curve graph? i know how to set the graph up and transfer the information from your bow to points on the graph. when your finished and the dots are connected, how do you read the line? what exactly does this represent? i always mean to ask but end up forgetting. now i see one posted and figured i'd ask while i'm thinking about it. don

don, here is a really poorly done fd graph.  On the plus side, it is actual data from my last project.  Downsides are;  It's done on a tillering string that was a bit short and thus starts out at 9" brace height measured from the back of the bow and it's done off a tillering stick on a bathroom scale.  The lower end measurements are not very reliable.

 

This is a 68 1/4" NTN long bow with an arced back set of 1 1/4"  Usually this much preload will start off with a bit plumper curve, but once again, it's that bathroom scale.  Guess it's used to my 270# hulk.    

Given the length of the bow it is clear that this bow does not stack even out to a draw length of 30"

Here is a hypothetical bow with some serious design or construction defects.  It would be fine for a youth or small person with a very short draw.

 

Notice how the fd line begins to rise more quickly between 24 and 25 inches.  From there it rises even more quickly.  For some design or construction reason, the bow has reached a point just past 24 inches where the angle between the bow hand, the string nock and the arrow nock is beginning to exceed 90 deg.

Now, remember how others explained above how the area under the fd line represents energy storage.  The more quickly the fd line rises, the less area there is under it if you draw a triangle there.  When the line becomes vertical (this is theoretical), there is no energy storage taking place.

Hope that helps and I'm sure others here with more knowledge and experience can further clarify what I'm trying to say.

[don s]

yeah, that helps me even more. thanks to everybody who posted. i'm sure there are others here who benifited from the info. don

[J3ff]

I did a force/draw graph for my ELB, i then calculated the area under the graph to give me the total force, then did some horrendous calculations that ended up telling me I could shoot a 25g arrow at 50 metres per sec. Two weeks ago I had a go on an arrow speed tester - result? 165ft/sec or 51 metres per sec.

No idea what to do with the information but it whiled away an hour or two.