Draw Force Curve - Please Explain

[Over&Under]

Ok so I have been hunting and shooting with Traditional equipment for 6-7 years and have seen lots of bow reviews in TBM and other mags, and think I have a decent grasp of how to read the "Draw Force Curve" graph, but could someone give me a little schooling on this?

I know I SHOULD know, and think I do, but have thought that about lots of things, and actually new alot less than I really did.... so I thought why not ask....right        

Thanks alot
Jake

[Shaun]

I know almost everything - unfortunately wrong about most of it.

So, here's my limited understanding of draw force curve; The larger the area under the curve, the more energy stored and hopefully released into the arrow and not your hand or moving heavy limbs. You can either figure this area by complex calculus or counting the little squares or just kinda squinting at the shape (my method).

More accurate measure of bow potential is crono with 10 grain per pound of draw weight at a standard draw length.

[Over&Under]

Hummmmmm..... If you are even close to correct, I was way off!!  I was thinking that the closer the curved line was to the straight line, the smoother the draw was, and it demonstrated how the bow stacked as it was drawn back.

Are we talking about the same thing, but in a different way???

Shaun - the squinting part made me chuckle...thats funny!

[BobCo 1965]

The draw force curve does not show the efficiency of the bow. What it shows is stored energy.

Usually it has the draw length across the horizontal axis and the pounds required to pull across the vertical axis. Everything below the line is stored energy. What it will show is the sweet spot on the bow just before it starts to stack (a sharp increase vertically). If it is already stacking at my draw length, then I know this may not be the bow for me. A good custom bow should have your the sweet spot just before stacking right at your draw length.

[Shaun]

Jacob,

the closer to the line would be an inefficient bow but nice to draw. A wheel bow would have a big hump in the middle of the draw and then fall off to let off weight.  - therefore more energy under the line. A good stick bow will have a slight bulge over the line and be close to the line at full draw. Then if you keep drawing you hit stack as Bob pointed out and the line will go up steeply.

A recurve or R/D longbow will have more pre weight (tension at brace) and start off pulling hard in the first inch of draw. This also adds more energy under the curve. A straight or string follow bow will have less tension at the start of the draw and be "sweeter" to shoot but less efficient.

[cbCrow]

Go to ACS bows look under Knowledge Base , there they explain all you would ever want to know about this subject plus more. Shaun is right in what he told you.  

[Over&Under]

Shaun

Great explanation! That makes total sense.  At least now that I look at the graphs in the future, I will have a better idea of how the bow performs.

Interesting concept about stored energy.  You begin to appreciate the science that goes into a good bow design that excels at both efficiency and ease of shooting.

Crow - thanks for the tip, I will check that out!

[Gil Verwey]

I never understood how a bow that is marked, say 50# @ 28, on the draw force curve shows that it's stored energy is less than 50# @ 28. Every test I have seen shows that the stored energy at a draw length is less than the draw weight of the bow. I will check out the A&H site and see how they explain it.

[Over&Under]

Couple of questions -

So how is the straight line of reference determined on which the "curve" is compared to?

It appears that it begins at the brace height, and shoots through the actual draw weight @28".

And once the two lines meet at say...25" +/- of draw, and follow each other for a few inches until it starts stacking, does that mean the bow is no longer storing energy?

That doesn't seem right as the longer you draw the more lbs you are pulling, thus providing more energy.  I may just have "stored" energy confused with the type of energy produced after the limbs are drawn past a certain point...

[John Havard]

Over&Under, you're asking most of the right questions.  Here's my take on the subject:  preload (the early F/D hump) is an EXCELLENT thing.  Store lots of energy early.  Let's say you want to shoot a bow that's 50# @ 28" and you have the option of having a bow that has a straight line for a F/D curve versus one that has a good amount of preload.  You end up at 50# either way.  There is zero downside to a bow that stores a lot of energy (presuming the bowyer designed it to be stable and very shootable too).  

What makes a bow hard or (relatively) comfortable to pull to and through your draw length is how much its draw weight is increasing from 27" to 28" to 29" (using the above hypothetical bow as an example and the archer "pulling through" his release).  

I'd urge you to scan our website pages in which I spent a lot of time trying to explain energy storage, dynamic efficiency, and then providing some examples that explain what's going on with each.  Be sure to click on the "for more information...." links.

John

[Over&Under]

John

I think the more I look into this, the more I am understanding.  I was looking at your website a little bit ago, and lots of info there.  When I get home, I plan on spending some serious time there, and reading the ins and outs of it.  

Thanks for your input and thoughts.

Jake