In another topic a member asked me about FDCs. The specific question was "How about starting a thread on that... how you do it, how you use the info, what you've learned?"

So, I'm doing that now. Can't say as I have much light to shed on the subject but it might be fun to talk about it.

First off, How do I do it? It can be done with a simple hand drawn graph or you can use a spreadsheet in excel such as the one at this site http://www.buildyourownbow.com/tag/fdc/

The ones I'm illustrating here were done with the spreadsheet but often I do them with a sheet of graph paper as well.

Your starting draw length point is the brace height which will be zero on the pounds scale. From there you draw the bow at increasing draw lengths and record the weight off the scale. The draw intervals are your choice but the shorter the intervals the more comprehensive the FDC will be.

The spreadsheet will plot the curve for you. If using graph paper you plot the points on paper and then connect the dots/

The next question is "how do you use the info"

My use is mostly to satisfy my curiosity. I do an FDC on just about every bow I can get my hands on. but since I also build bows I do it to get an idea of what a specific design is doing.

Then there is "What have you learned?"

I'm not sure that I've learned an awful lot other than getting some ideas on what I can do to get a little more out of a design.

OTOH, I've learned that a great FDC does not always predict performance exactly. Mostly it does but because a bow has a robust curve loaded with energy doesn't necessarily mean that it will translate into speed or that it will be "accurate" or pleasant to shoot or be quiet or will be with or without hand shock. You can throw in any of the adjectives we commonly use to describe our bows and the FDC will not necessarily be predictive of it.

Other than actual measured speed most of our favorite adjectives are very subjective and personal. One shooters perfect bow can be another shooters monster.

One other point about FDCs, The area beneath the curve tells how much energy is stored in the bow at full draw. It may or may not deliver that energy with efficiency.

The bows illustrated here are not currently available commercial products so there is no risk of bashing any bowyers work [except my own :-) ].

The first illustration is of a 56" NTN bow produced in the early 50's. The only markings on this bow are etched in the aluminum riser and say 40# @ 26". One look at the FDC and you will see that the bow does not draw 40# at 26". It may have when it was built but it does not now. It's a funny looking little bow that you would immediately take for a kids bow but it is not. It has an aluminum ambidextrous riser with solid red fiberglass limbs set into pockets in the riser. The limbs are contoured like a traditional ELB.

In the early 50's my dads lemonwood bow broke during the hunting season. The only bow available in the little sporting goods store was this red bow. Dad purchased the bow and continued to hunt and took a deer with it that season. In later seasons this little bow took another deer and a bear that I know of.

This chart is a good illustration of stacking. A bow stacks when it reaches a point where the angle formed by the nock point, the limb tip and the arrow rest reach approximately 90 degrees. It's not caused by the limbs suddenly becoming stronger or more resistant to bend. It's caused by the limbs running out of an ability to bend and they are now simply advancing at the full length of the limb and mostly bending at the fades.

The sudden rapid increase in draw weight is not contributing much to the bows ability to cast an arrow. Remember that the area under the curve tells us how much useable energy is stored in the limbs. The draw force is increasing rapidly from 26" to 28" which means that there is less area under that portion of the curve. Therefore less stored energy. If this bow was drawn much beyond 28" the FDC would begin to go pretty much straight up. Eventually the tips of the limb would be pointing straight back and the string would slip out of the nocks.

So, while our archer is applying more energy to drawing the bow that last few inches, the bow is not giving much of it to the arrow. You may love your bow and it may shoot wonderfully for you, but if it stacks it is not really a fit to your physical draw length.

Here is another FDC. This one is taken from a 68" NTN hill style bow with a bit of parabolic reflex. That little bit of reflex creates a small amount of preload which translates to a very slight hump in the early portion of the draw. Other than that the FDC is not really a curve at all but a straight line out to about 28.5". This is typical of a Hill style bow and if not drawn beyond the 28.5" mark it is what we like to call "smooth". The force is applied cleanly with no abrupt uncomfortable rise at the end of the draw. Since my draw length with this bow is 28.25", it fits me perfectly and is a pleasure to shoot.

And finally, here is an R/D bow that is 60" NTN. This bow is not finished yet but this is how it looks at this point. Originally it was beginning to stack at 26" but some minor modifications took it's effective draw length out to 28"

The R/D design generates an even larger hump in the early draw since the reflexed tips are holding more energy at brace and that translates to heavier early draw weight that stores more energy in the first 1/2 or more of the draw.

An R/D bow is expected by virtue of it's design to shoot more efficiently and with a bit more speed than a hill style. The area under the curve tells us why and how.

Longcruise, thanks for posting this. Great explanation and examples. I learned some things at least... and not necessarily what I expected to learn! The differences in types of bow limbs and what stacking really is.

How about a FDC for a full recurve and a static tip bow? Got any?

Thanks,

Shandor

I do this on some of my better bows or if one feels strange. I don't necessarily plot a graph but rather just record numbers at 20,24,27,28,29,30 and sometimes 31 inches. Looking at the numbers tells me the same thing as the graph does. I had a Whippenstick Phoenix that felt as if it was highly preloaded. I felt it, some other people felt it, so I did the draw force measurements and found that the bow was not preloaded any more than other top tier customs. Strange but no explanation for that sensation. DFC is interesting and is good information to have if some knuckledragger comes along and wants to know if the bow that you have for sale stacks at his draw length. Thanks for sharing.

Here's a fd curve done on a bow I built. If you have a chronograph it will calculate the efficiency of yorr bow. You can get it here http://tradarchers.net/forum/index.php/top ic,1522.0.html

Shandor, I have an older Shakespeare recurve that I can plot and a Howatt Cavalier static tip as well. I'll do those when I get some time.

Brad, I agree with you on just looking at the numbers. After looking at a few it is understandable without the graph.

Here's what the updated spreadsheet looks like

That's the one I was using but maybe not the one referenced in the link. I converted the chart only to a JPEG and just illustrated that. Do you have the correct link to the updated sheet? I think I got it from one of your posts on tradarcher.com

Sorry, I didn't see that you had already posted it.

Try this if you can't find it look in the bowyers forum as a sticky at the top. http://tradarchers.net/forum/index.php/topic,1522.0.html Here's a link to the spreadsheet I was able to download it from this link. http://tradarchers.net/forum/index.php? action=dlattach;topic=1522.0;attach=3390

That seems to be the one I have.

What I can't figger out is how to enter a fraction of an inch. It would only be applicable for the start point (brace height) and maybe I'm being too anal about the whole thing.

I know what you mean and I hate that weakness of that spreadsheet I do have a program called Force draw curve that will let you put in fractional numbers. I enter the data then copy and paste it over the chart on that spreadsheet. Here's the link to that download http://www-personal.umich.edu/~cdsnyder

FDC are real, but they really aren't good predictors of performance. You could have two bows with identical force draw curves and one would shoot 200 fps, and the other would shoot 50, or nearly 0. What it tells you is the upper limit of performance, because a certain amount of energy is required for a certain amount of performance, it tells you nothing about the output.

FDCs were made famous by Norb, and at any given period of time, you can find compound bows that are as nearly identical as possible, but may have very different cams. FDCs, were a pretty good way of illustrating how those cams would feel and likely perform. In a line like MAtthews, in any years you would have bows where the platform was the same, but the cam was different, or the cams were the same but the platform (bow length, limb angles, etc...) were different. And FDCs were useful there.

As illustrated above the differences on sticks are pretty subtle, which to me indicates there aren't as many points in a pure speed approach to sticks. That used to interest me because in instinctive archery, flatening the trajectory is directly related to making the shot easier to call. But for sight/gap shooters, unless you need to push the range a lot, a few fps extra is not that big a deal, particularly on known distance shots.

Ok, finally got to the recurve and static recurve.

First here is a Shakespeare Super Necedah 54" Vintage 69 or 70

And here is a Howatt Cavalier Static recurve. Not sure of it's exact vintage but guessing the 60's

So, there are those two for what it's worth. Both bows are pleasant to shoot althought the Cavalier has a rather bulky grip. I like the Howatt grips but this one is just to bulky for me. I'd take a rasp to it but just don't want to ruin it. I might build a duplicate someday.

Thanks, Longcruise and the others. I learned stuff! Shandor

Very interesting, thanks for the numbers.

we need to be a little careful when we use draw force curves as an indicator of bow performance ... for example ..

.. add a 1kg mass to the tips of your limbs. The force required to bend the limbs will remain pretty much the same, the force draw curve will be pretty much unaltered,the energy stored within the bent limb will be pretty much the same. but, the shooting performance of the bow will be dramatically reduced as limb speed is compromised by the increase of mass

Very informative thread ... thanks guys

Yes, Phil, the FDC is only one indication.

I did a curve on an RD the other day and it looked very good. However, it seems a little slow. OTOH, it's very pleasant to shoot and is easy to be consistent with. I'll run it over the chrono to see what it actually does. All in all, the bow is a very pleasant to shoot and would be my choice over many faster bows.

The only way to tell if the bow is efficient and performs well is to compare the energy of the Arrow when shot to the stored energy in the bow this gives you it's a dynamic efficiency. This requires a chronometer to get the Arrow speed and the scale to get the Arrow weight and a good scale to get the draw weight of the bow. If you look at the FD curve I've posted earlier you can see that it had a peak draw force of 53.25# at 29" stored energy of 50.32 ftlbs 94.44% of the peak draw weight was stored a 532 grain arrow drawn to 29" = 10gr/lb produced a speed of 203 fps and kinetic energy of 45.7 ftlbs and a dynamic efficiency of 90.2%. definitely a performer.

What's the meaning of that straight line in the diagram?!

Comparing different FDCs it's often placed in different positions and angles. How should it be placed correctly to get some information out?

Thx reini

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