F/D curves and SE/PDF

[NTD]

I just finished a BBI R/D my scale only goes to 50 pounds and I wanted to be in the high 50's to 60 lbs.  So I plotted an FD curve to get an estimate at a full 28 inches.( last reading I have is 50 @ 24")  I ended up showing 61 at 28" I'm happy with that euen plus or minus 2 or 3 lbs.  Any way after drawing the FD curve I got curious as to what it was telling me about my bow.  I calculated the area under the curve and it's 840.  How do I translate that number to SE/PDF? 

[avcase]

840 should be inch-pounds. Divide by 12 to get 70 foot-pounds of stored energy. Divide this by the draw weight (60-lb?) to get SE/PDF.

70/60 = 1.17

This is a extremely high value for a stickbow @28" draw. What does the bow look like?  Typical values are .9 to 1.0. 
Alan

[NTD]

Unfortunately my weight numbers from 24" up are only estimates from an extension of the curve so the Stored enery calculation is probably off but I can't imagine they are off by much.

I will post pics once the finish dries but it's 64" BBI R/D 1 1/4" wide until just past midlimb and then tapers to about 7/16's tips.  The reflex was glued in 16 inches from the tips, after tillering it kept 3 inches of reflex.  Has about 3/4" deflex.  I tillered the bow to bend much like a D bow except of course it doesn't actually bend through the handle.  I'm hoping to get my hands on a good bow scale so I can get more accurate weight readings but I do know it's fast.

Thanks for the break down on SE/PDF!

[Justin Snyder]

Your weight guesses should be pretty dang close. A 60# bow will add about 3 pounds per inch of draw toward the end. 3X4 inches = 12 pounds. You said 11 which is probably spot on. I know it is a lot closer than the scales I use could get anyway.

[NTD]

Thanks Justin.  Wish I had a chrono she sure seems fast.

[mullet]

 I can't wait to see it. I think the BB5 is going to need a new chapter on Ipe. It is really a performance wood, but, I still like yeller sawdust.

[NTD]

I'll tell ya Eddie, I can't stand working ipe but I sure do like the bow it can make.  Osage makes me happy, My friend laughs at me when I talk about working it.  I love the way it looks, feels, I like the way it feels under a sharp blade, he says I'm cheating on my wife when I play with the yellow stuff

[mullet]

  Don't get caught

[n2huntn]

 I'm partial to the Ipe, slim , dark, exotic and curvy what's. Not to like. Readily available too . Depending on stack you may only gain 2 lbs an inch. Can't wait to see it.
Jeff

[Badger]

   I have always had a hard time measuring fd curves. I started shooting extremely heavy arrows from the bows like 2 or 3,000 grains and assuming they are about 97% efficient. I call this energy extraction method of calculating stored energy. I feel it comes out closer than measuring. As Allen said, if you store about the same energy as the draw weight you are doing good, recurves will often store slightly more. Looking forward to a chrono report. Steve

[NTD]

After looking at the WTT results on FG bows I realized it was highly unlikely that I was getting 1.16 se/pdf, so I did my curve again and redid the math., got .96 se/pdf.  That still seems high but more likely.                                                                    Steve, can you elaborate on the heavy arrow method?  Are you measuring speed with arrows of that weight?

[Badger]

  Ntd, yea, I shoot a very heavy arrow through the chrono and then either figure out the kinetic energy or go to the online ballistic kinetic energy calculator and figure out the stored energy based on how much kinetic energy was in the heavy arrow shot. I don't know how to put a hyper link for the calculator here or I would. E mail me and I will send you the link if you want it. Steve

[jwillis]

NTD, the efficiency numbers from the latest WTT shoot out that I posted at Pirates of Archery are slightly off due to a problem with the spreadsheet. I've since reworked the spreadsheet and I can send you a copy of it. Just send me a PM with your email address and I'll send it to you. It is an Excel file and automatically calculates SE/PDF and draws a nice curve for you. You just need to plug in the draw force numbers. Jim

[avcase]

If you can get the speed of two or three different mass arrows, then there is a method to calculate the stored energy of the bow from this data. This method can be very accurate if the draw length and release is controlled with a shooting machine. In fact, You may consider shooting a 400, 500, and 600 grain arrow at future WTT events regardless of the bow weight and it will be pretty straight forward to pin point the speed for a 10 ggp arrow, stored energy, and efficiency from this data alone.

On a related note, someone is working on a new weigh-in machine for future flight shoots that will record the force-draw curve to a laptop in seconds.  I can't wait to see that!
Alan

[Badger]

   Allen, I have some things i purchased to put together a weigh in machine like that. If you think the person might be interested I would give it to them. Mine is compatable with excel. I have the s shaped load cell and the mter it reports to that send the info to the computer. I wanted to build a draw machine that would take a reading every 1/8" or so but never got around to it. Steve