PABowhntr

I think we tend to use averages for such things as increases in draw weight. Personally I have always used 1.75 fps for ever pound of draw weight however, I am guessing it is going to be dependent possibly on the spine of the arrow and possibly the relationship with the grains per pound of the arrow/bow combination.

Couldn't it be possible that the computer program is taking into account how a weak spined arrow might not fully utilize the energy being sent into it? Couldn't this also possibly be the case in real life?

In addition, as you increase draw weight while maintaining a consistant arrow weight you are lowering the grains per pound of draw weight ratio. I know many bows are more efficient at a particular ratio or within a particular range. I would think it possible that it may contributing here.

Just some things that popped into my head.

jsasker

302 fps if the bow is a 70# w/29" draw and shooting 5grain per pound of draw weight--your arrow is about 50gr. heavier and your draw weight is 23LBS. LESS.Very low 200's sounds pretty close.

Roskoe

I e-mailed Fred Bear Archery this afternoon - got a response back fairly quickly. They said low 200's sounded about right to them as well. Other than the Archery Hut calculator, which is sounding nearly worthless at this point; I'm also a little baffled at what I have read about the general loss of velocity as things vary from the 70#, 30" draw lenght, and 350 gr. arrow weight standard that is used in the IBO rating. Based on this, going down 23 lbs in pull weight, 1" shorter in draw length, and staying the same on arrow weight should result in a reduction from 302 fps to about 260 fps. Is there something about the IBO conceptthat I'm missing? Would my complete arrow weight have to correspondingly drop from 350 grains to 235 grains to get the 260 fps results at 47 lbs of draw weight?

TFOX

Your arrow would have to stay at 5 gr per lb.47# would = 235 grain.


You also will lose fps if you are adjusted down on the draw length of a cam module to a lower setting.The program is setup for high efficiency and that is only achieved when set at the optimum setting of a cam.Usually longest draw length or just below.This is one advantage of specific draw cams,they are always at the highest efficiency.


You will sometimes lose ibofps when at the lowest setting for poundage instead of bottomed out.

CONFUSED YET[8D]

Roskoe

No - actually things are getting clearer. Tomorrow, I'm going to turn the limbs back up to 60 lbs. (maximum for this bow) and see what the chrono reads. Are you saying that what the calculators say the speed is supposed to be will be accurate with the limbs cranked in all the way - orwould I also need to go with an arrow that was 300 grains (instead of 350) to maintain the 5gr. per pound of pull? If the Archery Hut calculator is worth its mega bytes, I should be in the vicinity of 280 fps . . . . Thx. Roskoe

TFOX

60# and 300 grain,also you would have to be at the maximum efficiency draw length setting of the bow.


If you would like,pm ALL you specifics and I will check your setup with my program to see what i come up with.

TFOX

Ther program should ask you for arrow weight and calculate the speed from that.

PABowhntr

Starting with a 302 fps speed rating.

-23 pound reduction in draw weight should result in 40.25 fps reduction in arrow speed.

-1 inch shorter draw length should reduce your speed another 10 fps

-String accessories, peep sight, string silencers, nocksets, string loops, etc... all reduce arrow speed a significant amount depending on which items, how many items and where they are located on the string. I did not go back to see if you mentioned if you had any of this on the string.

With just the first two calculations you should be at no more than 251 fps. The string accessories could easily result in a loss of another 15-20 fps, again, depending on the specifics. That would put you in the 230 something range. Also, as TFOX suggested many bows are more efficient at peak draw weight which means by backing the limbs off a full 13 pounds from peak you are probably losing a good bit of efficiency.

Sorry if I reiterated some folks' earlier points but I wanted to include everything in one post.

Roskoe

I just got back from the range - interesting session. Took the chronograph and the certified scale. I set the chrono a little closer this time - maybe four feet from the bow. Basically, I shotthree arrows at each full turn setting and averaged the speed - as well as the poundage. The bow was backed off a total of six full turns. Results were as follows:

47 lbs 209 fps
48 lbs 218 fps
51 lbs 227 fps
53 lbs 234 fps
55 lbs 244 fps
56 lbs 245 fps
59 lbs 256 fps

Results were about 25 fps lower than what the Archery Hut calculator said it should be - but a whole lot closer than what the calculator said for the 47 lb. draw weight!

Another interesting little foot note. While backing the bow off, one turn at a time, I decided to shoot some groups at each setting and see what happened. The 28" Easton ST Excel carbon arrows didn't shoot well at 59 lbs - maybe 4" -grouped high and a little right at 20 yards. As the pull weight came down, one full turn at a time; they shot better and better. When I got to 51 lbs (four turns out), the arrows clustered so tight that all the shafts were touching - broke two nocks! Then it opened up slightly at 49 and a little more at 47lbs. I have never seen this used as a tuning tool, but I came away a happy camper

TFOX

Now you see why I preach SPINE and tuning.You most likely hit the majic area of spine as well as found the sweet spot for you as far as holding weight.


You really had some strange numbers through the chrony.If you do not shoot through the same area of a chrony,speeds will fluctuate sometimes.