grains per fps (for you data heads).

[Doug Treat]

I decided to do a test to see what speed my bow shoots with different arrows.  I was shooting a 47# G&L Longbow with fingers @ 28" through a chronograph at a local archery shop. Arrow #1: I borrowed a GT arrow that weighed in at 420 gr.  I wanted to be as close as possible to 9 gr./#.  It shot @ 183 fps.  Arrow #2: My 2117s with a 250 gr. point- Total weight- 650 gr.  It shot an average 163 fps.  Now, I've heard that a norm is to loose 1 fps to every 5 grains gained or vise-versa.  I figured my bow is at 11.5 gr./fps. using those 2 arrows as a test. I was pleased with that because at 9 gpp it was pretty fast and it didn't loose much speed with the added weight.  I actually did some other math and figured that (with those 2 arrows) an increase of 55% in arrow weight caused only a decrease of 11% of arrow speed and that 11% decrease in speed nets an increase of 38% momentum!  That's what I like.  Not much trajectory difference but a LOT of potential penetration difference.  I wondered if others have had similar results with their bows or is 5 gr./fps more normal?

[Doug Treat]

This info. is useful when hunting larger (elk, moose, etc.) game.  My setup with a 47# recurve has more momentum than a fast compound shooting 350 gr. arrows @ 300 fps (that's a FAST compound), but Ashby's latest info. says that not even momentum is a good indicator of bone penetration.  He has found that 650 grains seems to be the bone breaking threshold whether the 650 grains is going 140 fps or 250 (which is why I'm shooting that heavy- almost 14 gpp for my bow).  You guys probably know all this...anyway the original question:  What have you guys found when shooting different weight arrows through the chronograph? 5 gr./fps or more?

[loco_cacahuate]

I havent chronoed my longbow , which is 50# @ 29". I was shooting 30" 2213's with 125 pt 380 gr total. I have switched to 2218's with 145 pt 520 gr total and have not seen any difference in my shooting til I got to 40 yards. I read a link on here about arrow weight, it said an olympic shooter found that 1 grain meant .1 inch at 90 yards.
Using that, I calculated that 100 gr more arrow weight the drop would be 2 1/2 inches at 22 yards
 So to me arrow weight has become the last thing I think about. I do want to chrono my bow as soon as I find one and see the difference you're talking about.

peanut  :D

[McK]

About 8gr./ft per sec., when shooting arrows of around 9-10 gr per lb.  With very light arrows shooting very fast it may approach 5gr/ft per sec with heavier slower arrows maybe 10 0r more.

[Doug Treat]

Ya lost me there, McK.  My question was what if you change arrow weight with the same bow say from 8 gpp to 14 gpp?  Is there a normal speed to weight ratio with traditional bows?  Let's say I know exactly how fast my bow shoots @ 9 gpp and I want to increase my weight by 100 gr. Without shooting through a cronograph, can I assume that I will decrease my speed by 20 fps?  It worked out to be much less than that for my setup.  With my bow, it would only be 8.6 fps difference with 100 gr. change.  Is there anyone else out there who has done more testing like this?  What were your results?

[Doug Treat]

I'm wondering if some bows are more efficient with heavier weight arrow than others.  If 2- 50# bows both shoot 450 gr. (9 gpp) arrows @ 190 fps, and then you shoot a 700 gr. arrow through both bows, will you get the same speed from both?

[McK]

Doug,
  If you shoot a 520 gr arrow, 180 fps then the same bow would shoot a 600gr arrow @170 0r lose 1fps for every additional 8gr.  This will vary with arrow wt. with very light/fast arrows an extra 5 or 6 grains might cost 1fps. with very heavy arrows it may take 11 or 12 gr to make 1fps difference.  Both bows 50# bows should shoot the heavier arrow the same speed.
  Look at some bow carefully conducted bow tests, Norb Mullaney tests trad bows with arrows of various wts and compare the speed differeces.

[Doug Treat]

Thanks McK, that makes more sense to me.  Is this Mullaney test data available on-line?

[KrEn]

I went out looked at old chrono data from my bows and did some quick calculations. Average grains per fps on 8 bows is -6.6. All the numbers are in the accompanying graph.
Bows from 40-55# arrows 350-690grs

 

Got numbers ranging from 1 fps per 4grs to 1 fps per 9 grains. But the extremes are with two data points only. 6 out of 8 bows between 5.7 and 7.5

With a compound bow, shooting the 590grainers 228fps, they hit about a fot lower than the 390 grainers at 18m.


K

[Doug Treat]

Thanks KrEn,  That's what I wanted.

[Taiga Recurve]

I just chrono'ed my set up the other night. I am shootin a 450gr arrow at 190fps. I have a 62#@28" recurve. How is that for speed vs arrow wgt?
I am going to make up some more arrows and get them up to at least 560-570gr. Any input on this?

[Doug Treat]

Tiaga, I'm not sure about how your speed compares to other bows (if that's what your asking).  Usually, the bows that I've seen chronographed are shooting about 9 grains per pound of bow weight.  A lot of the fast traditional bows I've read about are shooting 180s-190s fps. @ 9 gpp and drawn to 28".  Assuming you're shooting at 28", your arrow is at 7.3 gpp.  I would be interested to see how much the speed slows down though if you shoot the 560-570 gr. with the same bow. 558 grains will get you right at 9 gpp with your bow.

[KrEn]

Doug,

Take a look at OL Adcocks page, he has similar graphs for his ACX bows.
But with a little more impressing numbers !#¤£$-


K