? about calculating KE and TRUE FPS

[wvdeerhunter22]

This is my setup

Outback
70lbs
30 in. draw
NAP quicktune fixed rest
UltraknockII
Kisser button
No tube peep sight
Rubber whisker silencers
Zebra string
Doinker stabilizer
shooting 2315 arrows with 100 tips

I dont know the weight of my vanes but they are just the normal plastic ones
I dont know the weight of the knocs but they are standard to.

Can anyone help me calculate my KE and True FPS i dont have access to a chronograph

id also like to know how to find gpi on my arrows.

Let me know if i left out any necessary info
Thanks alot

[Mike from Texas]

OK I did some calculations for you. Bear in mind there are some assumptions being made here.

1st look HERE to see the GPI on your arrows.

1) I assumed your are using XX78 Super Slams

2) I assumed a 29" finished arrow length.

3) I used -5 fps for your Ultra Nok 2 since I am not familiar with that nock.

4) I assumed 4" vanes for weight on your arrows.

5) Since Easton does not state whether or not you have to add component weights to the XX78 or not I did a little guessing based off of the charts for ACC's and fudged a little. This will give you the lowest number instead of misleading you into thinking you have higher speed & KE than you really might. If you know the actual weight of your arrow, I can get more accurate.

Anyway, I come up with 240 fps and about 64# of KE. I hope this helps you out.

[jmac_or]

Well, assuming you are talking about XX75 2315's, and your arrows length is 30" as well, which it is probably not, your arrow weight would be around 524 grains. I would not be able to guess your speed, and as such, not your KE. There may be others on the site with a similar setup on the Outback, so they could give more input.

As a future reference, check out this site, has tons and tons of great info...
http://home.att.net/~sajackson/archery.html

JMAC

[wvdeerhunter22]

mike thanks alot i really appreciate the help and the link

wow thats alot slower than i thought

[Swamp Dawg]

wow thats alot slower than i thought

Yes, 2315's are heavy arrows (boat ancors), thus slowing em down. However, you are still getting very respectable KE. For white-tailed deer, elk, etc, you have nothing to worry about with that set up. If you shoot 3D, you may want to switch to a lighter carbon for more speed.

[wvdeerhunter22]

heres another question ive never shot carbons so i have no clue, will a carbon at for example 390gr finished shoot faster than an aluminum at 390gr finished?

[Jason N]

will a carbon at for example 390gr finished shoot faster than an aluminum at 390gr finished?

I was going to make a smart alec remark, but instead I'll just say NO! Same weigh arrow, same bow....you bow doesn't know the difference.

[Mike from Texas]

If your wanting to get your speed up and maintain your KE maybe you should look into the new Easton Axis arrows. In a 340 spine arrow, this would knock your finished arrow weight from 504 grains to 428 grains and would get you about 264 fps and about 66# of KE along with the bonus of flatter arrow trajectory. Also you could go up to about 270 fps by getting rid of the Ultra nok and replacing it with a string loop and replacing the cat whiskers with string leeches. This would also bump your KE up to around 69#. Just something to consider.

[TFOX]

I will add a little to the same arrow weight scenario.

If you have a small diameter carbon and a larger diameter aluminum,the carbon will maintain a flatter trajectory and maintain more ke downrange if we are splitting hairs BUT the difference is so minute it really isn't worth mentioning in a hunting scenario.

[JoshKeller]

I'll also take a stab at it..

IBO speed is 308.

308 - 10 fps for the ultra nok (aluminum is heavier than a string loop, and makes more noise)

298 fps. - 8 - 10 fps for the kisser and peep.

288 fps. - 5 fps for the whiskers.

283 fps

I, too, will assume xx78's and that you use the uni bushing.

I came up with a 534 grain arrow with 7.4% FOC.

I've generally found that 5 grains = 1 fps.

534 - 350 (ibo) = 184/5 = 37 fps.

283 - 37 = 246 - 250 fps, for a total of approx 70 - 73 ft lbs of KE.

Id estimate the AMO speed of this bow at 230 - 235 fps. Added your 10 lbs of draw weight, I think this is fairly close. I think you'd gain approx 10 fps by loosing the ultranok.