Help on FPS loss by grs of weight

[vixenmaster]

so what is the newest formula for figuring weight by grs per loss of speed i read years ago it was 1 fps loss for every 10 grs of weight added. would like to know ? if its 3 fps then my vixen shooting 650 gr arrows shoots 180 fps. actually using the 3 fps deal i am getting 171 fps. its so slow i may have to retire it from hunting.

[crazyfarmer]

so what is the newest formula for figuring weight by grs per loss of speed i read years ago it was 1 fps loss for every 10 grs of weight added. would like to know ? if its 3 fps then my vixen shooting 650 gr arrows shoots 180 fps

I'd like to know also since ive never really tested fps loss by 10 grain increases

[JRS]

I've always understood the accepted formula to be 1 fps change in speed for every 5-6 grains change in weight.

[Normous]

My testing in regards testing speeds and weights using my Excalibur agree with JRS. 1 fps for every +/- 5 grains.

[Michael Stogre]

Through my prochrono pal the loss is 5 feet per second for every 25 grain increase in weight. Or as some put it, 1 ft per second for every 5 grains of wt.

[shafferm]

Smoke,

I am not saying that it will be 3fps loss for every 10grs increase, I was trying to figure out why the posts on arrow weights being around 500grs total seemed to be the "sweetest" shooting weight. In order to get the math model for kinetic energy being maximized at this weight I used that as an approximate value and it matched up fairly nicely. It looks like that value may be closer to somewhere between 2 and 3 fps loss for every 10grs.

-M

[vixenmaster]

i under stood your post. sorry if i didn't state my ? right. i am asking what the latest formula was. as they seem to changed it. so the 1 fps = 10 gr is out the door. the new one is 5 gr = 1 fps loss in speed. ah so now i have went from 171 fps up to 204 fps. its above 200 fps so look out slow footed deer my vixen will clobber the slow one's!

[Woody Williams]

I've always understood the accepted formula to be 1 fps change in speed for every 5-6 grains change in weight.

Ditto...

[Big John]

I am believing the 1 fps. loss of speed for every 5 gr. increase to be the correct calc. seems to be the closest so far.

[bomtek44]

smokepolehall,

I have some recent figures as follows from my 225 pound Exomax with the Excel String (28 strands) and cat whiskers. Some were shot on different days.

Weights are total arrow weight including field tip:

368 grains / 335 fps
390 grains / 330 fps
437 grains / 319 fps
442 grains / 317 fps
446 grains / 316 fps
472 grains / 310 fps

The best shooter so far is the 442 grain weight which is a 20 inch 2216 with solid aluminum rear insert and aluminum Satellite front insert with a 125 grain field tip or 125 grain Sonic broadhead.

Thanks,

[steven in England]

smokepolehall
The formula in not at straightforward as you think because the heavier you go the more efficient the bow is and you actually loose less fps as you add more weight. I found this out with my testing with an Exomax when I got over 1000 grains the ke was 128fp. If I based the calculation on 1fps per 5 grains it would only give me 107ke. The bow will not loose ke as you go heavier so if you look at the instruction that come with the bow the Vixen gets up to 72fp of ke with a 478 grain bolt. Based on this you will be getting at least 230 fps with a 650 grain bolt in my opinion. Hope this helps.
Forgot to add this assumes you use a fast flight string, other strings react differently.