Kinetic energy ?'s

[mnbearbaiter]

With elk season on the way, i have a few KE ?'s! Im shooting my trusty Bear Montana #55@28" longbow paired with KK German Pine shafts that i oil soaked tipped with 160 STOS heads! In my eyes its an awesome setup with alot of punch and is extra quiet! Ive come very good with it the last 6 months, and am very confident out to 30yds! It chronos at 168 fps avg, and on a whizz bang KE calculator it has 42 ft lbs of KE! I recently switched to the STOS heads to maximize penetration, how much truth to this game, and how to factor in heavy arrow momentum? Im shooting a total arrow weight of 675gr!

[Looper]

Well, KE isn't necessarily an indicator of how great your penetration will be.  The fact is, with that arrow weight, that speed, and that Stos head, you're more than prepared for anything. What are you asking, exactly?  Will it work on elk? Certainly. A lot of elk have been killed with a whole lot less, both in speed and in arrow weight.

[mnbearbaiter]

Well i guess i shouldve specified, Easton has a KE chart, and they recommend 41 ft lbs as a minimum for stuff like elk! I think its probably geared more towards wheelie bow guys, who tend to get way to technical and spend way too much time with flim flam! I guess i was asking if anybody really hinges their setup on some formality such as this or is there some validity to it! Sure, the fancy dancy wheelie bows have upwards of 80 ft lbs KE, but i believe that our bows with the heavier/slower arrows are better for killing within reason once the initial puncture through the hide is made, am i right with this?

[Jim Wright]

Travis, if you think about a K.E. number is simply the answer to a mathmatical equation that manufacturers use to tout their products. If your loosed arrow has 42 lbs. of K.E. with your broadhead, it also has it with a field point or a blunt. KE kills nothing and by itself is a poor way to assess a broadhead tipped arrow's lethality. Read Dr. Ashby's research on the subject detailing all the factors in play such as arrow flight, broadhead shape, f.o.c./e.f.o.c. etc.. that can be found on this site.

[Bonebuster]

Awesome set-up.

The mechanics are there in draw weight, and arrow weight. The spirit is there in your dedication. Your confidence binds it all together and makes bowhunting what it is.

Hunt hard...and PLEASE post pictures!  :D    

[scedvm]

Travis, I can feel your enthusiasm in your posts.  I am on my 25th+ elk season and I am getting real "itchy" as well.  If you are shooting 675gr comfortable to 30yds you are ready my friend.  My arrows are usually 525-625gr out of 58-62# bows and perform very well.  Good luck and look forward to hearing about your hunt....Shawn

[mnbearbaiter]

I added a TS1 string from 3rivers to the bow, as well as another that i broke in to use as an extra should need be! It seemed to be very consistent, and seemed to add a lil snuff to the bow! I was dueling back and forth between my go to 160gr Ace Express heads and the 160gr STOS, and went with the latter as its a tad narrower and should aid in penetration, but looking back at some of the crude penetration tests ive done i think i wouldve been ok with either of em! I cant wait for the mountains and the elk, good luck to all this fall and thanks for the responses!!!

[straitera]

IMO, KE has little relevence. More important is penetration. I want my arrows through the boiler w/complete passthrough (bones included) & bury into the dirt 10" on the off side. Only thing better would include heartshot. IMO, you're good to go. Best of luck w/elk pix asap.

[wtpops]

Like said above KE not so important. What i use is momentum calculated like this momentum = speed X weight.

example:

say you have 3 arrow setups that fly well out of your bow, a 725 grain, 675 grain and a 575 grain ofcourse your speed will be differant with each arrow so i will pull speeds out of my head for this example except i will use your set up in this.

725 grain X 152fps = 110,200
675 grain x 168fps = 113,400
575 grain x 175fps = 100,625

Out of these three arrows i would use the 675 grain it has more momentum and will penatrate better.

With that said with your set up, if it is flying well add your are confedent with go kill an elk and post some pics.

[Bob B.]

For momentum you are looking for slug force, the unit of measure required to stop an object in motion. Bascially a number of around .4 slug and above is fine for elk, if using a high mechanical advantage broadhead.  The head you are using nets almost mechanical advantage of 3 which is excellent.

The formual used to calculate slug force is mass times velocity divided by the gravitaional constant of earth ( 225218 ), basically the pull fo gravity expressed as a number (7000, the amount of grains to a pound x 32.174 feet per second per second, the speed at which an oject falls to earth due to gravity.  This equals the number 225218).

So a 675 grain arrow times 168 fps / 225218 = .5034 slugs.  That with a cut on contact head that has a mechanical advantage of 3 is pretty darn good.  I think you knew that anyway, but now you have numbers to ease any creeping doubt.  Sounds like a killer set up to me.

   

Bob.

[Terry Green]

Originally posted by wtpops:

725 grain X 152fps = 110,200
675 grain x 168fps = 113,400
575 grain x 175fps = 100,625

Out of these three arrows i would use the 675 grain it has more momentum and will penatrate better.

Thank you....this is what I always show to dispel the 'heavier is always better' claim.  There is always a combo that will perform best with any set up.

[Zradix]

don't give KE any thought with your bow.
with a bullet yes.
not with an arrow.
Bullets shock an animal so it uses the KE while the bullet is mushrooming or fragmenting..

Arrows just need momentum/inertia to get through and hemorrhage....no expansion

Easy way to figure this..weight X Speed.

[awbowman]

You are good.  In short, as long as you have at least 9-10 grains of total arrow weight per pound of draw weight, just pick a balance between weight and trajectory and you will be fine.

[awbowman]

When I first started shooting a trad bow I had a million questions and theories (it's the engineer in me), until an old man with over 175 kills with his trad bow told me ....  "DANG IT FRED, IT'S TWO STICKS AND A STRING, QUIT COMPLICATING THINGS"            

It's funny now, and it's still true.  I think it's great to learn our craft, but I for one have been guilty of waaaaaaaaay overcomplicating things.  Go out and have fun.  It will make you love the trad bow even more.

[mnbearbaiter]

Im not a newbie at all, and ive killed several animals with great performance out of all my setups without ever questioning them! I believe that im looking for an excuse maybe to talk elk huntinh setups :-)

[Friend]

725 grain X 152fps =  110,200
675 grain X 168fps = 113,400
575 grain X 175fps = 100,625

Find the above example to be possibly misleading and implies the results are being generated from the same bow:

" 675 grain x 168fps = 113,400" as compared to the 1st set of values…….
What bow gains 16 fps with a 50 grain drop in arrow wt?

…This set-up would have req'd the same bow having  ~ 8 more pounds of draw wt.

**The higher momentum value of the second set of values  is primarily due to having come from a bow with ~8 pounds of increased draw wt.**


" 575 grain x 175fps = 100,625"
…This set-up would have req'd the same bow having  ~ 4 more pounds of draw wt.

Not at all disagreeing that a balance which meets the individual needs is imperitive.

[wtpops]

Originally posted by Terry Green:
 

Originally posted by wtpops:

725 grain X 152fps = 110,200
675 grain x 168fps = 113,400
575 grain x 175fps = 100,625

Out of these three arrows i would use the 675 grain it has more momentum and will penatrate better.

Thank you....this is what I always show to dispel the 'heavier is always better' claim.  There is always a combo that will perform best with any set up. [/b]

Yup there is always a point of deminishing returns, a little simple math will help you not to go to far or not far enough..

[wtpops]

Originally posted by Friend:
725 grain X 152fps =  110,200
675 grain X 168fps = 113,400
575 grain X 175fps = 100,625

Find the above example to be possibly misleading and implies the results are being generated from the same bow:

" 675 grain x 168fps = 113,400" as compared to the 1st set of values…….
What bow gains 16 fps with a 50 grain drop in arrow wt?

…This set-up would have req'd the same bow having  ~ 8 more pounds of draw wt.

**The higher momentum value of the second set of values  is primarily due to having come from a bow with ~8 pounds of increased draw wt.**


" 575 grain x 175fps = 100,625"
…This set-up would have req'd the same bow having  ~ 4 more pounds of draw wt.

Not at all disagreeing that a balance which meets the individual needs is imperitive.

You are correct my Friend, the first set and third set of numbers i just pulled out of my head, like i stated in my post, just to show an example of the math so he could get the idea how it works. You would have to set up and crono all three arrows and get real numbers and go from there.

[JimB]

Here are some real figures from one of my bows.

714  grs @ 155 fps,13.47 GPP=.49 slug ft per sec

619 grs @ 165 fps,11.67 GPP= .45   "   "   "  "

530 grs @ 175 fps,10 GPP= .41      "   "   "  "

I don't know where diminishing returns comes in or even if it does in this context.I have only tried arrows as high as 14 GPP and it doesn't show there.

[Bob B.]

Wow Jim, excellent numbers!  What kind of bow are you shooting?  that is pretty good for a 53 pound bow.

Bob.