Kinetic energy---- got my eyes opened!!

[macksdad]

I have always guessed at the speed of my bow and figured my kinetic energy from my estimation on speed. I am shooting a 550 grain arrow and my bow is 55# @28 and I draw to 29". According to my easton bow scale I am right at 57#. Well I took the guessing out of my speed estimation and shot my setup through a chronograph 165 fps ( I was guessing more like 190) . This calculates out to 33.3 ft lbs. of kinetic energy. I honestly always figured I was closer to 45- 50 ft lbs. I am sure my set up is more tnan ample for sny of my hunting situations , just thought I was packing a little more punch.

[Blacky]

That 165 fps doesn't seem right at 29" and a bit over 9 grains per pound. Just my experience.

165 fps would pretty much be the speed for a selfbow with those specs.

Blacky

[hybridbow hunter]

Buffalo bow shot at 10 gpi @ 28", braced at 8" with the stock string and string leech silencers under shooting machine is around 175-177 fps at 1 yard in front of the bow, if you add 1 inch of draw (thus 3# to the bow) with same bow and arrow you increase that speed about 6 fps, so 181-183fps. With a clean release you will loose something around 5 fps (little more or little less) so you could expect really something more in the 175-180 fps range.
Maybe real draw lenght when you let the string isn't 29" ?
Maybe release isn't so clean?
Maybe chronograph doesn't give the true speed (sometimes light related issues) ?
Maybe arrow is heavier than you think?

[Glunt]

Seems like you should be getting a little more speed, but your set-up is plenty for anything crawling around our continent.

[Elk whisperer]

try another crono

[Ragnarok Forge]

KE is a very poor measure of an arrows ability to penetrate.   Impulse momentum is the measure you are looking for.   KE is a measure of energy stored in the arrow during flight.   It tells you nothing about how the stored energy is released in living flesh.  Dr. Ashby covers the IM equation in his reports.   You have plenty of punch with your set up to kill NA game.

I shoot a mild R/D longbow -  55 lbs @ 29 inches.  564 grain arrow @ 165 fps.   I hunt elk and bear and have worries about penetration.

[Jeff Strubberg]

Exactly.  KE is not the way to measure penetrating power.  It's a tool for evaluating efficiency between bows.

[Zradix]

First off...Good on you for doing some thinking!    

How warm were the temps when you chrono'd it?

I just did a post about losing speed in the cold.
My setup was doing way faster in the summer.
Just tested it again last week and it was 20fps slower. WTH? right.

After checking for tuning and everything since all that would be off if the speed was that much slower.  

I found out the chronos will read slow when they're cold.
My setup was still shooting what it did in the summer.

I wouldn't be a bit surprised if you checked that bow at a warm range you'd be up in the 180's at least.

Your setup is fine with more than enough energy.

Most of this stuff was said by others for good reason.

Ashby does go in to the energy pretty in depth.

The easiest way to compare one setup's energy to another's is to simply take the total wt of the arrow and multiply it by the speed.

Doing this will also let you find out with what wt arrow your bow will impart the maximum amount of energy to.

Most of the time, bows throw the highest energy arrow at around 12-13 grains/#.

Might be too slow for you, but there is a trade off with everything.

Kinda fun stuff.

Good luck and have fun.

   

[wapiti792]

I use Stu's calculator and am often suprised by the KE I see in the far right box. Quite low...For kicks I shot my Talltines longbow through a chrono at a bowshoot. 52#at 28, 620 gr total arrow weight. 165 ft/sec. Start doing the calcs and KE is pretty low.

Then I go to WY and shoot through an elk at 25 yds, Manitioba and shoot through a bear that weighed around 400#, and a giant whitetail here. The bottom line is this: KE is a poor indication of momemtum and how well that stickbow performs. A sharp head, through the ribs with the set-up you described above will kill anything in North America.

[Zradix]

boom!

[TDHunter]

X3 on that speed doesn't seem right. I just shot my 50lb Dorado through my Chrono . I Draw 30 so was getting around 55lb at my draw and I'm using a 550 grn arrow as well and was getting 182fps.

[BradLantz]

KE is defined as

The kinetic energy of an object is the energy which it possesses due to its motion.[1] It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body in decelerating from its current speed to a state of rest.


Momentum is the key, like everyone else has said.

[LBR]

Formulas are only part of the equation.  An arrow that is flying straight (vs. one that impacts at an angle), sharp broadhead, broadhead design, shot placement, etc. all play a significant part in penetration.  

Everything can be perfect on paper and let you down in real life.  Know your equipment.

Chad

[ishoot4thrills]

Originally posted by Elk whisperer:
try another crono

Exactly. Check my longbow specs in my signature below with 26.75" of draw length and about the same weight arrow as you. I used my own chrono and it matches other high end chronos I have used before. I have used some cheap(Shooting Chrony brand) chronos before that weren't worth their weight in scrap metal.

[hvyhitter]

Or try another arrow of a different color....a few years back at a shoot with a crono that we found that a crown diped or brighter fletched arrow will give much different results........its all on how the machine reads the reflected light off the arrow.......we had one arrow that shot 200+ out of any bow from 45# to 65# and had a lot of fun with some of the compound guys........

[57HOP]

This won't answer any questions, but it may give you confidence in your setup!
From a thread in Hightlights Forum that I read today:
"I was using my 45lb 1966 Bear Kodiak, shooting Easton 2016's tipped with some single beveled 190 grain Tusker broad heads.

The shot took out both lungs, severed the heart clean from the arteries and cut the front leg bone in half just below the shoulder on the opposite side."

[macksdad]

Ok I did some more testing today and 170fps is the absolute fastest I can get with my hunting arrows. I tried some 2016 shafts that weigh in at 430 grains and they would shoot about 10 fps faster.  But the real kicker is that I shot a round out of my 50# dorado and I get the exact same speeds with this bow as I do with my 55# buffalo shooting the same arrows.

[JamesKerr]

Chronos are funny things. I just chrono tested my tomahawk the other day at an archery shop and with the same arrow I got readings from 121 fps - 171 fps. I am shooting an aggressive d/r longbow 54 @ 27.5" with a 620 grain arrow. I would guess my true speed is around 165 to 170 as I have bows that shoot around 150 and my Tomahawk is much faster.

[joe skipp]

We always used a 5 shot average when testing out the bows speed. I suggest you try another chrono if possible because no two are alike and you will get different readings from the chrono's.

[Ragnarok Forge]

Lots of folks get really involved in speed.  I have seen two bows at the same weight from the same Bowyer shoot different speeds.  Now throw in different bow makers and differing draw lengths for shooters and you can' t get any kind of standard.  I think the real key is for each shooter to find a set up that works and then optimize their arrows for what they like.  A perfect flying arrow with a razor sharp head will kill anything alive today.   Placement is the real key to successful kills.