Weight vs Speed = Penetration ?

[eugeneb]

Hi,

The speed vs weight concept relating to penetration fascinates me. (bowhunting)  I have read numerous articles of people professing KE vs Momentum to be the best indicator for penetration. I have read Dr. Ashby's research with great appreciation and I lean towards the momentum formula ... nevertheless, using either KE or Momentum as an indicator for best penetration ... results in using the combination of velocity and weight (mass) of an arrow.  I see many people saying heavier is always better?  (I will use my new compound as an example to bring across my question/point)
 
I have ordered a new compound bow (not yet received and did some planning so long) ... I will use it as an example, the way I figure the best penetration specific to the bow ... taking into account each bow's effiency will differ (i.e. power stroke and how the bow transfers energy to the arrow) Surely the same logic should apply to trad bows in determining the best momentum, which seems to be the best indicator for penetration.

The new bow specs as follow: IBO=330fps, #70 dw, 32" dl, 5 grain on string ... I have used 2 different arrow speed calculators to estimate the arrow speed at various arrow weighs (total arrow weight) ... took the average speed and calculated the KE and momentum ... this obviously is not the real world, but gives me an indication (range) where my bow will perform optimum for example around penetration (momentum)  
My question is ... whether heavier always better?  For me it is striking the balance between velocity and weight ... e.g. according to the momentum formula my penetration is better with a 700 grain arrow opposed to 1000 grain (obviously all else being equal ... broadhead mech advantage, shaft diameter, eFOC, etc.)

I would appreciate your comments on whether these assumptions are correct.  I have included some figures as example.

(Reason for my question is that I plan to hunt Aus Buff and would appreciate to understand whether momentum is a better indicator than purely using a heavier arrow)

 

Momentum below
 

KE
 

[Loren Holland]

all of the above charts show formulas and projected energy, but if you want to know what will really happen then test it.  i did this for my sons science project recently, showing 300, 400, and 500 grain arrows of similar construction, from a wheel bow to a consistent draw, from a set distance. we measured penetration, avged 10 shots from each group, and heavier won.  

you have to apply intent into the equation as another variable that you must interpolate intrinsically, meaning...we all know that the extremes on both ends of the above projected resultant energy charts are not useful hunting applications (example when the 220 Swift first came out, some idiot publisher touted that it was so fast and produced so much KE from speed that you could hunt lion with it...i'd like to see that...also on the opposite end of the spectrum pure weight alone won't do it either)

so analyze for intent...you must hit the animal, which is aided by speed and its affect on trajectory, and once that is accomplished you want the benefit of weight.  

maybe i just oversimplify things...but i have never seen the point in this argument...shoot the heaviest arrow that you can that allows you to fall within a viable spectrum of velocity (and if the result is not acceptable then you must change the platform, ie increase pull weight, etc). if possible actually test and manipulate both variable to optimize both...there is an optimum in the middle somewhere

[fentiger]

KE decreases with increasing arrow weight?

  http://www.huntersfriend.com/carbon_arrows/hunting_arrows_selection_guide_chapter_5.htm

 Above according to a chrono. Have been told that many of the online programmes using IBO speeds have significant issues;-] especially with heavier arrows.

 If my info is incorrect, apologies in advance.

[Col HJ]

If your new bow actually will spit a 700 grain shaft at 230 plus then there is little further thought needed, or I would think so anyway. To help answer your question a bit as to the reduced momentum with heavier shafts, well all tools eventually reach a point of diminishing returns and sporty new compounds are no exception. Actual testing will be much more revealing however, as any bow's efficiency will generally go up with increased arrow weight.

When I am able to put my long bows away (not common)I shoot a speedy 70# compound as well, using 620 grain shafts with 24% FOC. And they are penetrating 15" into one very tough crossbow target. Or just a scant inch or two more than my 80# longbow with 770 grain shafts.

[eugeneb]

Hi,

Thanks 4 all the info ... I actually posted the same question on another forum and someone also pointed out that the speed calculators are inaccurate on heavier arrows ... his bow is simular to mine (eg:  400gr arrow the same as my example) ... but he indicated his bow spits out a +1000gr at 214fps ... he actually inserted his setup into the same speed calculators I used with the result being 147fps ... as stated do not rely on speed calculators for heavier arrows ... perform actual tests yourself ... in essence this changes every for me, my best penetration seems to be way more than 700gr ... again thanks for your posts ... I will wait for my bow to check out actuals.  (in other words the figures posted relating to the heavier arrows is way off the mark)

[jrchambers]

looks to mee that the wheel bow doesnt tranfer energy to a heavy arow very well

[fentiger]

My pet peeve for 3 years has been the same computer whiz kids that gave us "toxic tranche mortgage bundling guaranteed not to fail" software also devised Bucklemke.

 Nothing wrong with wheelie compounds, eugeneb used a worthless programme that also fails in the real world.

 jrchambers, click on my link then compare with a recurve test from  www.bowreports.com  

 Energy in energy out add in momentum, Ashby and voila a passsionate debate. thanks

[eugeneb]

As fentiger said ... I used a worthless application (actually two of them) to calc the speeds ... it seems the speed calcs are fairly accurate at the lower arrow weights ... big difference between 214fps (real world) and 147fps (speed calc) on 1000 grain arrow ... I'll reserve my comment untill I have checked it out myself = lesson learnt!  ;-)

[Bjorn]

Why here? Are there really no 'compound bow sites' for you to have these discussions?

[miklvines]

Originally posted by Bjorn:
Why here? Are there really no 'compound bow sites' for you to have these discussions?

he won't get near the quality answers from them as he will get truth here.

[Don Stokes]

Bjorn, I wondered the same thing. It must be because Dr. Ashby was invoked early on.

[eugeneb]

Hi Guys,

I was contemplating whether I should answer.

Grapes is 100% correct in his comment.  Further my intention is not to discuss/promote wheelie bows, rather my interest and the topic objectives was to gather info around penetration and the related attributes.

More and more compound forums makes reference to Dr Ashby's reports and refer to the tradgang site ... and in my opinion, through his fascinating research is changing the views of compound archers (for the better) and more compound archers are going heavier arrows and two-bladed bh's.  Which I think is a good thing for archery in general and should be a feather-in-the-cap for the trad guys and this site.

As Grapes stated, I have learnt more from this forum in a short space of time than I would have from another ... that is why I chose it ... real world information!

My intention is not to invoke a huge debate around this and if you feel my post is offensive or out of line re Tradgang's policy ... let me know.

Cheers - Eugene

[TheFatboy]

I think this is as relevant for traditional archers as it is for compound shooters - almost. Traditional bows have a hard time reaching the speeds of a compound bow, therefore we emphasize more on weight than on speed, to maximize the joule. But I too have thought about this matter. I shoot 10GPP arrows, and speed is fairly good. A 700 grain arrow shot from one of my 70# bows should be enough to take out most game, so I'm not worried about that. I find that weight is important, as it maximizes the bows effiency, but I would probably not go heavier than 10GPP, unless I had to kill a buffalo. Versus quicker reacting game, I believe that speed is just as important.

[eugeneb]

Thanks Fatboy ... what made it more relevant for me was to understand whether my #70 can take a buff or whether it would be too light; and also ensuring I am not too light in the arrow department.  Lots of contradicting info on the WEB ranging from 550-1000 grain arrows, and professing draw weights minimum #80

I got all the info I required from this forum (thanks) and also met up with Ozy Clint

I selected the following:

- Easton FMJ DG (thin diameter) arrow;
- 75gr brass insert;
- German Kinetic bh 210grain;
- total arrow weight 850 grain @ 219fps.

(havn't checked my FOC yet)

Hopefully I can crack a buff rib with this and not return via the newspaper.

Thanks

[Ragnarok Forge]

fentirger.

Yes, KE decreases with increased arrow weight out of the same bow. The KE has velocity squared as a factor which pumps up the numbers based on velocity.  Increased arrow weight out of the same bow means decreased velocity = decreased KE.

[Northwest_Bowhunter]

So according to the calculator on the link above, my 68# longbow throwing a 547 gr arrow at 168 fps is barely adequate to drop a whitetail deer. At 34.3 ft lbs I have a better chance of injuring myself then a black bear and yet I know this is not true.  I think physicist don't hunt!!!

[Mike Vines]

Originally posted by Northwest_Bowhunter:
So according to the calculator on the link above, my 68# longbow throwing a 547 gr arrow at 168 fps is barely adequate to drop a whitetail deer. At 34.3 ft lbs I have a better chance of injuring myself then a black bear and yet I know this is not true.  I think physicist don't hunt!!!

I did the math a few years ago on my hill bow, and laminated birch arrows, and was right around 30 ft lbs, and the deer tasted good.  I do believe it is all about placement.  Yes higher odds will increase your numbers, but if the placement is bad, so is everything else.

[Mike Vines]

Originally posted by Ragnarok Forge:
fentirger.

Yes, KE decreases with increased arrow weight out of the same bow. The KE has velocity squared as a factor which pumps up the numbers based on velocity.  Increased arrow weight out of the same bow means decreased velocity = decreased KE.

I have always been a firm believer that speed kills, but mass destroys.  You could throw a brick, and yes it would hurt, but throw a bowling ball, and it will destroy it's target.  Yes it will be slower, but much more deadly.

[Northwest_Bowhunter]

I think a compound shooter made that chart, native Americans were killing buffalo long before they could achieve those speeds.

[S.C. Hunter]

I am still a little confused by all of this KE momentum debate. I know for a long time other hunters and myself have had great success with 45#-55# bows shooting arrows between 450- 600 grains with a good broadhead. I have had complete pass through shots on deer with arrows as light as 485 grains from a 50# and 55# longbow with good old zwickey 2 blades. I have never shot a arrow over 560 grains for deer or hogs and never had a issue. Plains indians were taking buffalo with 35# bows and stone heads I think we have a pretty good chance with what we have today. Just my 2 cents worth.