silentassassin

Well try to shoot a 3 pound arrow and the measure it and let me know your results There is also a point at which as you increase in arrow weight when you get above that point you begin to loose efficiency. (it works just like the light arrow thing only backwards)

So you are saying that it is impossible to design a cam that will shoot a light arrow more efficiently than it will a heavy one????????

Sylvan

That is correct!

A 3 pound arrow will shoot more efficiently than a 2.9 pound arrow and it will be less efficient than a 3.1 pound arrow. You will never achieve 100% eff however because as the mass increases the eff rate of change slows down and you will only approach 100% asymtotically. If you get too ridiculous with the size of the arrow some other factors will begin to play. Ultimately of course the arrow would be too heavy for the archer to lift but I think way before that you would have problems even constructing and arrow that would fit on the rest. If we keep things in perspective, then if the only thing that increases is arrow mass then efficiency will also increase.

BobCo19-65

Good post Cougar


Dr. Ed Ashby:

Sylvan

I believe the focus on KE is way overblown myself but to say that KE tells nothing about the potential to penetrate is idiotic. The laws of physics simply don't agree. Although KE is not the whole story it certainly is some of the story! If KE doesn't tell some of the story then neither does velocity and mass becuase that is what KE is.

BobCo19-65

Sylvan, I must say that you sound very intellectual to me. Maybe you would care to to discuss Dr. Ashby's ("idiotic" as you put it) theories directly with him. I must say that I would love to see that discussion take place.

The quote that I posted was written by him on TradGang late in March 05. He frequents there often. I'm sure you could get his attention with your brilliance.

silentassassin

However, there will reach an equilibrium point at which the cam is most efficient. Therefore, the 3 pound arrow will not be as efficient as say a 1 lb arrow. Using your analogy a 40 lbs bow shotting a 40 lb arrow (that it can't even budge) would have more kinetic energy than the same bow would shooting a lighter projectile than it could a 400 grain aroow that it could propel at 180 fps.

Sylvan

BobCo19-65 wrote: I appologize if I sounded arrogant but what I said was accurate. I suspect that Dr. Ashby's statement was an exageration for the purpose of making a point. The point being that there are other more important things to be concerned about regarding penetration on a live animal than KE. With that I would agree. I'm sure 1 sentence out of whatever paper this was doesn't tell his whole story. But back to the sentence as written; you needn't get past freshman physics or a short course in mechanics to know it is incorrect to say that KE has nothing to do with penetration. I don't need to discuss this with him as these discussions have already taken place as early as Newton in the 1600's. If there were any physicist or mathmaticians that argued Dr. Ashby's sentence they lost the argument a long long time ago.

BobCo19-65

Didn't think so.

Wasn't a paper, it was a reponse on a thread in the forum. Wasn't much more to it then that. But I'm sure that his years and years of actual testing on animals and extensive knowledge on the subject has little to do with your basis of solid reasoning.

Sylvan

BobCo19-65 wrote: I don't care how long he has been studying or what his degree is in I am merely pointing out that the sentence you quoted is not consistent with the principles of physics. To say that KE has nothing to do with potential penetration is simply incorect. If he is a PHD in any of the physical sciences I don't believe he believes that at all. Like I said, I suspect he was exagerating to make a point and it is now being taken out of context. I don't believe you believe that sentence is true either. Your own intuition tells you that this is wrong. You are actually arguing that increasing the mass of an object in motion or increasing the speed of an object in motion will have no effect effect on the resultant penetration during a collision. You might as well argue the earth is flat because you would be just as correct. This is not my reasoning BTW, you can learn this in any freshman level physics or mechanics book.

BTW its not very ethical to chop a persons words in mid sentence...
of which you only quoted Pretty cheap shot! At least you should have added the ...

Sylvan

silentassassin wrote: Bow dynamic efficiency(EFF) is defined as Kinetic energy / Stored energy * 100.

KE = m/2 x (SE / (m + mv))**1/2

where:
SE = stored energy
KE = kinetic energy
m = mass of arrow
mv = virtual mass of the bow

Now, as mv and SE are constants then you should see that KE is a function of m. As SE is a constant then EFF is a fucntion of KE making EFF=f(m)

I don't know what you are talking about but it isn't dynamic efficiency and that efficiency is the point of the thread. EFF is a function of arrow mass. That is a simple fact of bow mechanics. Heavier arrows shot from the same bow carry more initial KE than lighter arrows. The guys obsessed with speed would like to think otherwise but you can't get around the physics.

BTW, think about a recurve for a minute. A recurve follows the same equations and no cams at all!