Arthur P

Just floating around in the ether, Pinwheel.

Black Frog

Virtual Mass is a way expressing total inefficiency of the DYNAMIC bow and arrow combination. VM IS total inefficiency, but measured dynamically- not in static condition. How do we measure hysteresis? Compare the area under the draw curve and the letdown curve? right? That isn't a dynamic meaurement.

I see what you're trying to do, but it wouldn't be correct to say:
virtual mass + hysteresis = total inefficiency

The following wouldn't be technically correct either because we mixing apples and organes, but it might get the point across better. Assume that all the components are factors in energy loss, not different physics units:

Virtual mass = hysteresis + limb acceleration losses + cam acceleration losses + wind resistance on the limbs + wind resistance on the string + ....etc... anything else you want to add that may contribute to energy loss in the system

Virtual mass = total inefficiency of the DYNAMIC bow/arrow combo (expressed as an imaginary weight in grains)

Just keep in mind that Virtural Mass encompasses ALL the inefficiencies of the bow/arrow shot. It's the big-dog, head hancho, CEO of the energy loss and inefficiency department. ALL inefficiencies factor INTO the VM. VM accounts for ALL losses and is expressed dynamically on the shot.

clear as mud?

Arthur P

Actually, like I said on another thread, there are certain things my brain refuses to understand and this is one of 'em. My question was in response to a question on that thread about what virtual mass is and how to compute it in order to use the speed estimation program on Jackson's site.

If you must use a chronograph to find out the virtual mass of the bow in the first place, then you don't need to use the virtual mass number in the program because you've GOT a chronograph. DING DING DING!

I'm gonna go over to the freeway and throw my compound in front of the first 18-wheeler that comes along. That should be a good test of virtual mass.

ampahunter

Wouldn't that be the test for "Virtual Madness"?

Trushot_archer

You can borrow my snowshoes if ya want Arthur

Black Frog

Arthur, just another thought as long as this subject is fairly recent.

You mentioned hysteresis in some of your posts. Hysteresis is a static measurement. I checked with some info I had saved from Norb (like I posted about VM) on hysteresis.

As I mentioned, the hysteresis is measured as the difference between the draw curve and letdown curve. It is the STATIC friction of the bow- and it has nothing to do with an arrow of any kind.

Norb mentions that static hysteresis has no direct significance as related to the DYNAMIC performance of a bow since it is measured under static conditions. But generally, bow with high static hysteresis show lower levels of dynamic efficiency (higher VM).

Norb likes to keep the static measurements totally seperate from the dynamic end of things. I see why as you don't want to be mixing apples and oranges. Well, I don't know if I would go so far as to say that hysteresis has NO DIRECT SIGNIFICANCE related to the dynamic shot (which is what Norb said)- there's gotta be some sort of link to high friction in a static condition to poor dynamic performance.

Like we talked about, the VM accounts for ALL losses in the dynamic shot, so I would venture to say that the hysteresis has to factor into that somehow. I can't ever imagine how you could have poor static friction, yet have good dynamic efficiency.

just some more food for thought.....

Rangeball

Todd, what is the most efficient VM value you've seen? I'm assuming there is no arbitrary standard above or below which one could use to compare bows prior to purchasing, but generally, what VM value or below shows excellent efficiency?

Just wondering, as I'm sure this is moot as we'll never see these values published by companies as a spec to compare against the competition...

Black Frog

Range,

Problem is that VM will change with draw weight and arrow weight, and I would also guess draw length (power stroke). VM isn't constant when you alter any of these.

But what you can do is sample enough arrow weights at a set draw length and poundage to give you data to work with, and come up with an "average VM". The experimental data establishes a theoretically correct curve of virtual mass. From this curve, arrow velocity, dynamic efficiency, and kinetic energy can be calculated within the range of arrow weight tested. According to Norb, this technique minimizes experimental error and provides a performance profile for the bow. You think manufacturers want this comparative data published?

As far as what's a good average VM? I haven't kept a mental note on what is a "good" number compared to a poor value, but I know that Norb was quite impressed in his review of the Darton Rampage, but can't recall the number to compare with any other reviews. If you have kept copies of Bow Hunting World, start paging through some past bow reviews to have a look. Unfortunately, I pitched all my old copies....

Arthur P

Since Norb does all his testing to AMO standards - 540 grain arrow, 60 pounds at 30" draw AND 360 grains, 60 pounds at 30" draw, per ASTM1544-99 - then the VM figures he comes up with will be lower than they'd be if the bow was tested to IBO rating. So his ratings would show better efficiency than someone would actually get by shooting the same model bow with a 350 gn arrow at 70 pounds and 30" draw. Right?

PABowhntr

BF,

Great Info. I am glad to see you posting again. If I remember correctly, many of the VM figures were in the 135 to 145 range though I could be way off base. It has been some time since I read any of the Bowhunting World Mags.