pdq 5oh

On Jackson's sight, the speed calculator asks for SE/PDF ratio. A small change has a big affect. Other than a bow report, is there any way to find this out? What is virtual mass? Is there any rule of thumb as to bow efficiency? I have a Bear Vapor 300 set @ 55# w/ 30" draw. Any help is appreciated. I'm trying to decide on what broadhead to use, though I intend to test several different ones this summer. I'm leaning towards 125 grain heads. My bow's speed will enter into the final decisions. Thanks.


Phil.

PABowhntr

pdq,

Hmmm, surprisingly no responses even after 22 people looked at your question.

I will take a small stab at this one but I am sure JeffB, Len, Black Frog or one of the others can fill in the blank spots.

SE/PDF stands for Stored Energy divided by either Peak Draw force or Pounds of Draw force...I forget which one though.

My memory on virtual mass is even fuzzier, but I believe it has something to do with the way any given bow releases its energy into arrows of different mass weights.....but I think I am way off on that one. I do seem to remember that the higher the virtual mass the "better" the bow is in terms of transferring energy.

If anyone has one of the older copies of Bowhunting World laying around then Norb goes into a specific description of all of that info....

Sorry I couldn't help more.

pdq 5oh

Thanks PA. I'm trying to get an accurate estimation of my bow's kenetic energy to make a good decision on what type & weight broadhead to use. As a new bowhunter, I sure don't want to be irresponsible & use the wrong stuff only to find out after being lucky enough to hit a deer.

Phil.

sajackson

pdq,

What type of cam is on your bow? Typically solo and hatchet type twin cams run around 1.45 or so for an SE/PDF ratio, but this depends also on ATA of the bow in some degree. See if you can find any test reports in Bowhunting World magazine

A bow with lower virtual mass is more efficient. Norb Mullaney uses the virtual mass concept as a way to roll-up all of the inefficiencies in a bow.

The basic idea of virtual mass is if the bow were 100% efficient you could take the KE= 1/2mv^2 equation and solve for velocity directly. What Norb does is say what mass would I have to add to the arrow to make this true. He does this by taking several data points and averaging the result.

At this point you can take the Stored Energy(SE) and plug into the KE equation;

velocity = square root(2 * SE/ mass arrow + virtual mass)

Biteme/Paul Mohr

Go to a shop and shoot it through a chrono, that is the only way to really know for sure.

Or try this link.

http://dialspace.dial.pipex.com/town...d.applet1.html

Set the Paralax to 3.5 inches, and the fletchings to small vanes. You will aslo need to now your exact finnished arrow weight of the arrow you are shooting. Shoot at 10, 20 and 30 yards using the same pin for all distances. Measure the distance from point of aim to point of impact for each yardage and enter them into the calculator. It will be fairly close to what a chrono will read. I actually like this method better, because it measures the actual performance of you bow and arrow as it travels to the target at different yardages, not just a foot in front of the bow like a chrono. I don't know about you, but I have never had the chance to shoot a deer at point blank range before, so I don't realy care how my bow performs at that distance.

I think we came up with around 400 or to 410 grns for your arrow before didn't we?

Keep in mind you will need a fairly good sized target for this since you arrow will drop any were from 5 to 15 inches at 30 yards. I would think yours will be closer to 5 or under though. Mine drops around 10 at 230 fps. and around 5 or 6 at 270 fps.

You also need to be able to shoot a fairly decent group at 30 yards for this to work well, shoot 3 or 4 arrows at each yardage and measure from the center of the group. I only shoot 2 at 10 and 20 because I am afraid of wrecking arrows.

I hope that helped.

Phil, E-mail when you get a chance, I have a question about the epsilons. Mine are doing something odd and I want to see if yours do the same thing.

[email protected]


Paul

Biteme/Paul Mohr

Oh, Phil, you are shooting your bow at 30 inches with 55 pounds of draw. That is pretty close to AMO specs. The AMO speed is 223, I would think you should be able to shoot faster than than that because of the lighter arrow. I am going to take a guess at some where around 240 fps if the advertised specs are correct. That would put you right around 50 to 55 ft/lbs of energy. That sounds low to me for a 30 inch draw though. My Yukon does close to that with a 26 inch draw and 60 lbs. I think they are fudging the IBO spec a little. Let us know what you find out.

And the difference in speed between the 100 and 125 grain head won't be that much that I would worry about it. I would go with what ever gave you the best spine and foc.

Paul

pdq 5oh

Saj & Paul, thanks for the help. This will get me started in the right direction. I was hoping for around 250 fps. My bow may be close to that it appears. I can always crank it up to 60# if need be. Thanks again.

Phil.