Jerry/NJ

Thats some impressive numbers Russ! Has your bow been pumping iron?

Sylvan

walks with a gimp,

60.7 pounds 29"

================91gr=============70gr============= =53gr======
-wt------vel----SEP----predicted-----SEP---predicted------SEP----predicted
301.4----317----1.44------------------1.37-------------------1.30------------
365.4----297----1.47----293.9--------1.41----292.8--------1.35----291.7
365.4----297----1.44----296.7--------1.38----295.7--------1.32----294.8 **Corrected**
391.4----282----1.40----285.9--------1.34----284.4--------1.29----283.1
444.3----267----1.40----171.4--------1.34----269.4--------1.30----267.6
484.0----257----1.39----261.9--------1.34----259.6--------1.30----257.5
518.0----249----1.38----254.5--------1.33----251.9--------1.30----249.7

Note the value for the 365.4 grain arrow appears to be out of whack with the other test results.

I couldn't get a descent "fit" of predicted to actual until I took the virtual mass down to 53 grains. I would expect a bow to have close to 100 grains virtual mass so this is extremely low virtual mass. In terms of efficiency, this bow would be shooting a 301.4 grain arrow at 85% !!! and a 518 grain arrow at 91% !!!.

If you want to put this in perspective, check out the February 2005 issue of Bowhunting World pg 63. There are effiency test results for a High Country bow. You will see that for a 500 grain arrow the efficiency is under 78%. 90% would be truly remarkable. I won't use the word unbelievable but I'm pretty close to it.

It's interesting that when I adjust virtual mass for good theoretical fit to the raw data both Jerry's bow and your bow settle in to an SEP of about 1.3. This is a pretty normal value for a cam that would be considered a "fast" bow. Like I said, 1.4 is kind of upper end. Note pg 62 of the magazine and you will see the High Country bow SEP is about 1.21 to 1.31 pending on the cam module they tested. They refer to SEP as S.E./P.D.F.

To sum up, in order to get the "physics" of all this to balance with the measured data, virtual mass has to be lowered to a point that is significantly below what would be considered normal.

pdq 5oh

I think one thing that greatly increases the efficiency of the E cams IS the force draw curve. With the majority of the weight at the front end, the cam continues to deliver energy to the arrow longer into the power stroke. With a conventional single cam, the weight peaks more at the end of the curve. Energy is transmitted initially, then reduced further into the power stroke. A poor analogy: mash the throttle on your car. Then let off at 60 ft. The initial hit is hard, then you decelerate for the next 270 ft. Now feed the throttle in smoothly to the floor all the way to 330 ft. You are, of course, accelerating the whole 330 ft.

Sylvan

pdq 5oh,

You bring up a facinating topic of bow mechanics. Force/draw curve effects on overall efficiency. I've been told that there have been some papers on the subject but I haven't seen them. I don't believe its a subject that has been explored very far. Most of the research I have read aside from overall stored energy is with regard to ergonomics of the draw. Archers tend to like the upfront loading because we are physically more capable at the beginning of the draw and we loose mechanical advantage as our elbows bend and the hand comes back. I wish you hadn't brought this up because now I'll have to ponder the subject and get frustrated because I don't have the equipment to measure how the arrow accelerates during the power stroke. Thanks alot! (just kidding of course)

Sylvan

I'm new here so I'm alowed at least 1 dumb question. Why is it that everytime I post it says I'm replying to buckeyebuckhntr? It doesn't do that for other. I'm hitting the reply button thats associated with the post I'm want to reply to. What am I missing?

walks with a gimp

Sylvan, what does virtual mass mean as in 53 grains virtual mass?? How do you measure virtual mass?

500 fps

This is just a little tidbit along these same lines that some might find interesting:

I don't know which cam it applies to, or if it is applicable to bow cams in general (single and dual), but Kevin Strother has a patent on the maximal amount of efficiency that can be designed into a cam.

He used to come here occassionally. Maybe he could shed some more light.

Matt / PA

I should have stayed awake longer in Physics 202 in college.......it may have helped. I reach a certain point with math where bodily systems begin to shut down.[:-]

I do find all of this quite interesting........and also just a note on these calculations: all of the speeds posted are with full string loads. Hush kits, loops, peeps etc.........these are not bare(or minimal) string loads that you might see in other raw bow tests.

In reality the actual rated speeds compared to other manufacturers testing methods would probably be 5-7fps FASTER than what is being quoted.

Carry on, I'll keep up if possible.

Sylvan

If you really want to learn about virtual mass try this web site http://homepage.ntlworld.com/joetapley/

I think of it as the amount of mass the bow has to accelerate in addition to the arrow. Virtual mass is the reason the bow string doesn't go to infinite velocity when you dry fire.

To measure it you need to know 3 things. Arrow mass, arrow velocity and stored energy
then:

SE = 1/2(m1+m2)V**2

where m1 is arrow mass, m2 is virtual mass, V is velocity and SE is stored energy.

Solve for m2 and plug in the measured values and you've got it.

Sylvan

I think what you are talking about here is a different efficiency than what I have been refering to. Some when they say efficiency they are talking about how much stored energy the cam provides relative to the peak weight. This is also referred to as stored energy to peak ratio or SEP.

The efficiency I am referring to is the percentage of stored energy that is transferred to the arrow. The camming effects it but so do a plethera of other things like friction, limb design, material etc.