D.Parsons

How do you figure Peak draw fource ratio?
How do you figure Bow efficiency in %?
How do you figure Virtual Mass (GR)?

Sylvan

"How do you figure Peak draw fource ratio?" - I assume you mean Stored Energy to Peak ratio. Divide the energy stored in the bow in ft/lbs by the peak weight in pounds.

"How do you figure Bow efficiency in %? -" Divide the kinetic energy of the arrow by the stored energy in the bow and multiply by 100.

"How do you figure Virtual Mass (GR)? " -

SE = 1/2(m1 +m2)v^2
where:
SE = stored energy of bow
m1 = mass of the arrow
m2 = virtual mass
v = velocity of the arrow

Measure SE, m1 and v then plug in and solve for m2. If you want the answer in grains you'll need to take care in keeping the units appropriately i.e. convert arrow weight in grains to slugs, SE in ft/lbs and velocity in ft/sec.

Arthur P

You have to graph the force draw curve of your bow to find out how much SE you've got before you can do the calculations. Plotthe curveand then count the number of boxes under the line. The total is your SE.

After your cams roll over, it's very important to make sure you take the weight on the pull cycle. If you let up even the slightest bit you will get a false reading because the poundage drops off on the letdown cycle due to static hysteresis. It's nearly impossible to do when pulling the bow by hand.

Along that same line, it's always interesting to also plot the bow's force draw curve on the letdown cycle too, in order to see how much hysteresis your bow has.

Elkcrazy8

Many of the targeting programs will have these graphs. The nice thing about having them. Is you canmake changes to your bow on the computer and see the differences without having to calculate it every time. I beleive that Pinwheel software still has the 10 day free trail. You can download it and mess with it. The graphis on there andit also shows the hysteresis that Arthur is speaking about.

Sylvan

Arthur is right on. Static hysteresisgives arough indication of the frictional losses in a bow. The lower the number the better. To calculate it simply find the diferrence in the amount of SE in the "draw" vs "let down" cylcle in ft/lbs. Of course virtual mass is a much better indicator of the bows performance loss as it isunder dynamic conditions and that's where it really counts.

D.Parsons

You all are talking Greek to me!!

Sylvan

Sorry D. Parsons. Arthur and I have a track record of boring people with technical jargon and details probably few give a rat's butt about. Anyway, Static hysteresis is just a fancy term that refers to the frictional losses in a bow under "static" or at rest conditions. It's measured by determining, as Arthur described, the difference in the stored energy in a bow as measured when the bow is drawn back vs when it is let down. It's expressed in ft/lbs. As Art warned, you get lower weight values at the same draw length when you are letting a bow down then when you are drawing it back so you have to be carefull not to "mix" the 2 when you are doing measurements. Hope this helps!