That is the amount of energy you get out of the bow for all the energy you have to put into it. Ive been testing a lot of bows over the last 5 years and I keep seeing efficiencies going up (good news for aging hunters cranking down the weight) I don’t think the industry directly uses this number since it does not rely on an unrealistic 70/30/350 (assuming that there are actually bowhunters that hunt like that)but instead this works for everybody since it takes your exact specs (your draw weight, your draw length, your arrow weight) and gives you a value so you don’t have to try to extrapolate from the industry number. Work smarter, not harder. Here is how I arrive at it. KE/draw weight (which is the energy you get out of the bow divided by all the effort of draw weight you have to pull back) This allows you to find the sweet spot for performance based on tweaking draw weight and arrow weight, My current 2006 Bowtech has an efficiency rating of .883 based on my setup which is 64lbs draw weight Arrow wgt. of 7 grains per pound of draw (not grains per inch) x 64 lbs = 420 grain arrow. 29” draw Arrow speed: 238.3 FPS KE: 56.4 ft/lbs Efficiency factor (KE divided by draw wgt.): 56.4 divided by 64 = .883. I have to draw back 64 pounds to get 56.4 ft/lbs of KE. As I look to get a new bow (looking at the Bowtech RPM 360) and watching personal reviews where hunters provide projectile weight and video'd speeds , I see people are getting performance efficiencies of 1.28 to 1.46. Another member here mentioned his RPM 360 is delivering the following 58lbs draw weight 30" draw 413gr arrows 304fps. 84.7 K.E. That’s an efficiency of 1.46. He is only pulling back 58 lbs (im pulling 64) yet he is getting K.E. of 84.7 compared to my 56.4 ft/lbs. I have to work harder to get much less killing power. (sort of like getting less gas mileage and HP but expending more energy for it) To get more K.E. I either have to crank up my draw weight (im actually going the other direction as I age) or I can use a more efficient bow. What is your efficiency? Do you know? Im all for getting more KE without blowing out my shoulder.
Most of the 2013 and 2014 bows I have tested are in the 1.0 range and higher. This new RPM 360 is pretty impressive. What are your particulars?(Draw wgt, speed)
None as chrony speeds were tested at point blank before the effects of drag come into play. These values could be measured (and efficiency checked) without fletching if you wrapped tape around the back of the arrow to mimic the weight of fletching.
BowTech Carbon Overdrive 62 pounds, 29 inches, 401 grains (6.442 gpp), 71.75 KE, 285 fps, 1.157 BowTech Insanity 62 pounds, 29 inches, 323 grains (5.223 gpp), 75.95 KE, 325 fps, 1.225
Yes, that's what Im talking about. Those are efficient bows. Its been my experience that you will get even more efficiency as you increase projectile weight (up to a point of diminishing returns) which for me was 11 grains of projectile weight per pound of draw weight.
My last bow was 2014 Bowtech RPM 360. 58#, 28.5" draw, 397 grn arrow, 301 fps. 79.79 KE, 1.38 eff. Current bow: 2014 Hoyt Faktor 30, 60#, 28" draw, 375 grn arrow, 272 fps. 61.54 KE, 1.02 eff. All fps were measured and not estimated.
Make my 2006 Bowtech (.83 effic) look meek. I have to work that much harder and I will never get the KE you guys are getting at lower draw weights wich is part of the reason for my search for a more modern bow that I can crank the weight down on and enjoy performance greater than I have now.
27" and about 71-72.5 pounds. Somewhere between 294-298 fps. I used 295 fps. Even if I used 298 fps I would get just over .94
So you are on the lower end (but still not so poor in efficiency as me). It will still kill any game it encounters, you just have to do more to get less out of your setup.
I am good at shooting dots but I can't hit a 3d deer at 50 yards to save my life. One day I was trying to shoot at 50 yards and I missed the target. I shot through my shed with aluminum siding and when I looked for my arrow it was through my friends derby car door. Yeah it had a field point but I wasn't the first person to shoot through the shed and I was the only one to make it into the car . I would never take a deer past 30 yards because after that I can't consistently hit the heart and lungs past 35. Anything closer I should be good on penetration.
No doubt. As I said, It will pass through any deer it hits for sure. Im just into getting the most out of what is my steady decline of draw weight as I age. Since I can max out on projectile weight, the only other option I have is to get a bow that is very efficient.
How did you determine the bows stored energy (the energy you put in)? Different cam configurations results in draw curves. Did you measure holding weight at 1" increments of the draw cycle? Did you take the max holding weight x length of stroke. I wondered how you would do this before. I had no answer except I need all the help/efficiency I can't get.
Its not so involved or difficult as I think you are making it. The "energy in" is simply the peak draw weight you have to muscle back and the "energy out" is the KE (Kinetic Energy) your bow imparts into the arrow. KE divided by Peak Draw weight tells you how efficient your bow is at the current setting. You can most likely increase the efficiency by increasing projectile weight (in already constructed arrows that means increasing tip weight). Your bow is what it is. You can do a lot to effect efficiency ratings as it is a product of the bow's design which is why I am shopping for a new bow that is already efficient at the heavy arrow weights I like to shoot. When I bought my last bow (9 years ago) I had an efficient model but in that time span, designs and materials have changes enough that my 9 year old bow is not even close to today's models. That means I am drawing more weight and getting less performance than today's bows with lower draw weights.
Is this a real equation or something you figured out. It may give you a relatively consistent number to base things off but I don't think you can call it efficiency. When you say energy in and use the draw weight, that isn't a valid quantity of energy, its a force. Energy requires you to have a force, distance and acceleration. I don't know how you arrive at the KE out but that seems reasonable to me. Things get hairy again when you divide the KE out by Energy in, which is the correct formula, but your saying KE with units ft/lbs divided by Energy in with units lbs end up being unitless, which is what efficiency should be. When it really ends up being ft/lbs^2. So you do get a ratio of the two numbers and like I said previously, could be used to chart how bow correlate with one another but I wouldn't call it efficiency.
There is a formula to calculate efficiency, I do not recall what it is. I do remember it is a long equation. Norm Mullaney always referenced it in his bow reports. The formula usually used differes from DIY's, it is Stored Energy divided by Peak Draw Force (SE / PDF). It yields a number very similar to what DIY was using. KE is MUCH easier to calculate than stored energy, which is why I assume he uses the formula that he does. You do not need a draw board to use his formula like you do to calculate stored energy where you would need to start at brace, measure the draw weight per inch of draw length and chart it all. DIY's method is easy to calculate and gives a very good representation of the performance yield by the energy or force that we use to draw the bow.
Interesting. The kinetic energy will be less then the stored energy due to energy loss in vibration and such. I think the kinetic energy method would give you a slightly lower number then the stored energy calculation but it sound like the difference is minimal.
You are exactly correct with your thoughts... Per DIY's formula for my bow that I calculated for him yesterday: BowTech Insanity 62 pounds, 29 inches, 323 grains (5.223 gpp), 75.95 KE, 325 fps, 1.225 My bows actual ratio of SE / PDF: BowTech Insanity 62 pounds, 29 inches, 323 grains (5.223 gpp), 75.95 KE, 325 fps, 1.483
