Rule of thumb for stored energy

[Badger]

    Measuring stored energy is kind of a pain. But if you are interested in knowing where your energy is going it really is a necessity to get a fairly accurate reading of how much energy your bow stores. I am going to propose a simplified method for fairly accurate assessments. One person doesn't really make enough bows to supply enough data so I am asking the community here to help us out and see what we can come up with in a short amount of time. The method I am proposing involves taking one simple draw weight measurement at exactly one-half your draw stroke. Example: bow draws 50# @ 28", bow has a 6" brace height. So the power stroke is 22". So we measure 1/2 of the draw stroke plus brace height equals 17". This is where we measure the weight.

  If someone here knows excel they can track this for us and give us the data. Before I go into detail, I want to show what I am thinking it will produce.

   Measurement @ 1/2 draw,        Stored            SEPDF                 Backset              style         Bow Length
    50# bow, 28" draw                  Energy
   
                          22#                  40.3#              80.6%               -1"                   straight         65"
                          23#                  42.06               84.1%                -5"                  straight         64"
                          24#                  44.00               88.0%               0                      R/D              65"                     
                          25#                  45.80               91.6%               .5"                    R/d               67"
                          26#                  47.6                95.5%                 1"                   recurve          64"
                          27#                  49.5                 99%                   2"                     R/D              67"
                          28#                  51.3                 102%                 3"                     recurve         66"
                          29#                  53.1                 106.2%               3"                    R/d               74"
                          30#                  55#                  110%                 6"                      recurve        65"


    These are just theoretical examples of what I think it will look like. If we can get in about 100 samples from the crew here we could establish a credable data base to figure where we were at and what the different numbers might mean to an everyday bow maker.

  The info I will need is . Draw weight, #" at 1/2 draw.  bow length, style and back set. I will probably add some optional rows just for reference.

[Tuomo]

I think that estimating stored energy is quite straightforward. Take the maximum draw weight (DW) and the power stroke (PS), then use the formula 0.5 × DW × PS.

For example, consider a 50# @ 28" bow with a 6" brace height. This gives 50 lb draw weight and a 22" (1.83 ft) power stroke, so: 0.5 × 50 × 1.83 = 45.8 ft-lb.

For a straight bow, this is a good estimate because the draw-force curve is almost linear. For reflex and recurve bows, the stored energy can be multiplied by a factor of 1.05–1.25, depending on how much backset the bow has (more backset gives a higher factor). This factor arises from the draw-force curve being “fatter” for bows with greater backset. For a straight bow, the factor is 1.0, and for deflexed bows it can be under 1.0.

In my opinion, defining a draw-force factor would be the best approach for estimating stored energy for a specific bow. This factor can be determined by measuring the draw-force curve of real bows and/or by modeling it using a program such as VirtualBow.

[Badger]

In my opinion, defining a draw-force factor would be the best approach for estimating stored energy for a specific bow. This factor can be determined by measuring the draw-force curve of real bows and/or by modeling it using a program such as VirtualBow.
[/quote]

   I agree with you 100%, the chart is not to be used by the bowmaker. That is just for the purpose of building a database that would help us establish the factors we need to use. The bow maker would simply take the draw weight and multiply it by his factor. He would know that is what he is looking for at 1/2 draw. Years ago I built what I called a super reflex with giant hooks and 12" backset. That used a factor of .6 X 50 = 30. So I would be looking for 30# at 1/2 draw. A typical r/d might use a factor of .52 for example. I think it would be a helpful barometer for those interested in such things.