if I have a 2 rubbers set same diameter with different length ,50 cm /80 cm ,and each stretch to 300 % (150 cm / 240 cm) ,each will have same stored energy ,if we check separately that energy for example by pulling scale ?the formula in spearit show energy for different cross section but what about the length ?it seem like length factor won't be considered ,is it true ?
what is the rubber diameter?
The amount of calories you expend loading the bands should be an indication of the potential energy stored in the elongated band. Longer pull length means more effort to load.
band length should not matter, as long as %elongation is the same. They will both reach 0% elongation a the same time and have the same power stroke. ( Im assuming these bands are in the same muzzle hole but anchored on different loading tabs)
This is in a vacuum, however.
In practice. a longer and heavier band will throw more weight forward and probably have a bit more drag.
I like to stick with the least amount of band mass possible using small ID rubber achieves this.
In my opinion the most efficient set up on a traditional multi band gun is allowing your bands to have the longest power stroke possible. having your bands and your loading tabs as far apart as possible.
In my modest opinion
the most effective stretch (K) for 14mm diameter should be 5 to 6 ( 500 to 500% )
the most effective stretch (K) for 16 mm diameter should be 4 to 4,5 ( 400 to 450% )
for example 50cm band should fit with about 100 cm length (from muzzle gun to the last fin of the shaft)