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« Last post by Badger on Today at 01:31:41 pm »
Virtual mass is easy to calculate. It helps if you know the stored energy but if you don't you can usually get it by shooting a few different weight arrows.
Here is how to calculate it. His bow shot 256 fps with a 463 grain arrow. You go to a KE chart and look it up It shows 67.39 ft pounds. Lets say I estimate the bow is storing 93% of draw force. So it would be 93% X draw weight ( 98# ) That would give me 91# of stored energy. So now I simply add enough imaginary weight to the arrow that we know the speed of which is 256 fps to give me 100% efficincy. On the icase if I add 160 grains to the 463 grain arrow and then check the ke at 256 fps it will give me about 91 Ft#. So to check speeds of arrows I have not shot simply add 160 grains to the arrow I want to the know the speed of. So if I want to know how fast a 320 grain arrow would be I add 160 to 320 and see what speed gives me 91 ft#. So 320 + 160 + 480. How fast would a 480 grain arrow have to be to get 91. The answer would be 292 fps.
I am guessing at the stored energy in this case because I only hvae 1 arrow sample. If I had two samples I could be a lot closer. A 250 grain arrow should do about 316 fps.