SA-

And to answer your question about the spine of the arrow- you are correct. The arrow only "sees" the amount of energy stored under the draw force curve, not just peak weight. That's why many shaft charts have different columns for round wheel, soft cam, hard cam, etc... The charts are trying to compensate for the draw force curves in combination with peak weight.

And I think SA and JeffB may be talking about two different "efficiency" terms. There's the efficiency having a 70# peak weight, but a draw force curve that doesn't take advantage of much of the peak weight. I think that is what SA is referring to when talking about speed bows being more efficient. The use more of the peak weight to store more energy. For total "efficiency" of the draw force curve, you would have to compare the draw force curve of the bow in question to a absolute draw force curve. In that I mean that the absolute bow starts out at the PEAK weight, and stays at that peak weight throughout the entire draw. In other words, a draw force curve that is a straight line across the entire draw cycle.

There is also the efficiency of the bow as it is, as a system, regardless of the drawforce curve. Norb expresses these values as VIRTUAL MASS and DYNAMIC EFFICIENCY. The virtual mass takes into account the bow and arrow combination, because as the arrow gets lighter, the effiency goes down. Virtual mass is the weight in grains of an imaginary mass shot fro the bow at the same time and the same speed as the measured arrow. VM represents all the inefficiencies in the bow and arrow combo. The sum of the KE of the arrow and the KE of the imaginary mass equals the stored energy of the bow (area under the draw force curve). Norb's idea of VM is not constant (but close), and will change with different arrows on a given bow, but allows good representation of what's going on in the system.

Dynamic Efficiency is the ratio of the KE of the arrow at launch to the stored energy of the bow- expressed as a percentage.



Edited by - Black Frog on 11/07/2002 12:05:24