The smaller the shaft, the faster the bow will be; however, it will also mean the bow will produce less kinetic energy and the result is less penetration. If you take a small gravel and throw it against a fence, it will be quick. Sometimes, even so quick you won’t see the gravel moving, but you will only hear a “peck” noise. If you would grab a much bigger rock, it might travel a little slower, but you will hear a much heavier “bang.” That is because the heavier the rock, the more kinetic energy produced. Don’t get caught up in too much speed. Kinetic energy is very important.
Kinetic energy is probably the most under-looked factor by archers to this day. Everyone is so caught up in how fast the chronograph measures their bow, that they lose the idea on how much penetration and kinetic energy the arrow is producing. In boxing, kinetic energy leads to knock outs, while speed leads to points.
Kinetic energy is measured in pounds per foot. And the formula to finding the kinetic energy your bow is generating is [speed * speed * arrow weight divided by 450240]. That’s to say that speed is important for your total kinetic energy produced, but the arrow weight is also important. You want to produce fast speeds with a heavier grain arrow. Most people that are not seeing a lot of kinetic energy is due to lack of arrow weight; rather than lack of speed.
People often mistake the “spine” of the arrow to be the grain. The “spine” is the big number on the arrow often written “500” or “340” or “400” or somewhat. The heavier your draw weight, the lower the “spine” number should be. The spine number is nothing more than the total bend-stiffness ratio. The technique used to determine the “spine” of an arrow is to take a 29 inch arrow, hold it at 28 inches on each end and hold a 1.94 pound weight in the middle of it, the amount of bend determines the stiffness. A heavier draw weight arrow would have a harder landing and more impact, and the result is the need for less bend in the arrow, so that the arrow doesn’t break or bend upon impact.