I got the following email from my best student this morning:
“Okay coach, explain this one to me. Increasing my bow weight seems to make my arrows shoot more to the left. Compounding my confusion is that tonight I got the groups to move back to the right by tightening my plunger. Count me confused and dazed!
If this has never happened to you, you haven’t been in archery very long. The student in question shoots Olympic Recurve, so you have that as background. Here is what I answered, expanded for this post).
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A bow is a closed system, when you change one part, many others are affected. (Memorize this!)
You got two counterintuitive responses to things you did. The problem is that ceteras parabus was nowhere to be seen. (Ceteras parabus is the principle that “everything else was the same.”) When you make a single change to a bow, you make other changes, too … always! There is no such thing as “everything else was the same” when working with bows.
For example, you increased your draw weight. I do not know how much but it was not a fraction of a pound is my guess. When you screw in the limb bolts, you change the angle of the limbs to the bow (making the limbs more upright as it were). This results in a lower brace height. (Plus more tension on the string at brace, plus …) The brace height is one of the determinants of the point in space at which your arrow’s nocks separate from your string at the end of the power stroke. Since the string’s path toward the riser is a flattish “S curve,” the change in the point of separation of the string and nock is complex. If the nock comes off more to the right from where it did previously, the arrow ends up pointed more to the left (the point has enough inertia that it doesn’t move as much as the nock end). If the nock comes off more to the left, the arrow will be pointed more to the right. (Think about it.) I have also to point out that when the arrow separates from the string it is no longer touching the arrow rest.
“Coaches need to expect counterintuitive responses to equipment changes.”
When you change the bow’s draw weight, you are also changing the efficiency of the bow due to a spine match or mismatch. I think I told you about the compound archer who lowered his draw weight (just a half turn on each limb) only to have his arrows hit higher on the target. What happened when he lowered the draw weight, he created a better spine match (arrow to bow), which created a more efficient transfer of energy from bow to arrow which made up for the energy loss from the change in draw weight and more. These are the kinds of counterintuitive things that can happen.
If we had created a perfect spine match for your bow before (unlikely, such things take a great deal of time and effort), we no longer have that spine match. When you finish your draw weight changes, a complete re-tune is necessary because so many things have changed.
A general consequence of this situation (reality actually) is coaches need to expect counterintuitive responses to equipment changes. This is because of the reasons stated and because what you were taught were often oversimplified rules of thumb. For example, “weak arrows fly to the right, stiff arrows fly to the left.” and “If you lower the nocking point, you will raise the hit point of the arrow on the target.” (All of these are for right-handed archers.)
These equipment aphorisms were intended to get you down the road until you could think through such problems without needing them. From a perfectly tuned bow, if the nocking point is lowered a slight amount, the arrow will hit on the target lower than it did previously. But if you lower the nocking point enough, the rear of the arrow will start hitting the rest or arrow shelf and where those arrows land is anybodies guess.
All of those pithy little rules need to be taken with a grain of salt. And, they need to be thought through as they are all true … up to a point. By thinking them through they provide an entry to better understanding of archery equipment. If you do not, they become unreliable crutches. (I am speaking from experience here. If I had a nickel for every mistake I made, I could have retired earlier.)