Let's look at a brief rundown of bow history; Someone came up with the basic bow and arrow system, using a wooden bow with a string attached to shoot straight sticks at higher-than-arm throwing speeds. The bow basically stayed the same until Hollis Wilbur Allen, a person with a gift and desire to improve machinery of various kinds, decided that the bow and arrow system could be improved. He was an inventor and a bow hunter and was thinking about ways to improve arrow speed since he built his first bow in the 1920's. In 1965, he studied a borrowed high-school physics book and confirmed his own suspicions about stored energy in the bow and arrow system and the physics information gave him ideas on how to get more of that stored energy dedicated to providing greater arrow speed. In early 1966, he built the first compound bow in two days. He used a pulley system to flex the bow limbs when the string is drawn and, by drilling the axle holes for the pulley wheels off-center, he was able to decrease the holding force at full draw. The shape of the string groove on the wheel or cam affects the amount of this decrease. The amount of draw weight would increase quickly as the bow was drawn and then drop off (let-off) prior to full draw. This can be seen graphically in the draw-force curve referred to in reviews of compound bows in hunting and archery magazines. The draw-force curve is a line on a graph which represents the amount of draw-force in pounds that the archer applies to the string as the bow string is drawn back from its resting point to full draw. The term "let-off" is how we refer to the amount of draw-force or pulling weight that decreases when we reach full draw on a compound bow, in other words at full draw, a portion of the peak draw weight "lets-off" or decreases so the archer is holding a small percentage of that total peak draw weight.

Let-off is a very common term among the advertised specifications for compound bows. Bow advertisements typically include AMO speed, IBO speed, let-off and some details about the built-ins that the bows possess. The most often quoted let-offs are 65% and 80%. This means that, at full draw when the full amount of let-off is realized, the archer will be holding either 35% (65% let-off) or 20% (80% let-off) of the total draw weight, also known as peak weight. Example: If the bow has a peak draw weight of 70 pounds and a let-off of 65%, at full draw the archer will be holding 35% of the 70 pounds or 24.5 pounds. With the same peak weight and a let-off of 80%, at full draw the archer will be holding 20% of the 70 pounds or 14 pounds. You can calculate the full-draw holding weight(HW) of your bow by multiplying your bow's peak draw weight (PW) setting by 100% minus your bows let-off (LO) percentage (PW - (100%-LO%)) = HW.

I became curious about let-off and started some research on the subject. As part of the research, I surveyed archers on the Archery Talk website about their preferences for let-off. I was not surprised to find that 65% and 80% were the most common two percentage preferences. This was not surprising because most of the compound bow manufacturers offer these two percentages in the majority of their bows. Let's face it, many bowhunters who purchase compound bows don't do alot of tweaking of their bows or get that involved in the mechanics of the bows and work to maximize the bows performance. Most bowhunters purchase a bow, do what they need to get it tuned and leave well-enough alone. If the bow "ain't broke" they are not looking to "fix it". They walk into a shop, pick up a bow and shoot it and, if it feels right, purchase it, get it equipped with their preferred accessories and don't concern themselves on the bow's let-off spec. I'd put myself in this group but I'll admit that I will sometimes dig deeper into the mechanicals in order to better understand the physics of the bow and arrow system. Some of the other responses during the survey came up as having different let-off preferences than the two "standards" of 65 or 85%. Some of the percentages went as high as 90+%!

So what is the reason that archers have a let-off preference? Some like a higher holding weight (lower LO %) because there is more tension in the archer's muscles and the bow. These archers like the feel of this higher tension because they feel a harder "back-wall" to their draw and it helps them with shooting with proper back-tension. Other archers like a lower holding weight because they feel they are more relaxed at full draw and can concentrate more on aiming and keeping pins steady on the target. Each archer is individual and their preferences in equipment are as individual as the archers themselves. Some like high percentage, some others prefer low. Some of the surveyed archers had multiple bows, each set-up for a different let-off percentage, each for a different purpose (example: one bow at X percentage for hunting and another bow at X percentage for 3-D and target shooting).

One note concerning bow let-off's and their respective draw force curves: Draw force curves, as noted above, are graphed out by comparing the amount of draw-force in pounds to the number of inches the bowstring is drawn from relaxed to full draw. At the relaxed point, there is 0 draw force applied on the bow string. At 1" of draw, the draw-force increases and continues to increase as the string is drawn back until the point comes in the rotation of the cam where the string-groove changes position and the amount of draw force begins to decrease or let-off. Archery manufacturers will publish the let-off specs based on the percentage of draw force let-off at a specific draw length. IF you draw to a full-draw length other than the manufacturers specified length the actual let-off percentage may change because you are at a different point in the draw force curve.

Changing the let-off on your bow may alter your arrow flight. As the let-off is increased, the resulting draw-force curve indicates that their will be a higher increase in force applied to the arrow in a shorter period of time. Whenever making any adjustment to your bow and arrow system, be sure and check the bows tune for any changes that may result.

The bottom line as always in archery is shoot what feels best to you. Comfort and confidence are key factors in shooting accurately. Best of luck to you, whether shooting paper, 3-D targets or hunting.