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I have found 3 applications for the word “stacking” to be used. In my answer above, a bow being drawn in it’s designed draw length range will commonly increase draw weight by 2 to 3 pounds per inch. So it builds up to it’s rated draw weight at a given draw length.
So the first ( and incorrect) use of “stacking” is usually by converted compound shooters who are not used to the weight build up as opposed to the let off of a compound. So they refer to the natural weight build up as stacking.
So there are really two causes of true stacking. The first is caused by the string angle to the limb tip. Bows vary but as the string angle approaches 80-90 degrees you start to pull on the limbs (try to stretch them length wise) and loose the leverage required to bend them. This causes a very fast jump in draw weight from that 2-3 pounds per inch to something noticeably much higher like 5 or more pounds per inch. You are loosing leverage for bending and wasting effort on trying to stretch the unstretchable.
Recurves (due to design) can maintain a much lower angle for a longer time than a longbow. That’s why most straight ended bows are generally longer than a recurve to serve a longer draw length.
The second true stacking example is when the actual elastic limits of the limb are reached. There is a physical point where the limb material simply cannot bend any more and material failure is the next and only option. At this point draw weight increases really fast per unit of draw and if the draw is continued the limb will fail.
Here is a simple experiment as an example. Take a piece of common wood lath – I think they are about 36 inches long (but doesn’t matter). Grab each end of the lath and slowly bend it. For a while, that bend will be smooth with just simple bending resistance. But you will get to a point where all of a sudden the bending feels like you hit a wall (it just doesn’t want to bend any more) — NOW bend it more — what happens? BOOM??!!
Long answer to a short question. To determine which it is you need to look at that string angle at full draw and evaluate it. Over 80 degrees means you are getting too close to the designed draw limits of the bow. That’s the first check.
The second is simply being practical and asking yourself if you are asking too short of a limb to bend too far and if you are reaching the material limits. This is why most bowyers have recommended draw ranges for given length bows. String angle and bending limb length.