* * *
First, a separation must be made between modern and post-modern bowstring materials (my definitions). I say this because no one is using traditional materials (silk, hemp, linen, cotton, sinew, etc.) in target archery so we do not need to talk about them. (Yes, primitive archers are using such materials but not to seriously compete against others in ordinary target archery venues.) And, there is a big difference between the first-generation modern string materials and those that came after.
So, the first “modern” string material and the only one worth considering is Dacron® (still available for purchase, by the way). Dacron® was much stronger than most of the materials and cheaper and more regular, etc. But Dacron® was also “stretchy.” So, when a new Dacron® bowstring was placed on a bow, several procedures were used to remove some of the stretch. One common technique was to place the strung bow, back down, in one’s lap and press down on the limbs. After some of the stretch occurred, the string would be twisted up and shot. Unfortunately, Dacron® never stops stretching so one needs to keep twisting over the life of the string.
The first post-modern string material (in my scheme of things) was Spectra® marketed under the brand name “Fast Flight.” This string material was primarily ultra-high-molecular-weight polyethylene, a slightly modified version of the material used to make black plastic garbage bags. And, there was Kevlar® (poly-paraphenylene terephthalamide—Aramid fiber), and yes the stuff used to make bullet proof vests, but Kevlar strings had the nasty habit of breaking … at full draw so achieve only a temporary popularity.
Most newer bowstring materials are made from two such materials, their fibers twisted together to make “blended string materials.”
The point here is that once you get past Dacron®, the “stretchiness” is much, much less. (There are technical terms used, such as “creep” which have technical definitions (stretch that doesn’t recover, stretch that does recover, etc.) but my argument doesn’t require a foray into the itty-bitty details.) Because of the low level of stretch in these materials “string break in” is a simple procedure: string the bow and shoot it.
The “old” rule of thumb (Fast Flight came out in the late 1980’s, so we are talking about the last 30 years or so) was 100 shots and you were good to go. Basically 30 will probably do it. After that adjust your brace height or eccentric positions and you are good to go. Yes, you still need to check these things regularly because things do go wrong, but don’t expect large changes after break in, they just don’t happen with these “post-modern” materials.
And, if you keep your ears open you will hear old-timers talking about things like “sinking in” their new bow by shooting their heaviest arrows. These are traditional self-bow archers who are not talking about their strings so much as they are their bows. But keep listening, just be sure you associate what you hear with what is really being talked about and don’t just extrapolate that to you and your archer’s archery.
PS I still use Fast Flight string material for compound strings and cables and the occasional recurve or longbow strings. Good stuff. But (Warning!) do not use modern string materials on older recurves or longbows that do not have limb tip reinforcements. These materials are so unyielding that they can cut right into the bare wood of such bows. Most archers use dacron strings on those older bows as the springyness of the string lessens the shock on the limb tips. (No charge for that tip.)