I have a student I am teaching to make bowstrings (recurve, not compound) and he has a great many questions, quite a few of which I cannot answer. Those questions, though, got me speculating about a number of things, specifically nock fit and string diameters, in that arena. I would love somebody more knowledgeable about these things to comment on this!
One of the questions involved a discussion of the numbers of strands used to make a bowstring and hence its diameter. Please recognize that I am speculating more than quite a bit here.
Back in the old days (when string materials were natural and more easily broken, the pattern became: heavier drawing bows required thicker strings. the string materials: silk, linen, hemp, etc. were not particularly strong and so strings broke fairly easily. (I remember one war bow researcher stating that a string on one of those heavy bows might last six shots!) Thicker strings were made of more strands. Arrow nocks were generally just a slot cut in the back of the arrow (self nocks) and the fit wasn’t particularly a concern as the arrow was pinched between the fingers.
When manufactured nocks came into being, they were made in four different throat sizes to allow for this. Heavier bows required larger arrows (to get more stiffness, wooden arrows were just made thicker), and larger arrows required bigger nocks (think glue-on nocks at this time) and the larger nocks had bigger nock grooves because the bows had thicker strings. Smaller shafts used smaller nocks which had smaller grooves because those bows had smaller strings (in diameter/numbers of strands).
Fast forward to modern bowstring materials (Fastflight and beyond). These materials are much stronger and more resistant to wear/abrasion and breakage than the old materials but they were introduced into a system that was already created for the older materials. If you look at the breaking strengths, it only takes a few (roughly four) strands to carry the load for any recurve bow. The additional strands tend to be for nock fit. You have an example of this in aluminum arrows. Easton sells its line leader X7s with inserts for Super Nocks (large groove) if 20/64˝ diameter or larger and G Nocks for the smaller shafts (both large and small grooved G Nocks have smaller grooves than the one in a Super Nock). The assumption is still that the larger arrows are stiffer, going to be used on a heavier bow, with a larger diameter string.
I think modern materials have provided us with all kinds of options we aren’t taking advantage of. We can use strings of fewer strands on heavier bows, which would make them (marginally) faster. Some barebow archers use a triple center serving in lieu of using a tab (or a thicker one). We can use heavier strings on lighter bows but there are few advantages to this. We can put smaller nocks on bigger arrows. (Before Easton started making G Nock inserts for their larger arrows, I took ACC 3-71 nock inserts and epoxied them into Super Nock bushings to create the equivalent (so I wouldn’t have to change the bowstrings I was using to use those arrows with Super Nocks).
I think this is one of those examples of a system created under vastly different circumstances but which is maintained now to create some continuity with the past. I suspect that the number of strands recommendations by the various string material vendors is based more on nock fit than it is on having a strong enough string. Maybe secondary to nock fit is a thicker string distributes the pressure better on the draw fingers and therefore makes shooting more comfortable. I’d even place that in front of string strength as a guide to how many strands are needed to make up a string.
Another example of these phenomena is aluminum arrow sixes, e.g. 2114, in which the first two digits is the size of the shaft in 64ths of an inch. Sixty-fourths was chosen because the common wood arrow diameters were: 1/4˝, 3/8˝, 5/16˝, 11/32˝, etc. All of these are translatable into 64ths (being 16, 24, 20, and 22 sixty-fourths respectively). The benefit of any such connect (between aluminum and wood shafts) has dubious value in my mind, other than making aluminums more acceptable to those who were used to the sizes of wood shafts.
The situation regarding nocks and strings is that I think you can use any nock on any string for any reason and still be safe. I know Larry Wise preferred small groove G nocks over large grooved ones as they presently less of a target for another arrow’s point and a glance-off costing him points. Any reason can be good, the possibilities are very large, so feel free to experiment.