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There are at least two common types of bowstring used in traditional archery: Flemish strings and endless loop strings. Both are described in detail by Baker (2000c). Baker's article is a very good read for anyone wanting to understand the basics of bowstring making, although it does not describe making of endless loop strings in much detail. Although Flemish strings were (and are) used extensively in selfbows, they are not well suited for crossbows with heavy draw weight and countless number of individual threads. Endless loop strings, on the other hand, can be easily made to any strength. Even so the sheer number of threads can be daunting and the string making takes quite a lot of time. There are also some other challenges described in the articles below.
In most cases you can get away with a simple endless loop strings. There are, however, a few reasons why you might want to consider making a reinforced endless loop string:
- If the string has to be as light as possible. Reinforcing the loops helps as they are somewhat weaker than the main skein (Baker 2000c: 219). I've noticed this in my own tests. If the loops are not reinforced and a too light bolt is shot, the string always breaks at one or both loops. If the loops are reinforced significantly (e.g. by 50%) the string tends to break at the middle. So, contrary to the popular belief - which I once shared - the loops are not half the strength of the main skein, even though this seems counter-intuitive.
- Some threads materials weaken considerably if they have to bend arounds the nocks in a sharp angle. In this case adding loop reinforcements increases bowstring strength considerably without adding a lot of mass. The alternative is to make the string much stronger overall, but this results in a much heavier string and hence lower bolt velocity. Apparently most synthetic fibers (e.g. Dacron, Dynema, FastFlite) don't have this problem, whereas some natural fibers are problematic (e.g. linen). Note that this piece of information is from some Internet forum and need to be taken with a grain of salt.
Making of various bowstring types is described in below articles. Note that you can make strings for selfbows, crossbows or two-armed siege engines using exactly the same procedures:
With very weak bows or crossbows it may be possible to make strings from thin rope (e.g. polyester), but the performance of these strings is suboptimal for any given string weight. They also tend to stretch a lot in use, although this depends on the rope material and type of knot used to form the loops.
As a string is made of several threads, the choice of thread makes a big difference in the performance of the bowstring. You should use thread with highest possible breaking strength (per weight) and least amount of stretch (Baker 2000d: 73-74). The weight of the string matters for a simple reason: it's only purpose is to transfer energy from the bow to the bolt (or the arrow) - otherwise it's just dead weight. The heavier the string, the less energy is left for the actual missile.
There are many different kinds of natural fibers that can be used for making bowstrings:
Baker describes the qualities and strength of many natural fibers extensively in one of his articles (2000c: 187-258). If you're fine with using synthetic threads, there are several good alternatives, listed here in rough order from weakest to strongest:
Dacron seems to be roughly the same strength as highest quality linen, whereas Dynema is (according to my own tests) at least 3 times as strong as Dacron for it's weight. I noticed this when I had broken two carefully made, loop reinforced 22 gram Dacron strings in a row when chronographing light 27 gram bolts. Normally I shot bolts in 40-50 gram range. With the 27 gram bolts both strings broke after two or three shots. I could shoot the same bolts with a simple 8 gram Dynema endless loop string without any problems. According to some sources Dynema is 40% stronger than kevlar, although I have not done any testing myself. In any case the best synthetic fibers are much stronger per mass than the best (available) natural fibers.
When making strings it's always a good idea to make thorough description of each string you make. The key things you should probably write down are
- Thread material (e.g. Dacron B-50, Linretors #12 linen thread)
- Thread or loop count
- Reinforcement loop count (if any)
- String treatment (e.g. wax type)
- Serving (e.g. thick linen thread, polyester thread, leather)
- String weight
- Any notes (e.g. "two loops were left a little slack")
- How did the string end it's life (e.g. "broke from the middle when shooting 27 gram bolts")
Keeping accurate notes allows you to avoid making the same mistakes twice and notice how various details affect string performance and durability. This allows you to tune your strings perfectly to your own crossbow and bolts.
If you're making strings from new materials or experimenting with especially light bolts you should shoot the first bolts with a "remote control". An easy way to do this is to attach the crossbow to a vise and pull the trigger with a strong cord. This safety precaution is especially important if you've reinforced the loops of the string as then then the string is most prone to break at the middle. This can be very dangerous for one's eyes, as I learned the hard way. The story goes like this...
I was using my third reinforced loop string when I was chronographing my crossbow after having made enhancements to it's steel prod. The loop reinforcements were pretty strong, the loops being about 75% of the thickness of the main skein. I had shot at least a hundred shots with the string using bolts weighing between 40 to 67 grams and the string had held up exceptionally well. I had chronographed all of my heavier bolts when I finally started chronographing my lightest (27 gram) bolt. I could loose three shots without any problem. The fourth shot, however, almost punched a hole in my right eye. Something flew off from the string backwards, struck the top of my eyeglasses, bend them a little and punched deep hole just below my eyebrow. Lots of blood gushed out, as you can see from this picture. Fortunately my eye was not damaged, but it was a close call.
The reason for accident was not immediately obvious, even though it was clear that the string had partially broken at the exact middle. After thorough study of the string it became obvious that the broken Dacron strands had somehow managed to tear away a 2 centimeter wide section of the middle linen serving and send it toward my eye at high speed. I'm not sure if this was an isolated, very rare incident, or something that happens often. Just in case I strongly suggest the following precautions:
- Use a wide bolt holder - it prevents stuff from the middle of the string from flying directly backwards.
- Wear some form of eye protection - at the very least until you've found the correct string/loop thickness.
Simple (non-reinforced) endless loop strings tend to break at the loops rather than the middle. This is potentially very dangerous if one loop breaks instantly after the string is released and the string slashes the shooter to the face like whip. However, in all cases when a simple endless loop string has broken on me the breakage has occured simultaneously at both loops. Also, the string should in most cases break after the arrow has left the crossbow and not before. This reduces the risk of serious injury a lot.