A bow string joins the two ends of the bow stave and launches the arrow. Desirable properties include light weight, strength, resistance to abrasion, and resistance to water. Mass has most effect at the center of the string; one gram of extra mass in the middle of the string slows the arrow about as much as 3.5 grams at the ends.
Most bow strings may be described as either simple, reverse-twisted, or looped.
Simple strings may be made of any fiber, twisted into a single cord. Such strings have been used in many parts of the world and are still effective and fairly quick to make. However, they tend to be weaker for their weight, and they may also come apart if not kept constantly under tension. They are normally secured to the bow by a knot at each end.
Reverse-twisted strings are traditional in Europe and North America for most natural materials. Linen and hemp fiber have been widely used. The form is also used for modern materials. A reverse-twisted string is made of separate bundles, each bundle individually twisted in one direction; the entire group of bundles is then twisted in the other direction. The result tends to be stronger for its weight than a simple or looped string, and holds together better than a simple string. A further advantage is that the full thickness of the string passes around the nocks, where wear is usually greatest. Additional threads may also be laid in at the nocking points for the bow stave and for the arrow, which are sites of likely wear. The string may be secured to the bow by a knot at each end, usually a timber hitch, also known as the bowyer's knot.
The traditional "Flemish" string has a laid-in loop at one end, which is easier than most knots to fit over the nock of the bow when stringing and unstringing. It is more trouble to make; the short length, towards one end, that will form the loop is reverse-twisted first. The ends of each bundle are then laid in to the main length of the bundles, which are reverse-twisted in turn. The Japanese bow string is made by reverse-twisting in different directions in the core and outer layers of the string. See Kyūdō.
Looped strings are made of one or more continuous loops of material. Modern strings are often made as a single continuous loop: this is then served to give the final form. Disadvantages include the lesser amount of fiber at the ends, where wear is most likely; this may be overcome by serving the string. In many parts of Asia, traditional strings have a single loop in the center, with the ends made of separate lengths tied on using a special knot. This design allows extra fiber to be used at the ends, where weight is less important and wear more likely.
Traditional materials include linen, hemp, other vegetable fibers, sinew, silk, and rawhide. Almost any fiber may be used in emergency. Natural fibers would be very unusual on a modern recurve bow or compound bow, but are still effective and still used on traditional wooden or composite bows. Sinew and hide strings may be seriously affected by water.
Widely-used modern materials are stronger for their weight than any natural material, and most are unaffected by water. They include:
Dacron B50 - (strength per strand = 22.5 kg. , stretch = 2.6%), a polyester material. Because of its durability and stretch, Dacron is commonly used on beginners' equipment, wooden bows, and older bows. The relatively high stretch causes less shock to the bow, which is an important consideration for wooden-handled recurves. Dacron strings are easy to maintain and can last several years.
Kevlar 7-11 - (strength per strand = 31.8 kg. , stretch = 0.8%), also known as Aramid, is a liquid crystal polymer material with a higher density and smaller diameter than Dacron, which results in a faster arrow speed (approximately 2 metres per second faster). There are two problems with this material. First, its limited stretch causes increased stress in the bow limbs. Secondly, a Kevlar bowstring may only last 1000 shots before breaking as it tends to fatigue due to bending at the nocking point. Failure tends to be sudden rather than gradual.
Fastflight - (strength per strand = 45.5 kg. , stretch = 1.0%), introduced in the 1990s, is a high modulus polyethylene material, also known as Spectra, which results in a "plastic" look and feel. It is very slippery, so the servings have to be wrapped very tightly to prevent sliding. Special serving material has been developed to help overcome this problem. Fastflight has largely displaced liquid crystal polymers like Kevlar for bowstrings as it is more durable and fails more gradually.
In 2006, all US-based Spectra production facilities were requisitioned by the US Government, since Spectra is also used in body armour. Production of Spectra-based Fastflight has thus ceased, but the manufacturer (Brownell) has launched several alternatives, such as the Dyneema-based Fastflight Plus.
Fastflight S4 - (strength per strand = 73 kg. , stretch = less than 1.0%) is made from a composite of 50% Fastflight and 50% Vectran making the strands thicker. Therefore approximately half the number of strands are required as for a Fastflight string. Vectran is a liquid crystal polymer similar to Kevlar. Mixing it with Fastflight avoids many of the durability problems associated with liquid crystal polymers.
Dyneema is a high modulus polyethylene material. It has very similar characteristics to Fastflight, though with a little more stretch. For this reason, many recurve shooters prefer Dyneema and find it more "forgiving" than Fastflight.
Serving a bow string refers to the use of an additional thread, commonly wrapped round the main string at the nocking points where abrasion is most likely, and also used on looped strings to keep the two sides of the loop together.