Bowstrings part II: modern bowstring materials

In part 2 of his series on bowstrings, Cleve Cheney looks at the various materials used for making modern strings.

Modern synthetic bowstring materials are far stronger, weight for weight, than any natural material and have been developed to minimise creep and stretch and to maximise durability and strength. There are four basic materials: polyester, liquid- crystal polymers (LCPs), high-modulus polyethylene (HMPE), and composite fibres.

Polyester materials
Examples of polyester materials are Dacron and Fortisan. Early experiments with a synthetic polyester bow material called Fortisan were conducted in the 1940s and 50s. Attempts were also made to manufacture bowstrings from nylon, but this proved disastrous as the amount of stretch in nylon made the bows almost like catapults.

Dacron, introduced in the late 1950’s, was the first synthetic material to be successfully used for making bowstrings and immediately set the standard for the industry as its performance far surpassed anything else available at the time. It is still widely used to this day on traditional bows (recurves and longbows) and on older compounds because the slight amount of stretch reduces the shock on the bow limbs. Using modern non-stretch bowstrings can cause breakage in wooden bows as the shock imparted to the limbs is often too much for the limb tips. Dacron strings are easy to maintain and can last for years. However, Dacron does creep over time, and this will cause changes in the brace height on traditional bows and in the wheel/cam timing of older compound bows. The advantages of Dacron are that it is safe on all bows, is relatively inexpensive, and is available in a wide range of colours. Its disadvantages are its high creep and stretch properties (stretch is 2,6 per cent), variations in performance with changes in humidity and temperature, and its relatively low strength when compared to other modern synthetic bowstring materials.

Dacrons’s breaking strain is 22,5 kilogrammes (50 pounds) per strand. It has a relatively low tensile strength: B50 – 22,5 kilogrammes (50 pounds) per strand, and B75 – 25 kilogrammes (55 pounds) per strand. Diameter is 0,018 inch.

Liquid-crystal polymers (LCPs)
There are two basic types of LCPs: aramids and Vectran. Examples of aramids are Twaron, Technora and Kevlar. These LCPs were important in archery in their day – especially Kevlar. They continue to have important uses outside of archery, but have largely been superseded on bows by newer and more reliable fibres.

There might still be spools of Kevlar lying around in some bowhunter’s cupboards, but they should not be used as they have a relatively short lifespan (even when new) and have a tendency to break without warning. Kevlar was especially vulnerable to bending stresses at the nocking point. Using old Kevlar bowstring material can be dangerous – especially when it has been exposed to sunlight. Kevlar 7-11 was also known as Aramid and is a liquid-crystal polymer with a higher density and a smaller diameter than Dacron. Its limited stretch made it unsuitable for traditional bows with wooden limbs, but when used on modern compounds resulted in an arrow velocity approximately three feet per second faster than could be achieved with Dacron. Thus Kevlar’s advantages were that it was lighter than Dacron and gave increased arrow velocity. Its disadvantages were its low creep and stretch properties, its short life, its tendency to break suddenly and without warning, and its price (higher than Dacron). It is no longer used as bowstring material. Kevlar’s breaking strain is 31,8 kilogrammes (70 pounds) per strand, and its stretch is 0,8 per cent.

Vectran, like LCP material, was prone to sudden and unexpected failure and should not be used on its own in modern compound bows. However, Vectran is an important component in the current bowstring family of composite fibres. It is heavy and if used on its own would result in reduced arrow velocity. It has the lowest creep of any synthetic fibre. Under archery conditions,
its stretch is negligible.

High-modulus polyethylene Examples of HMPE materials are Fastflight, Fastflight Plus, Dyneema, Spectra, Dynaflight 97, D75, TS Plus and BCY 8125.

HMPEs are currently the most important family of bowstring materials. They are used on high-performance recurves and on most compound bows – either alone or in combination with other fibres. The two basic fibres used in this family are Spectra and Dyneema, manufactured under licence with slightly different finishes to produce a number of different brands of string material.

What is the difference between Dyneema and Spectra? The answer is: not much. Both products are HMPE (high-modulus polyethlene) materials developed originally by DSM in Europe and licensed to Allied Chemicals for production in the United States.

The slight difference between Spectra and Dyneema really does not affect the performance of an archery bowstring, as they are in the number of filaments and the actual makeup of the yarn.

BCY introduced Dyneema to the archery market in 1995 with its Dynaflight bowstring material, which was made from SK65 Dyneema. In 1997, DSM began
manufacturing SK75, a higherstrength Dyneema with less creep, and BCY introduced this with its Dynaflight 97 bowstring material in 1997.

The main benefits of Spectra and Dyneema over other fibres previously used in archery bowstrings are their extremely high strength and durability. The high strength results in very low creep.

Fastflight was introduced in the 1990s and was manufactured from Spectra fibres. It had a “plastic” look and feel about it and was very slippery, so serving had to be wrapped very tightly to prevent sliding. Special serving material was developed to overcome this problem. Fastflight larg-ely displaced LCPs such as Kevlar, as it was more durable and failed more gradually. Spectra fibre is also used to produce body armour.

All Spectra production facilities were requisitioned by the US government in 2006 and production of Spectra-based Fastflight has thus ceased. The manufacturer, Brownell, has however produced several alternatives such as the Dyneemabased Fastflight Plus.

Examples of Dyneema-based strings are Fastflight Plus and 8125. Dyneema is a high-modulus polyethylene material with characteristics similar to Fastflight but with a little more stretch, which also makes it suitable for high-performance recurve bows.

Some figures on a number of HMPE materials: the breaking strain of Fastflight is 43 to 45 kilogrammes (95 to 100 pounds) per strand. Its stretch is 1,0 per cent. The breaking strain of Dyna Flight/Dyna Flight 97 is 44 to 54 kilogrammes (97 to 120 pounds) per strand. Stretch is slightly more than 1,0%. The breaking strain of Angel Dyneema is 50 kilogrammes (110 pounds) per strand. TS Plus is made of 100% HMPE material. Diameter is 0,014 inches. D75 is also made of 100% HMPE material. Diameter is 0,015 inches. D75 thin is likewise made of 100% HMPE material, but its diameter is 0,011 inches. BCY 8125 is constructed of 92% SK75 Dyneema and 8% SK75.

Composite fibres
Examples of composite fibres are BCY 450 Premium, BCY 450 Plus, BCY 452, Brownells S4, and Ultracam.

Fastflight S4 is a composite string made of 50% Fastflight and 50% Vectran. The addition of Vectran makes the strands thicker, requiring only half the number of strands to make a bowstring as compared to Fastflight. Mixing Vectran with Fastflight overcomes many of the problems associated with liquid-crystal polymers. The breaking strain of Fastflight S4 is 73 kilogrammes (160 pounds) per strand.

Stretch is less than 1.0% Some other composite fibres: Xcel is constructed of 33% Vectran and 67% SK75 Dyneema. Ultracam is constructed of most compound bows – either alone or in combination with other fibres. The two basic fibres used in this family are Spectra and Dyneema, manufactured under licence with slightly different finishes to produce a number of different brands of string material.
What is the difference between Dyneema and Spectra? The answer is: not much. Both products are HMPE (high-modulus polyethlene) materials developed originally by DSM in Europe and licensed to Allied Chemicals for production in the United States.

The slight difference between Spectra and Dyneema really does not affect the performance of an archery bowstring, as they are in the number of filaments and the actual makeup of the yarn.
BCY introduced Dyneema to the archery market in 1995 with its Dynaflight bowstring material,
which was made from SK65 Dyneema. In 1997, DSM began manufacturing SK75, a higherstrength
Dyneema with less creep, and BCY introduced this with its Dynaflight 97 bowstring material in 1997.
The main benefits of Spectra and Dyneema over other fibres previously used in archery bowstrings are their extremely high strength and durability. The high strength results in very low creep.

Fastflight was introduced in the 1990s and was manufactured from Spectra fibres. It had a “plastic”
look and feel about it and was very slippery, so serving had to be wrapped very tightly to prevent sliding. Special serving material was developed to overcome this problem. Fastflight larg-ely displaced
LCPs such as Kevlar, as it was more durable and failed more gradually. Spectra fibre is also used to produce body armour.

All Spectra production facilities were requisitioned by the US government in 2006 and production of Spectra-based Fastflight has thus ceased. The manufacturer, Brownell, has however produced several alternatives such as the Dyneemabased Fastflight Plus.

Examples of Dyneema-based strings are Fastflight Plus and 8125. Dyneema is a high-modulus polyethylene material with characteristics similar to Fastflight but with a little more stretch, which also makes it suitable for high-performance recurve bows.

Some figures on a number of HMPE materials: the breaking strain of Fastflight is 43 to 45 kilogrammes (95 to 100 pounds) per strand. Its stretch is 1,0 per cent. The breaking strain of Dyna Flight/Dyna Flight 97 is 44 to 54 kilogrammes (97 to 120 pounds) per strand. Stretch is slightly more than 1,0%. The breaking strain of Angel Dyneema is 50 kilogrammes (110 pounds) per strand. TS Plus is made of 100% HMPE material. Diameter is 0,014 inches. D75 is also made of 100% HMPE material. Diameter is 0,015 inches. D75 thin is likewise made of 100% HMPE material, but its diameter is 0,011 inches. BCY 8125 is constructed of 92% SK75 Dyneema and 8% SK75.

Composite fibres
Examples of composite fibres are BCY 450 Premium, BCY 450 Plus, BCY 452, Brownells S4, and Ultracam. Fastflight S4 is a composite string made of 50% Fastflight and 50% Vectran. The addition of Vectran makes the strands thicker, requiring only half the number of strands to make a bowstring
as compared to Fastflight. Mixing Vectran with Fastflight overcomes many of the problems associated with liquid-crystal polymers. The breaking strain of Fastflight S4 is 73 kilogrammes (160 pounds) per strand. Stretch is less than 1.0% Some other composite fibres: Xcel is constructed of 33% Vectran and 67% SK75 Dyneema. Ultracam is constructed of 56% Vectran and 44% HMPE. Diameter is 0,014 inches. BCY 450 is constructed of 67% Dyneema and 33% Vectran. BCY 452X
is constructed of 67% SK75 Dyneema and 33% Vectran.

Composite fibres are the definitive synthetic fibres for modern compound bows. They consist of a combination of Vectran (for creep resistance) and HMPE (for strength and durability).

Their advantages are negligible creep, high strength and durability, and increased arrow velocity. Their disadvantages are that they are expensive, and render slightly lower arrow speed than the best HMPE materials. Their breaking strains are all in excess of 68 kilogrammes (150 pounds) per strand.

All the foregoing might be somewhat confusing but may be better understood by referring to Table 1.
Serving materials Serving a bow refers to the use of an additional thread, commonly wrapped around the main string at the nocking and mounting points where abrasion is most likely and also on looped
strings to keep the loops together. The commonest serving materials today are nylon and HMPE.

Until quite recently nylon was commonly used for all serving and it still has wide application. Nylon serving has one excellent quality – it grips the string well and is less prone to slip or to open up than modern fibres. The one disadvantage is that it is not as durable as modern materials and must be replaced more often.

Soft twist nylon is the cheapest and is adequate for any endserving that is not subject to heavy wear, such as on recurves and on compound bows where the bowstring does not run over cams. It is not recommended for centre serving as it wears through too quickly. Braided nylon is easier to work with and thinner than standard soft twist, and it fits better into the grooves of cams.

Monofilament nylon is not suitable for end servings as it slips too much, but it is an ideal centre serving material. It is smooth, providing a clean, fast release for finger shooters. It also forms a screw thread that allows for fine nocking point adjustment, and it is available in many thicknesses to provide the best nock fit. The one disadvantage with monofilament is that it becomes brittle and weakens with age and should be replaced fairly regularly.

Most HMPE brands are available as unwaxed serving threads as well as bowstring threads. Most are twisted and are intended primarily for end servings as they are hard-wearing and durable. Their intrinsic wear resistance makes them especially suitable where the serving must pass over cams. Some are braided and intended for use as centre serving material. Angel Dyneema and HALO (braided Spectra) have a good reputation as smooth and durable centre serving. Both are however very
expensive. The main disadvantage of HMPE serving materials is that they tend to slip and open up on the string. A general rule is that the thinner the serving thread, the better it will grip. Some threads are coated with a light, adhesive coating to reduce slipping problems.

An example of some of the serving material available is shown in Table 2. Table 3 shows the recommended serving material for different bowstrings.

In the next article we will look at how to design and make an endless-loop bowstring.

Updated: Thursday, December 10, 2009 11:19 AM