Bowstring Tech-Tips

Bowstring Tech-Tips

Easton Tech-Tips #1-14

By George Tekmitchov, Easton Sr. Engineer & International Technical Advisor

The Bowstring is one of the three fundamental components of any modern recurve bow.  Limbs, riser and the string are all called out in the rulebook as the fundamental components of the bow.  Without its string, a bow is useless.

Your bowstring can have a profound impact on many aspects of shooting, from tuning and grouping, to noise and vibration. 

Material

Competitive archers have seen more progress in the past 50 years than in the past several hundred years when it comes to bowstring material technology.  Waxed linen strings dominated our sport in the west for more than 600 years, right up through the 1950’s.  Other materials found across the globe include horsehair, cotton, catgut, silk, and Chinese grass fiber. 

In the later 1950’s and early 1960’s the explosion of synthetics quickly displaced traditional linen and similar natural materials.  One of the first of these, still in some use today (mostly on beginner bows), was Dacron polyester.

In the 1970’s and early 80’s, in high level target archery, non-stretch Kevlar strings displaced Dacron.

These materials had many advantages over natural materials.  Proof against moisture, more forgiving of large temperature swings, and more durable (in the case of Dacron).

However some of these also had serious disadvantages, compared to modern Spectra or Dyneema based string materials.  Dacron will continually stretch and change over time, while Kevlar will always, eventually, unexpectedly break, often in the course of a competition. 

Spectra/Dyneema based materials changed all of that.  In fact, when the Hoyt Archery Company introduced Allied Signal Spectra fiber string material to the archery industry in the mid 1980’s, it proved to be one of the biggest single performance leaps in recurve archery in the past 30 years.

Spectra and Dyneema are both, technically, Ultra High Modulus Polyethylene (UHMPE) fiber.  There are some small differences between the two materials, but they are, for the most part, physically similar.  Practically impervious to solvents, normal environmental conditions and moisture, the weak point with these materials comes into play only at greatly elevated temperatures well beyond what one can expect to encounter in the course of a competition.  Under tension and in high temperatures these materials will exhibit “creep” and stretch under high heat and high tension.

Blends

In the mid 1990’s the “creep” characteristic of some UHMPE strings and compound cables led to the development of blended fibers using UHMPE and Vectran, a liquid-crystal polymer related to Kevlar.  The blend helped arrest creep, until the gradual failure of the Vectran component in the string or cable would manifest itself.  With good maintenance and improved string waxes, these issues can be managed.  However, many shooters feel these materials are too harsh in terms of shot reaction.  Accomplished shooters who choose to use these materials typically run very high string twist counts to compensate for the “abrupt” feel from these materials.  As the blending processes for these materials has improved, however, a somewhat larger number of shooters have gravitated to these materials.  For longest string life, silicone based synthetic waxes are recommended for these materials.

Construction

Even the very finest string materials can be wasted, if certain elements of string construction are overlooked.   Well-constructed bowstrings feature a number of key features which enhance performance, durability, and reduce variability over time.  One of the most important of these is the consistency of strand tension during construction.  Incorrect or uneven individual strand tension can cause the string to change in unpredictable ways, especially when going from cool to warm conditions.  Incorrectly tensioned strands can even cause an imbalance which can cause the string to load at different rates from shot to shot, resulting in grouping difficulty.  One trick to help ensure your string is going to have good strand tension distribution (assuming only minor differences) is to take all twists out of the string, string your bow (yes, the brace height will be low) and let it sit overnight.  You can even take a few shots with the untwisted string with an arrow on a close target.  This will force the tighter strands to stretch a little and allow the looser strands to take up their share of the load.  When re-twisting to achieve correct brace height, be sure to twist in the direction that tightens the center serving.   You can determine this immediately simply by grasping each end of the served area between thumb and forefinger, and twisting.  One direction will be harder than the other.  Twist into this direction.

Twist rate (twists per unit length) can also have a significant impact on string performance.   Too few twists often makes for a noisier, harder to tune string, because any slight strand tension variability is amplified, and the string has less “give” to help with reducing noise and vibration after the shot. 

On the other hand, too many twists may reduce performance and may lead to a string which is more prone to stretching with use.  With that said, strings can work well with a fairly wide range of twists, for example for a 68” string, anywhere from 20-60 twists will produce acceptable results as long as the bow’s brace height (string height) and correct.  Generally more twists will slow the bow slightly and produce much less noise, while fewer twists will add a tiny bit of speed, but potentially more noise.

Serving materials are another important and integral part of the bowstring.  However, servings have two very different functions on the bowstring.  The end-loop servings must be abrasion resistant, but not abrasive themselves, so as to avoid damage to limb tips and underlying strands. 

End-loop serving should be low in mass, and must tightly grip the underlying string strands to prevent separations, loosening or fraying. 

On the other hand, the center serving has multiple functions with potentially different requirements than those of the end loop materials.  Center servings must provide for a smooth and consistent release from the shooting tab surface, and also resist separation and loosening to prevent the nocking point from changing.  They must also often be called upon to resist wear from brushing the armguard after the shot.  Finally, they must keep their diameter over a long period of time, so as to prevent a change in nock fit.

Certainly, there are a number of materials which function well as recurve serving, with the original Angel braided material (and the similar BCY Halo) still excellent choices for center servings, and material blends of Spectra/Dyneema and polyester such as Brownell Diamondback are popular end loop material choices.  Interestingly, however, extensive testing shows that Angel Majesty serving material is actually the best material for both of these applications, due to a unique combination of smoothness and tenacious locking ability on the underlying string strands.

Strand effect on tuning

It’s a simple matter to say that strings with “more strands are slower and less strands are faster”, but the fact is when we change the number of strands in a string, we change more than one variable.  For instance, a smaller diameter (less strand count) string also generally has a looser nock fit (if not compensated for).  This can cause big changes in tuning, even beyond the effect of the strand count change.  Obviously a strand count increase can cause the opposite effect.  Experienced shooters are careful to account for this by using larger or smaller size throat nocks, or by using larger or smaller serving diameter, to achieve a predictable and correct nock tension.  Generally, nock tension should be set to allow an arrow to hang off a strung bow with the string hung parallel with the ground, but not so tight that the arrow can’t be dislodged with a sharp tap on the string.

 

Nocking points, kisser buttons

It’s not always obvious, but a small change in string mass weight at the center can have a lot more impact than a larger change distributed over the entire string.  In fact, adding 10 grains to the center of the string has about 5 times the effect of adding 10 grains to the overall string.  So, we need to choose accessories such as nocking points and kisser buttons with a bit of care- and, at least be sure to have these in place as you progress with tuning, since adding them afterward can cause a substantial change.

Maintenance

“Wax is to an archer what tar is to a sailor; use it often, and always have two strings to your bow.”

Excerpt From: Saxton Pope, “Hunting with the Bow and Arrow”, 1922.

Modern strings are well served by at least some maintenance.  In some ways, especially for more traditional bows, it’s hard to beat one of my favorites, natural beeswax.  But for more modern string materials, particularly the Vectran blended materials, string lifespans can be dramatically improved by using more modern synthetic waxes.  You can even take it to the extreme of a total string maintenance kit, consisting of cleaners, restoratives and waxes.  A number of manufacturers offer these total maintenance kits, including Easton.  Visit the Easton YouTube channel to see how to use this (or any) string kit and pick up a few tricks of the trade.

(Photo:  Easton string kit)  Caption:  The Easton “Dr. Doug’s Complete Bowstring Maintenance Kit” features a cleaner, revitalizer and wax to restore and protect strings and compound cables

Conclusion

If you have come into archery in the past 20 years, you may not remember the “bad old days” of Kevlar and Dacron.  Very few archery-related items have made as much progress as string materials in the modern era.  But, even as good as modern materials are, accidents can happen.  And, any bow is useless without a good bowstring.  So, most top competitors always maintain at least two well-used strings, ready to go and interchangeable on any given bow.  As a competitive archer, this is really very inexpensive insurance for your competition performance.