STRINGS

Tim Baker

Although we struggle with words to express it, each of us senses the magic in a wooden bow. And we all share the ever-fresh thrill of the flight of an arrow. But archery has a third area of wonder, the effect of which is not often felt in modern times, originating as it generally does far out of sight in distant areas of manufacture. This third wonder is the simple string.

This may be difficult to accept until you see a string suddenly alchemized into existence, appearing from the magician's gesticulating fingers, rising up from the seemingly useless linear dust of animal or vegetable fibers.

During any bowmaking class there are moments when eyes suddenly brighten with insight, but never as brightly as when grasping the concept of string.

"Useless" fibers are taken in hand. The students are told a bow string is being made, but they don't quite see how this can happen. Not a real string. Not from such refuse!

The twist-and-reverse action is visible, but the results are concealed for a time by the hands, then suddenly revealed to the skeptics. They are startled by abrupt comprehension; by such perfection of form rising from such rude makings; by the elemental mechanics of the process, yet the far-reaching implications of the results.

There are exclamations of surprise, sublime smiles, and suddenly-eager fingers. There is magic in the air.

THE PRINCIPLES OF STRING

If loose, parallel fibers are twisted together into a single-ply cylinder or cord, the resulting internal friction prevents the fibers from slipping past each other when strained. In addition, the cord tends to elongate when pulled, causing its diameter to contract. This applies even more friction against internal fibers, somewhat like a Chinese finger tube toy. As a result, the breaking strength of the twisted cord can approach the combined breaking strengths of its constituent fibers.

A crude bowstring can be made from such a simple, single-ply cord. Its main body will hold together surprisingly well, but at the nocks it quickly frays and weakens. If single-ply cordage is not kept actively and permanently twisted, it tends to untwist into its original useless disconnected fibers.

To make durable, practical cordage early man had to invent a way to prevent

The miracle of reverse twisting transforms apparently useless chaff into a cord having both strength and beauty.

Such untwisting. And to our good fortune this very important development would have occurred not just accidently, but inevitably.

Twist up a cylinder of fibers. Continue twisting the cord until it begins to kink. Then simply let go of both ends.

The cord will instantly wrap around itself, creating a two-ply cord now having neutral twist. Some of the original twist will be used up in the process but enough will remain to supply necessary friction within each ply. A stable, durable, practical cord results.

Once this inevitably occurring principle of reverse-twist cordage revealed itself little refinement would have been needed to produce uniform, durable cordage of any length.

Useable cordage can be made simply by twisting loose fibers into a single-ply cylinder.

Stable, durable, reverse-twist cordage in the act of inventing itself.

To understand how cordage is made in practice it will be helpful to look closely at what happened when the primitive single-ply cord untwisted spontaneously around itself.

Each ply is twisted tightly clockwise, then allowed to untwist. If both plies are held side-by-side while untwisting they spiral uniformly around each other. If each separate ply was to continue unwinding, its constituent fibers would soon feel no friction, and there would soon be no cordage. The single plies do not continue unwinding, however, because the double-thick cord they are creating resists being twisted, and having greater diameter, it has greater leverage with which to resist. By the time a stable balancing of leverage is reached, the primary plies have used up only about 25% of their original twist. Sufficient friction remains in each ply to yield full-strength cordage.

Without this fortunate balancing of twists, practical rope, string, and thread would not have been available for early mans' clothing, shelters, containers, weapons, snares and nets.

It is unlikely that we have benefited more from, or taken anything more for granted, than this ancient, vital invention. Cordage is so useful that, if invented early enough, it no doubt was interactive in the selective processes that produced our very natures. It is possible that we are what we are partly as a result of cordage.

Perhaps the earliest evidence of cordage is seen on "The Venus of Lespugue," a 27,000 year-old small ivory statue of a European woman. She is wearing a loin cloth, its surface bearing the characteristic pattern used today to depict reverse-twist cordage. Perforated beads have been found in 35,000 year-old Cro-Magnon sites; their extremely small hole diameters suggests the possible use of cordage.

But there is reason to believe cordage is possibly far older; Prehistorian Dr. Errett Callahan was recently asked to examine a collection of 300,000 year-old stone artifacts from Africa: Acheulean hand axes housed at the Lowie Museum here in Berkeley. These are hand-sized, multi-purpose, bifaced stone implements made by Archaic Homo-Sapiens. Errett examined several axes, turning them at different angles to the light, running a thumb along first this flake scar then that. He was obviously in deep thought. Even an observer with

Twist up a length of single-ply cord from loose fibers. Twist until the cord attempts to kink. Fold the twisted cord into side-by-side plies. Note both plies are twisted in the same direction but both are struggling to untwist around each other in the opposite direction.

Slowly, as one, let the two plies begin to untwist together in your hand. At this point they are absolutely hungry to become string, wrapping around each other with certainty and eagerness.

Only apprentice-level knapping skills could see that much forethought and preparation went into the knapping of these tools. Later, I asked him what he had been thinking. And he said he had been looking at the work of a brother knapper.

Making an Acheulean hand axe requires more knowledge, planning and effort than making cordage. Cordage would have been as inevitable. And cordage would have been as useful. Maybe some day we will find reverse-twist impressions in pre Homo Sapien-era clay.

There are many techniques for making cordage. Most are difficult to master through text and photos alone. But they are all based on the twisted-fibers-will-make-themselves-into-cordage principle. Once you twist fibers tightly all you have to do is get out of the way and string happens. Remember the common-

Sense of this when having difficulty grasping some particular cordage-making technique.

MAKING FUNCTIONAL CORDAGE

SINGLE PLY CORD — a cylinder of parallel fibers twisted tightly enough to function as cordage.

SIMPLE PLY — a single ply cord used as the primary building block of reverse-twist cordage. It can be thin or thick, an entire ply, or one of many in a simple parallel ply.

SIMPLE PARALLEL PLY — many small, simple plies, the sole purpose being to give uniformity; they are used in parallel lines as if a simple ply.

SIMPLE CORD — a cord made by reverse-twisting two or more simple plies.

PRIMARY PLY — one ply in a simple cord, when this simple cord is one ply in a complex cord.

COMPLEX CC5rD — where each ply is itself a finished simple cord.

Crafting a superior bowstring requires more than simply selecting the strongest fibers. It matters very much how the fibers are assembled.

Here are the variables commonly understood to affect cordage strength:

Finer fibers have more surface area, therefore more points of contact, therefore greater internal friction. When given the choice, select or shred fiber as thinly as possible without damaging the fiber. This is more important for vegetable fibers because cellulose is far less elastic than the keratin and collagen of animal fibers.

Smooth-surfaced fibers slip past each other more easily, and therefore must be twisted tighter. This weakens the finished cord.

Short fibers must be twisted tighter than longer fibers, also weakening the finished cord. Apart from weakening the string, excess twisting also shortens the string. This means a longer string is needed to begin with, which increases mass somewhat.

Excess twisting also makes a string more coilspring-like, causing a bowstring to stretch and absorb energy as it slams home after release. Energy absorbed by an elastic string is unavailable to the arrow, thereby reducing cast.

Beginning on page 203 in Archery-The Technical Side, Nagler, and other contributors, are puzzled when stronger threads sometimes make weaker strings, and vice-versa. I believe the theories and tests reported below explain this phenomena.

On the face of, it one could hardly imagine a more straightforward act than a string breaking. But when viewed in mental slow motion, a breaking string is a complex series of events involving many interacting forces and processes. But even a cursory understanding of the events in a breaking string will lead to better bowstrings.

Strings made up of many small-diameter plies, properly twisted together, are stronger than those made from fewer larger plies. But why?

The outer layers of a thicker cord have a larger diameter than inner layers. Therefore, when twisted, its outer fibers are asked to stretch and travel a longer, more spiraled path than inner fibers — the stripes on a barber pole, for example, are longer than the pole itself.

These outer fibers try to relieve strain by shortening their path. They accomplish this by: one, squeezing and contracting the cord's diameter, and two, by shortening the cord — central fibers are actually telescoped into negative tension.

When such a cord is strained in tension, its pre-strained outer fibers must necessarily break first, leaving fewer and fewer near-surface fibers to resist the load. A case of divide and conquer.

Inner fibers of thicker cords have not been twisted as severely as outer fibers, relying on compression from the more-strained outer fibers to create their cordage-making friction. Once these outer fibers break, inner fibers are able to pull apart more freely.

Since these inner fibers are, in effect, dead weight, such a string has less strength per mass.

Thread-thin cords, on the other hand, have smaller inner cores for outer fiber to wrap around. When twisted, outer and inner fibers therefore feel nearly equal strain, and nearly equal cordage-making friction. As a result, outer fibers do not break much more quickly than inner fibers. Thinner cords therefore have a lower percentage of central dead weight. They are stronger per mass.

As a visual clue to the more-equal behavior of inner and outer fibers in a thread-thin cord, notice that when spun very tightly, such threads do not become much shorter. A thick cord, on the other hand, shortens quickly when only lightly twisted.

Here is evidence of the greater strain placed on a thicker cord's outer fibers: The fibers in such a cord are smooth and parallel before being twisted. Once twisted, then untwisted, these outer fibers become slack because inner fibers now carry the load.

Outer fibers of thicker cords are more strained than inner fibers. This is easily demonstrated by twisting a thick cord of parallel blades of grass. You will see its diameter constrict, its length shorten, and its outer layers begin to break in tension. Notice that surface fibers are oriented at a strong angle to the cord, inner fibers progressively less so. Central fibers are almost parallel, and feel less friction once outer fibers fail. Meanwhile, a very thin cord of the same grass will accept considerable twisting without shortening or breaking.

Since a small-diameter cord can be twisted more revolutions per inch, fibers wrap around themselves more frequently. More points of fiber contact equals more fraction. More friction equals less slippage. Less slippage equals greater strength.

For graphic proof of this, cut unspun flax fibers to one-inch lengths. Twist up a thread-thin strand from these short fibers. This thread will be frizzy, but if tightly twisted will make useable cordage. Then try to twist up a half-inch thick cord. A cord this thick can not twist sufficiently for fibers to grip each other. No points of contact, no gripping can take place. No cordage can be made.

This extreme example illustrates the friction/strength advantages of smaller diameter cordage. If fine enough, and if twisted enough, thread strength will approach the combined strength of its constituent fibers. This thread will have a very high strength-to-mass ratio, precisely the requirement of an efficient bowstring.

Because outer fiber are more strained, primary plies should be twisted no more than necessary to prevent inner fibers from slipping. But they must be twisted up tighter than initially needed, because they will unwind somewhat when untwisting into a finished cord. And when making complex cordage, twist the simple plies slightly tighter still, because plies in the primary cordage will receive a slight net un-twisting in the process.

A cord's strength is due primarily to the twisting within its smallest component plies. Subsequent reverse-twisting adds less strength. Even the largest ropes could be made up of countless small, parallel, reverse-twisted plies. But such ropes would be ungainly, disorganized, and fray easily. Second and third level reverse-twisted plies in complex cordage and rope are there largely for convenience and durability, not inherent strength.

Ply diameter decreases as ply numbers increase. Smaller diameter plies will permits tighter twisting with less strain on surface fibers, but from four plies on, plies must be progressively more distorted as they attempt to fill an ever-larger hollow core. It seems likely that ply strength would be compromised by such distortion. With seven plies the problem solves itself: a single ply fills the void. This central ply becomes largely dead weight, but by equalizing stresses in the six outer plies, higher net cord strength results. With eight or more plies the cord's hollow core is filled with ever more dead-weight central plies.

But why doesn't larger-diameter reverse-twisted, simple and complex cordage suffer from the thicker-is-weaker problem?

For two reasons; One, the reverse-twist is not as severe as the initial simple-ply twist. Two, each ply is much smaller in diameter than the entire cord; this is especially true with cords having three, four, and more plies.

It seems reasonable that there should be no more than seven simple parallel plies in a ply, no more than seven plies in a simple cord, and no more than seven cords in a complex cord. If more than seven the cylinder becomes too thick, causing some plies to remain internal. When twisted, internal plies will not be strained in unison with external plies. They become merely dead weight. The thicker-is-weaker problem again.

Equally important to mass/strength is the uniformity of the simple plies. The following multi-strength property of string has been overlooked in the past. But an understanding of it is necessary for efficient bowstring design:

A spool of high quality, wet-spun, single-ply line linen had an "average breaking strength" of 5 lb.

But when a 50-inch long strand was tested, breaking strength dropped to 3 Ib.

And when a series of 5-inch long sections of a long thread were tested, breaking strength varied from 3 to 7 lbs.

Even in this evenly-spun thread, strength varied by at least 3 to 7 lbs along its length.

If seven such plies are kept separate and parallel, as with an endless string, they will each break at their weakest point of 3 lb. Their collective breaking strength being 21 lb. But this 21 lb. string has the mass of a 35 lb. seven-ply twisted string.

If these seven plies are twisted tightly into cordage, with weakest-points placed next to strongest points, weak and strong will average out, and combined strength will be 35 lb. But the law of averages will not arrange things so perfectly.

The likelihood, however, of having advantageously distributed weak points increases with the number of plies. This is likely the reason the strongest-tested shoemakers' cord has 7 simple plies.

Here is a simple experiment to demonstrate the thicker-is-weaker/no-more-than-seven-plies theory:

Using single-ply natural fiber thread of 5 lb test, twist up simple cords containing 40, 20, 10, 7, and 5 threads. Twist each tightly enough to create cordage-making friction. Test each cord for breaking strength.

Results will be approximately as follows:

What we conclude from all this is that cordage made up of several, very small diameter simple plies will be considerably stronger per mass.

When commercial cordage is examined, stronger-per-mass samples prove to be made up of many small primary plies. For example, shoemakers' lock-stitch cord, the strongest-per-mass string yet tested here, has seven sewing-thread-sized simple plies, even though total diameter is little larger than kite string. And the same holds true for complex cordage. Final plies should be smaller in diameter, and greater in number, but not exceeding six or seven.

Testing Cord Strength, Stretch, and Set

When breaking a string to measure its strength, it's important not to disrupt internal strains when gripping or securing the string. The weakened string may not break where gripped, but as with a knot, it may break nearby, and at lower strength than it would otherwise.

Secure a thread or string to be tested around a smooth, round surface. Take a couple of turns before tying off. If secured properly breaks will occur randomly along the string's length. If secured improperly, breaks will occur near the fastened ends. Do not begin measuring until confident your method of securing the string is not contributing to its failure.

If a string is to be used by itself, or in parallel with others — as in an endless string — test section several feet long. This will reveal the strength of its weakest point.

If a string is to be twisted together with others in a cord, determine its average breaking strength: first break long sections to find its weak-link strength, as above. Then break several three to five-inch section to find its strongest-link strength. Take several readings, then average them.

When testing animal fiber, higher, truer readings will be obtained if the load is applied and released a few times, gradually building to the breaking point. This is true of vegetable fibers too, but less so.

To measure stretch and set — the amount a string will remain stretched when tension is released — lay out fifty inches of thread or cord. Fifty inches is long enough for accurate readings, and short enough for convenient handling. And each half inch of movement equals an even f %.

Wrap one end of the cord twice around a smooth, round dowel before tying off at a nail or such. Attach the other end similarly to a scale. Place a ruler beside the scale. Pull the scale, applying tension slowly and repeatedly, building up to point of failure. Note the amount of stretch, and the amount of set as you proceed, as well as the point of failure.

Being more elastic, animal fibers such as sinew, gut, rawhide and silk can tolerate somewhat larger diameter primary plies. American Indian bowstrings were often a single-ply cord when made of sinew, but more often three-ply when of plant origin. This is true of weaker fibers, such as palm strings of Brazil. And it is true of the strong dogbane strings of the north.

To restate, a bowstring made of large, simple plies cannot be as strong as one whose plies are themselves made up of smaller simple plies. This is true for two reasons: 1, small diameter plies are inherently stronger, and 2, many small plies will average out weak areas inherent in simple plies.

Obviously there is a practical limit to the number and fineness of simple plies.

Of 5 lb. plies. But plies of up to 10 lb. are not weight the thicker-is-weaker problems becomes it's more difficult to arrange plies to add up to a — exact change is more easily made with nickels

Shoemakers' linen was made seriously weaker. Above that more serious. And above 10 lb. designated bowstring weight - than quarters.

You may decide its too much needed for a superior multi-ply But try spinning up your own at

Trouble spinning the 100 or so yards of primary ply string. If so you can buy pre-spun thread instead, least once. Spinning is easy, fast, and enjoyable.