A Writer’s Guide to Bows, Part One

Posted: January 30, 2013 in Archery, Articles

It occurred to me recently that many people who write historical or fantasy stories could benefit from some information about how bows actually work, as well as how to speak knowledgeably about them. This series of articles is the result.

Part I: How bows work and what they’re made of

A bow is a fairly simple machine. In essence, it is a spring with a string attached.You may also hear them termed “a bent stick and a string,” which is not altogether incorrect for most types of bows.

The bow itself can be broken down into two distinct elements. The first is called the Riser. The riser is the part of the bow where the grip is taken. In most types of bows, this element is rigid, and does not flex during the action of the shot. Some longbows “flex through the handle”, but even those only deflect a few degrees within the span of the bow grip.

The active part of the bow, the element that flexes during the shot, is the limb. There are two limbs, and they are usually symmetrical in resistance and dimension. Some bow designs, specifically the Japanese Yumi, are quite asymmetrical. Modern theory dictates that a bow’s limbs should be within certain tolerances in terms of degree of flexion during the shot, but archaic or primitive designs would often diverge from this.

When a bow is strung, the material of the limbs is put under pressure. This is often called “preload.” As the bow is drawn back, this load increases. With a bow of traditional design, the draw weight should increase at somewhere between two and three pounds per inch. Any limb design has a finite amount of flexibility, however, and as they reach that point, the draw weight will climb rapidly. This is termed “stacking”. Stacking is generally to be avoided, as it tends to put extra stress on both the bow and the archer.

The bow, when drawn back, will accelerate the string as it springs back to its initial preload. If an arrow isn’t fitted to the string when it is released, this is called a “dry fire”. These are damaging to the bow, as all the energy of the spring is channeled back into the limbs. Bows can fail suddenly and spectacularly when dry fires occur.

When an arrow is present, the travel of the string accelerates the arrow and sends it on its way. Generally, heavier arrows will absorb the energy more efficiency, but there is a limit to this line of reasoning, as an arrow that is too heavy will have very poor range, due to traveling too slowly. As with any projectile (object propelled by initial thrust, rather than having propulsion systems of its own), an arrow is falling to earth at the same speed as an object you drop from your palm. The only difference is, the arrow is propelled quickly on another axis, and will go some distance before it falls to earth. The faster the projectile’s initial speed, the further it will fly before its trajectory degrades and it falls to earth.

The amount of power a bow can impart to an arrow is determined by a few factors. The first is the peak draw weight. This is the maximum level of spring tension that the bow achieves during the draw. The second is the linear length of the power stroke. The third is the efficiency of the bow design. With two bows of the same design, the one with the heavier draw weight will create more power. Weight for weight, the bow that is pulled back further (one archer has longer arms than the other, for instance) with give more power. If weight and draw length are unchanged, only an improvement to bow design can increase power.

The simplest bow designs are essentially straight sticks of wood that conform to a long D shape when strung. A bow that is made of a single piece of wood is called a “self bow”. These are the easiest to make with simple tools, as a piece of pliable wood that bends uniformly is the only requirement. The downside of the selfbow is that it tends to be inefficient, fragile, and have a short service life.

Not all types of wood are equally suited for bow making. The wood has to withstand compression on the belly (side of the bow facing the archer) and tension on the back (side of the bow facing the target). The wood has to be pliable and light. It has to resist bending, such that it doesn’t quickly conform the shape the string keeps it bent to. It has to not shatter in your hands, as archers uniformly find this disagreeable. The best wood known for self bows is Yew. Several other woods work well. Maple and Oak are common woods that make useful bows.

Historically, most bows have been made of wood, either self bows or multiple laminated wood pieces. That said, the shape and type of bows in any given area was primarily dictated by the materials available. Europeans had access to many good or great bow wood species, and so they created longbows that helped them succeed in warfare and hunting. In climates where the woods are not ideal, archery tends not to be a big factor in society, or the bowyers learn to use other materials. This is the case with the horn bow that the Mongols, Magyars, and Huns used. Horn is a material with excellent strength to mass for a natural product. It is also very tough and can be used in the limbs of bows.

One can also notice the sinuous curve of a horn bow. These curves are not simply for show. No, by designing recurve or reflex into a bow’s limbs, preload can be increased. Also, because there are multiple vectors of stress and flexion, a shorter bow can be pulled back further. The recurve bow is often more efficient than a straight bow.

In the days before modern adhesives and finishes, bows had to be very carefully maintained. If they were left to get too dry, too damp, or even shot when the material was too cold, they could be damaged or ruined. The strings, often made from twisted linen in the case of the longbow, had to be carefully treated, and needed considerable “shooting in” to hold their proper length. A coating of wax protects strings from damage and keeps their fibers from fraying.
Since the advent of fiberglass, it has been found to be a fine addition to bow making materials. The majority of bows use fiberglass in their limbs today. For recurve or longbows, thin laminations of fiberglass are used on the belly and back of limbs to protect the wood, keep it correctly shaped, and add to the efficiency of the design. The great majority of recurve and longbows today use fiberglass over a bamboo or maple core, to good effect.

Today, we have the pinnacle of bow engineering, the compound bow. These bows have aluminum or even carbon fiber risers, and high tech laminates in the limbs that allow them to be more efficient than ever. Beyond this, a compound bow uses cams and pulleys at the end of the limbs to manipulate the rate at which draw weight rises and falls during the draw cycle. Modern compound bows can rival firearms for level of lethality. With science, the bow has been made to be about twice as efficient as archaic designs.

Arrows through history have usually been made with wood or bamboo. Good woods for arrows include cedar, poplar, walnut, and so on. Though bows can often shoot well with crooked limbs, arrows are fairly exacting. Without straight, uniform arrows that are the right stiffness for a particular application, accuracy can be hard to achieve. During the shot process, an arrow must flex somewhat, then oscillate back and forth for the first several feet of travel. An arrow that flexes too much will generally fly too far right on the target, while an arrow that is too stiff will often land to the left of the desired target. These generalities assume a right handed shooter. Left handed shooters will expect the opposite result. An arrow that suits a particular bow and shooter is said to be “properly spined.”

Modern arrows are often made of aluminum or carbon fiber, though wood is still used and can still work well. The difference between the new arrow materials and wood is in their consistency. Aluminum and carbon fiber are both inert materials that do not change much in their behavior between hot, cold, dry, or wet conditions. The same cannot be said for wood. Archers who use wooden arrows must always examine them for straightness and damage, as wood will tend to warp and require attention.

The fletching on an arrow works to stabilize it in flight. It can, in some cases, impart a helical spin, but that is not necessary. Basically, the fletching will add resistance if the arrow flies in a non-straight path. Until the advent of flexible plastic vanes, which are often used today, feathers were the only way to go in terms of fletching. Even now, feathers are the lightest material with which to fletch. Their only drawback is that they don’t respond well to dampness, and can be degraded when an arrow passes all the way through a target.

The feathers for fletching are culled from the flight feathers of good sized birds. The classic feather is the turkey feather, but any large bird that has stiff flight feathers can be useful. All the feathers used on a particular arrow must come from the same wing-side. That is to say, all from right wings or left wings. This is because the shape and curvature present on wing feathers will cause erratic flight if mixed. Feathers are usually glued onto the shaft of the arrow. This must be done with care. In the days of old, when adhesives dried slowly, fletchers (people who made arrows) usually tied the feathers on with thin twin.

The method by which arrows do damage to targets is by cutting through them and compromising their materials, rather than conferring massive blunt trauma. To put it another way, an arrow is not a bullet. The most effective arrow points for damaging living things are termed “broadheads.” These are essentially razor sharp blades on the front of the arrow, often hooked so they will not easily pull out of the wound. Traditional broadheads were two-bladed, as this was a shape that could be created by the manufacturing techniques of the day. Modern heads can have three, four, or even more blades.

Broadhead-equipped arrows work by compromising internal organs and causing massive blood loss. Short of an arrow piercing the central nervous system or going directly into the heart, death is not instant. Arrows do not knock people/animals backward like being hit by an NFL linebacker. In many cases, the shock of injury will cause a fall, but the actual impetus of the projectile is not that high.

Arrows don’t pierce hard objects well. A thick piece of wood will stop an arrow, as will iron, brass, etc. Only the heaviest of War Bows could pierce plate armor, and they could only do this with an armor piercing point, often called a bodkin point. To give an example, an English War Bow drawing over 100 pounds could, at close range, punch through a sheet of ¾” plywood. It could also go through pretty much any armor that was in use at the time. Some bows of the Medieval era had draw weights of up to 180 pounds.

Keep in mind, however, that these bows required years of rigorous training to gain the strength and skill required to use them well. Even a strong man will find it difficult to draw a bow of more than 60 or 70 pounds without practice. Long-term use of super-heavy bows can actually cause curvature of the spine and marked asymmetry in the body’s muscle structure. Through most of history, the best evidence is that bows for hunting and fighting were often between 40 and 70 pounds of draw weight. The power provided by these bows would pierce light armor and cause lethal wounds when shot accurately.

Next time, we’ll talk about effective range, lethality and usage of bows in warfare and as tools for hunting.

Advertisements
Comments
  1. Awesome article. Thanks, Pat. I never knew about the feathers having to come from the same wing. Fascinating. I learned a lot.

  2. […] Part One addressed how bows and arrows work, and what they are made of.  You’ll learn here what types of wood a believable bow could be constructed from, and how an arrow wound differs from a bullet wound. […]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s