The Tuning of Atlatl Darts

by Dick Baugh

Introduction
How many of us made our first atlatl dart or spear shaft out of some stiff, unyielding piece of wood or bamboo and concluded that spear throwers weren't very effective because the back end of the spear ALWAYS kicked down when we threw it? Enlightenment came later. If you go today to an atlatl throw-in it should be pretty obvious that the "kick-down" problem has been solved and most of the darts or spears have nice straight trajectories. The purposes of this article are to present a systematic procedure for tuning atlatl darts so that they fly with the least up-and-down perturbation and to provide a firm theoretical foundation for why they behave the way they do.

Relationship With Archery
One common feature of shooting arrows and throwing atlatl darts is the fact that the force is not applied in a straight line collinear with the flight of the projectile. A bow string applies a force in a fairly straight line aligned with the centerline of the bow whereas the arrow has to bend around the handle of the bow. This was especially the case with Robin Hood's yew longbow which had a draw weight of 100 pounds and a handle diameter of 1 1/2 inches (3.81 cm) [1]. The arrow had to snake around the handle. Consequently archers are very aware that in order to achieve a clean trajectory with no "wigwag" the arrow shafts must be "spined" to the weight of their bow. People today who sell arrows have charts which specify the appropriate arrow stiffness for a given bow draw weight. There are also spine testers available for people who still enjoy making wood arrows which are spined for a particular bow weight. The same would be appropriate for atlatl darts.

Javelins used in tack and field events are an exception for several reasons. The javelin thrower is able to exert a force in a fairly straight line along the path of the javelin so the force tending to deflect the xxx is small. The international organization which governs track and field events specifies the dimensions, weight and weight distribution of a competition javelin very rigorously so there is no room for adjustment. Initially the rules allowed javelins to have large diameter and a balance point (center of gravity) behind center of force. This gave an aerodynamic trajectory instead of a ballistic trajectory. In other words the javelin sailed through the air like a glider instead of like a rock. The result was greater distance with a good throw but the possibility that a bad throw would swerve off into the crowd. The newer rules insured that the javelin was more "rocklike" than "gliderlike".


Dart Tuning
We can employ an old archer's technique to adjust atlatl darts. Archers are admonished not to depend on the arrow's fletching (feathers) to insure an un-wavering trajectory. In order to tell if your arrows are matched to your bow shoot them before apply the fletching but after you apply the points. Get close to a target, a few yards (meters) and shoot your fletchless arrows. Do they go straight in or do they go in at an angle? For a right-handed shooter, an arrow which is too stiff will have the back end deflected to the right and conversely for an arrow which is too limber. Arrows can be made less stiff by reducing their diameter. Typically one buys shafts which have already been selected with a spine or stiffness tester.

The same applies to an atlatl dart. Throw it before you apply the feathers. Use the same throwing effort you normally use. If the back end kicks down the dart is too stiff. If the back end kicks up it is too limber. Enough said. The atlatlist has a couple of choices for fixing a dart with an errant trajectory. If the dart is too stiff get out your wood working tools and reduce the diameter. If the dart is too flexible you can either shorten it or make a new one out of stiffer or larger diameter material. After it flys well you can add feathers to the back end. Figure 1 shows the three possible conditions.

How carefully do you need to tune your atlatl darts? It isn't that critical. Remember, a very hard, fast throw should require a fairly stiff dart whereas a slow gentle throw should require a more flexible dart with a longer vibration period. You can test your darts by throwing very hard, with a moderate effort and with an easy toss. My observation has been that a well tuned dart works well for a hard to moderate effort but almost always kicks down for an easy toss.


Theory
A flexible shaft such as an arrow or atlatl dart will have a natural vibration frequency analogous to a bell or a guitar string. The period or amount of time needed for a complete cycle of vibration is independent of the amplitude of the vibration. The trick in tuning atlatl darts (or arrows) is to adjust their vibration period to match, in some way, the time needed to complete the throw. Because of the complicated nature of the throwing motion it is impossible to give a simple relationship between vibration period and time required to complete a throw. You simply use the theoretical relationship between period and throwing time, test a featherless dart and adjust its vibration period according to the way it flies.

Strictly speaking, a flexible shaft has a large number of different vibration periods, depending on how it is excited and whether or not it is held at one or both ends. The exact details are not important. The only features we take advantage of are the facts that the vibration period gets shorter if the shaft is shortened and it gets longer if it is made thinner or additional weight is added. Reducing the diameter of a wood shaft makes it lighter and less stiff at the same time but the stiffness is reduced more than the mass so the net effect is an increase in the vibration period.

Extravagant claims have been made about the efficacy of making the atlatl flexible so the vibration period of the atlatl matches the vibration period of the dart. The claim is that this "resonance" causes a "tremendous" increase in the dart's velocity. Horsefeathers! When the dart and atlatl flex during the throw energy is stored in the flexure the same way that energy is stored in the bending of a bow limb. That"s where the similarity ends. a bow is very cleverly designed to transfer its stored energy to the arrow with high efficiency. [2] This is a doubly minor effect in atlatl dart flexure. First, the amount of energy stored in the dart flexure is very small compared with kinetic energy of the dart (4.3 %). [3] Secondly, very little of the energy stored in the dart flexure is available for increasing the dart velocity. The simplest analogy is to think of a kid bouncing up and down on a bicycle seat. All that up and down motion contributes absolutely nothing to making the bicycle go forward. In order to test this set your atatl on the ground, spur up and hold it in place with your foot. Hold your favorite atlatl dart vertically, point up. Place the butt end on the atlatl spur and flex the dart as much as it would normally flex when being thrown. Now suddenly let go and see how far it springs into the air. The vertical distance it jumps is a measure of how much energy was added by dart flexure to its forward motion. Hardly any height at all! Energy added by dart flexure is a small fraction of a small fraction of the total.
Flexure in the atlatl is a different story. Again, the added energy is small, 6.7 %, compared with the total kinetic energy of the dart [3] but it is available for increasing the dart velocity.

Conclusions
In order to have a good trajectory an altatl dart muse have sufficient flexibility to compensate for the fact that the throwing force is not applied in a straght line colinear with the dart. There is a fairly wide range of flexibility which is acceptable.

[1] W.F. Paterson, "Mary Rose" - a second report, Journal of the Society of Archer Antiquaries, 1981, pp 4-6
[2] C.N. Hickman, "Archery, the Technical Side", various articles.
[3] Baugh, Richard, "Atlatl Dynamics", Lithic Technology, Volume 23, no. 1, pp31-41.
These reference articles are available from the author at cost.

E-mail your comments to "Richard A. Baugh" at richardbaugh@att.net