Evaluation of atlatl darts made with

Gold tip 75-95 carbon fiber tube.

by Christopher Oberg, e-mail oberg@msu.edu

Objective

To find a synthetic dart material suitable for use on 3-D archery ranges. The dart shaft must have sufficient stiffness (spine) to prevent over flexing when balanced between 32-38%. To prevent damage to valuable 3-D targets, the outside diameter of the dart should be no greater than a conventional arrow shaft. Balance must be achieved through internal adjustment. The finished dart must be durable, accurate and easy to produce with a minimum of tools or expertise. Cheap would be nice too!

Materials

Gold tip "HUNTER" 75-95 carbon arrow shafts were selected based on advertised reports of superior strength and light weight compared to aluminum. The Gold tip shaft is comprised of 5 cross-laminated layers of carbon fiber wrap so strong that conventional point inserts can be used without danger if spitting the thin walled carbon tube. The outside tube diameter is 5/16" and the inside diameter is .245". The shaft length is 32.5" so two shafts need to be joined to produce one dart. Gold tip carbon tubes come in 3 sizes or degrees of stiffness. Only the 75-95 tube was selected for this test on the assumption that it would allow for greater weight behind the tip without becoming over flexed during the power stroke of a throw.

Dart Construction Methods

Two methods for joining the Gold tip tubes were explored.

Permanent one piece easy joint

A 1/4" diameter hardened 2" long motor mount stud was epoxied with "JB Weld" between two tubes to make a simple permanent joint.

Permanent method of joining "gold tip hunter 75-95" arrow tubes.

The tubes were prepared by gently abrading the inside of the tube with 100 grit sand paper then swabbed with acetone on a q-tip. The threads at each end of the motor mount were filed down slightly to remove any sharp edge that might damage the inside of the tube as the dart flexes. Test-fit the motor mount! The center non threaded section may need to be reduced slightly so that it fits into the arrow shaft. I just chucked it up in an electric hand drill and used a flat mill file to remove enough material to make it fit into the arrow shaft. The motor mount was then degreased. "JB weld" was applied to cement the motor mount in place between the two carbon tubes. Because of the inherent strength of the Gold tip carbon wrap, no additional support to either tube was necessary.


Note: a more durable bond can be achieved if a 1/4" threading tap is screwed into each arrow shafts a short distance to score the inside of the arrow shaft before glue is applied. I have had zero seperations or loose joints after switching to this technique.


A two part TAKE-DOWN JOINT can be constructed by modifying a female point insert (comes in a bag with the arrow shafts) and a screw-in field point.

MALE PART - Reduce the diameter of a 145 grain 5/16 screw on field point to .245" diameter so that it can slip into an arrow shaft and set aside for later use. This job is best done on a lathe but an electric hand drill and a flat file will work in a pinch.

FEMALE PART -The lip of a female point insert was also removed or reduced to .245" so that it could be inserted 1/4 inch below flush into the arrow shaft.


JOINING TAKE-DOWN ARROW TUBES

If a number of take down darts are to be made, it would be advisable to install the reduced female point inserts the exact same depth into each arrow shaft. This will allow for an interchange of components so that if you damage a front section of your dart you can screw-in a replacement shaft that fits just like the original. To do this you need to install the reduced female point insert at the exact same depth on every dart!

Here is how I do it.

Installing the female point insert after reducing the lip.

Don't mix your epoxy yet...

- Screw a standard unaltered field point (any size) into a reduced female point insert.

- Make a small mark with a fine point felt pen or knife blade anywhere on the lip of the female point insert.

- Slowly unscrew the field point paying close attention to the exact instant where the field point sepetates as it is released from the threads of the reduced female point insert. Make a mark on the standard field point that lines up exactly with the mark on the female point insert at the point where the two parts seperate.

Note - The two marks should line-up exactly where the two parts seperate when they are being unscrewed.

Congratulations, you have just established the origin of the threads on the reduced point insert.

Next - line-up the marks on the reduced female insert and the standard screw-in field point and screw them together exactly 2 revolutions. This arrangement will allow you to seat the reduced point insert about 1/4" into the tube below flush were it will be stopped from further penetratation by the base of the standard field point.

Now you are ready to glue the female point insert into a shaft.

Lightly sand the inside of the tube and clean with acetone on a q-tip.

Mix a pea sized blobs of parts a and b of JB weld and smear some on the outside of the reduced female point insert but first plug the hole in the bottom of the female insert so epoxy can not migrate into the female insert threads. Use a tooth pic to apply the remainder of the jb weld to the inside of the arrow shaft.

Insert the female reduced point insert into the arrow shaft using the above technique to seat the female insert into the arrow shaft 1/4" to your predetermined depth. CAREFULLY unscrew the field point and CAREFULLY wipe-up any exposed epoxy on the inside lip of the arrow shaft with acetone on a q-tip. Don't bump or move the female reduced point insert. Set the arrow shaft aside so the epoxy can cure undisturbed for 24 hours.

Repeat this process with as many shafts as you want to make front sections for take-down darts. Note- all of my dart breaks have been on the front section so I make a few more front sections than back (fletched) shafts.

The Male screw apart joint is constructed from a 5/16" dia. 145gr field point reduced to .245" on a lathe. A groove can be added to the field point to accept cement for a stronger bond. The reduced filed point will be used on the fletched shaft of the dart.

Bonding the take-down joint

Prior to cementing the reduced male field point in the tube, the inner wall of the tube MUST be lightly sanded and swabbed with acetone. Running a 1/4" threaded tap a short distance up the shaft helps make a better bond, see info above.

Preparing the reduced female insert shafts before final gluing.

IMPORTANT - Swab the inside of the female threaded tube with vaseline. The vaseline allows the screwed-in field point to be removed even if a little epoxy seeps in when the two shafts are bonded together.

- Screw the reduced field point into the reduced female insert shaft.

WARNING - don't let any vaseline get on the exposed part of the reduced field point!

Smear JB weld inside another tube (don't forget to abrade the inside) and on the exposed portion of the field tip then join the two shafts. Shafts must remain straight while curing. I had good results by doing 3 darts at one time then taping them together for support. Let the darts cure over night taped in place, standing up in a corner with the freshly glued joint on top so that any cement will flow onto the tip of the field point. If you have already fletched your shafts you will have to tape them together in a staggered fashion to avoid crushing the fletching.


ADDING INTERNAL WEIGHT TO ADJUST BALANCE POINT

All testing was done using a 100 grain 5/16" field point screwed into the tip insert. Additional ballast weight was added inside the tube behind the insert in the form of 1/4" threaded rod cut to length and JB weld applied to the entire threaded surface area just prior to cementing the point insert. A 7" piece of threaded rod produces a balance point around 32%. A 5" threaded rod balances at about 35%. A 3" threaded rod behind the insert will balance close to 38%. These results were measured on a dart with 3 plastic 5" vanes and an insert installed as butt cap. Switching to a 145 grain field point shifted the balance point about one percent further forward.

Fletching

A Bitzenburger fletching tool set-up to produce a straight fletch was used to apply 5 inch plastic fletch at 120 degree increments. Tubes were degreased with acetone and dipped in white fletchlac before applying the vanes. FletchTite fletching glue was applied to the vane after the base was lightly sanded. . A temporary nock was made from a short section of 1/4" threaded rod to index in the nock receiver of the fletching tool. After fletching the temporary nock was removed. A point insert was added as a butt cap and thrower spur receiver.

Balance

The most import factors determining flight characteristic are dart balance and spine or stiffness. Through trial and error, it was observed that a dart balanced between 30 to 40 percent from the tip of the dart would produce acceptable flight characteristics. Performance tests conducted at long range allowed prolonged observation of dart flight characteristics and were helpful in establishing an optimum long range balance point. Ideal dart balance was evaluated by comparing dart orientation in relation to the arc of the dart flight path. It was observed that darts thrown a distance of 70 meters were most stable when balanced at 35% from the tip.A preliminary method of viewing flight performance is a technique also used to check the stability of Estes rockets before a maiden flight. Tie a strong line at the balance point of the dart and swing the dart around your head as you let out more line until the dart is orbiting on 10 or 15 feet of line. Be sure to keep all observers at a safe distance! As the balance point was shifted further back fletching became necessary to keep the tail in line with the arc of the dart. 3" of threaded 1/4" threaded rod will create a 38% balance using a 3 fletch design with 5" plastic fletch. When the desired amount of ballast is established it can be glued in place using JB weld and followed by the point insert.

Tuning

For the purposes of this test, a dart/thrower formula was used as described by E.W. Nelson in his review of "The Eskimo About Bering Strait". The throwing stick used by the Unalit Eskimo are made of a length proportioned to the size of the person who is to use them; this is determined by the measurement of the forearm from the point of the right elbow to the tip of the outstretched forefinger. the ordinary length of the seal spear used with throwing sticks by the Unalit is calculated as three times the distance from the point of the maker's elbow to the tip of the outstretched forefinger, with the added width of the left thumb for each of the first two cubits and the width of the left hand added to the last. A dart balanced anywhere between 31 to 38% will fly consistently. Weight added to achieve a balance point of 31% will create the heaviest dart practical with the gold tip 75-90 tubes. It will weight approximately 2.75 oz. The other extreme would be a dart balanced at 38% that weighs around 2.25 oz. The light dart can be thrown with greater force and to a greater distance without over flexing than can a heavy dart.The question becomes what balance point is best. Several factors need to be considered when deciding where to balance your darts. Generally speaking, the velocity of a light dart will be greater which translates into a flatter trajectory than a heavy dart. Due to its low mass, the light dart is also less susceptible to impact damage. A light dart offers practically no resistance during the power stroke and is most comfortable for the atlatlist with shoulder problems. Light darts can utilize oversized fletch causing the dart to perform like a fluflu arrow. Children may also enjoy a lighter dart.

Heavy darts have relatively few advantages over a lighter dart. The primary advantage of a heavy dart will be the stabilizing effect to dampen small movements or effects of a mild cross wind while aiming. Also, the transfer of kinetic energy is more efficient with a heavy dart causing it to hit harder and penetrate deeper than a light dart. Finally, it is easier to cause a heavy dart to flex during the throwing sequence which translates into energy as the dart springs off of the thrower.

Conclusion

Gold tip multi directional carbon fiber tubes are extremely well suited to the production of effective and durable atlatl darts. When compared to AFC 2540 carbon tubes the Gold tip darts weigh 20% less for identically balanced darts. Informal long distance throws with Gold tip darts averaged 10 meters further travel than AFC 2540 darts. Take-down jointed darts needed to be checked for tightness after each throw. Lock-Tite would solve that minor inconvenience. A Balance point of 31 to 38% is effective on a fletched dart. Unfletched darts require a balance point between 30 to 32% from the point for best results but the thrower should be aware that without the aid of fletching, small variations in throwing style will cause exaggerated deviations in dart performance. My overall impression was very positive! The Gold tip multi directional carbon fiber wrap produces a very strong and unusually light dart, lighter than any other dart material tested to date. for more information on Gold tip arrow shafts go to http://www.gold tip.com/

Acknowledgement

I would like to thank Dan Hovather of the Chelsea Rod and Gun Club for introducing me to and sharing his experiences with Gold tip arrow shafts as a dart material. Dan is an experienced traditional archer and a creative free thinker when it comes to atlatl design and dart production. Without Dan's outgoing and helpful nature this report would never have been produced. I would also like to convey my deep thanks to John Jacobsen who has been building custom arrows for over 35 years. John graciously supplied Gold tip tubes and inserts for preliminary testing and offered encouragement and a genuine interest in furthering the evolution of atlatl tackle. John also can supply archers or atlatl dart builders with arrow making materials or custom fletched shafts in a wide variety of brands and sizes for a fraction of the cost charged by other custom arrowsmiths. I have included a link to his price list so that do-it-yourself dart builders can take advantage of his great bargains. I have not found better prices for Gold tip shafts anywhere!

Check out Artesian Arrow Company.

 


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