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A Note on Indian Bow Making |
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Written by Dick Baugh
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Page 2 of 2
The engineer in me took over. What are the material properties
which will yield a superior bow and how can I measure them? The things
which matter are the elastic modulus (how much it stretches with a
given tension), the tensile strength (how much tension is needed to
break it), and how much it shrinks when it dries. In addition, it helps
to define some other useful terms:
Potential energy: the ability to do work. When you pull the
bowstring back you store potential energy in the bow limbs. The
available potential energy is equal to the distance you pull the string
back multiplied by the average force that it took to pull the string
back to full draw. When you release the string the potential energy is
transferred to the arrow, giving it . . .
Kinetic energy: the energy of motion. A perfectly efficient bow
would transfer all of the available potential energy stored in the bow
limbs into kinetic energy of the arrow.
Elastic modulus: a measure of how stiff a material is. Make a
one inch cube out of the material and stretch it with a known force.
The cube will get slightly longer. The elastic modulus is the force
times the length of the block, divided by the area of the block times
the distance the block stretched. Steel has an elastic modulus of 30
million psi (pounds per square inch), hickory has an elastic modulus of
2.2 million psi, black locust has 2.1 million psi, and the measurements
I have made on yew wood give a figure of 1.2 million psi.
Tensile strength: keep pulling on that one inch cube of
material and eventually you will pull it apart. The force per square
inch that it takes to pull something apart is the tensile strength. For
tempered steel the number is 400,000 psi, for hickory it is 20,000 psi.
For those of you who wonder: yes, it is very impractical to make these
measurements on a one inch cube of material. The one inch cube was
cited to emphasize the force per unit area nature of the experiment. In
actual practice a much skinnier specimen of the material would be
tested.
My measurement of the elastic modulus of a dried, solid horse tendon
gave a figure of 411,000 psi. Similar measurements on yew wood yielded
1.16 million psi. This said, much to my surprise, that under the best
of circumstances sinew had only 21 to 35 percent of the elastic modulus
of wood. Put in other words, and leaving out the mathematical formulas,
if you make a yew wood bow of 50 pounds pull and add more yew wood on
the back to make the limbs 5 percent thicker, the resultant bow will
have a 15 percent stronger pull or 57.5 pounds. If, instead of adding
more wood on the back of the bow, you make the bow limb 5 percent
thicker by adding sinew, the increase in draw with would only be 2.2
percent or an additional 1.8 pounds. Why
bother adding a material to the back of the bow which doesn't add much
to its strength? The other 'secret' ingredient must be shrinkage. |
 I was pretty well convinced that sinew shrank while it dried and this
put the sinew backing under great tension. Did the amount of shrinkage
depend on the type of glue used? The experiment to find this out was to
glue sinew on the backs of two identical strips of 1/8 inch balsa wood.
On the first one, the sinew was glued on with hide glue, on the second,
the sinew was glued with Elmer's carpenter's glue. The two samples
behaved identically. As the sinew dried and shrank it pulled the wood
into a curved shape. This experiment showed little difference between
the two types of glue, only that the sinew shrank as it dried. Again I
took two identical 1/8 inch strips of balsa wood and put a thick strip
of hide glue on one and a similar strip of Elmer's on the other (no
sinew on either). This time there was a pronounced difference between
the two. The hide glue shrank and curved the wood just as much as the
sinew, and the Elmer's glue did not shrink at all. Moral of the story: don't use anything but hide glue for applying the sinew.
Furthermore hide glue is 'compatible' with sinew since on a molecular
level they are identical. The last experiment with sinew was to see
exactly how much it shrank when it dried. I pinned one end of a strip
of wet sinew to a piece of plywood, and pinned the other end to the
short end of a stick that pivoted at one end. Now, when the sinew
shrank, the long end of the lever would move through a greater distance
and make the shrinkage easier to see. The result was that the sinew
shrank 3 percent upon drying. |
In conclusion one can say that the benefits of sinew backing on wood
bows come from a combination of several effects acting together. They
are:
1. As the sinew dries and shrinks it puts the back of the bow under
compression. As a consequence, the wood fibers on the back of the bow
are not stressed as highly when the bow is drawn.
2. The sinew protects the back of the bow where it doesn't follow the grain.
3. The back of the bow, which is stretched a great deal at full draw,
is now a material which can stretch 5 percent before breaking (wood can
only stretch about 1 percent before breaking). |
REFERENCES
Saxton T. Pope
1980
Bows and Arrows
University of California Press.
Reginald and Gladys Laubin
1923
American Indian Archery
University of Oklahoma Press
This article was first published in The Bulletin of Primitive Technology (Vol. 1 Spring 1994, #7)
Many thanks to Dick Baugh from www.primitiveways.com for supplying this article.
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