Follow-up Discussion of the 'Khotan Bow'
What would be
the mechanical advantage, if any, of non-lift off siyahs?
Posted by:
Dale Yessak 07/18/2002, 11:01:20
As I posted in
a separate question concerning the Magyar bow design, what would be the
mechanical advantage, if any, of non-lift off siyahs? I can see that these are
essentially siyahs that are braced so as to have already pre-loaded when braced
to beyond the lift off stage, i.e. leverage optimized while at brace height,
yet such an arrangement would seem to be less efficient than siyahs of a
greater angle that provide an actual "camming" action at lift off. In
the Kotan bow design, we can see that the long, thin siyahs needed additional
bone reinforcement -- perhaps as lateral stiffeners? -- so would this indicate
that the design NEEDED to be non-lift off so as to make it more laterally
stable?
A separate
issue might be the question of whether or not this design petered out as a
technological dead end because the benefits of such a complex design did not
justify the additional work needed when compared to simpler composite designs.
Posted by: Adam Karpowicz 07/19/2002, 06:57:40
Good to see
you here, Dale. You are right in saying the long siyahs would lead to the
"high-braced" bow and non-contact siyahs. From my experience it is
difficult to make a bow with angled, contact siyahs and at the same time to
keep the siyahs long. Such bows are prone to twist and string shedding even at
brace, due the leverages involved. Note, other designs with long siyahs, such
as some Indian for example, have the angle between string and siyahs very small
at brace, no more than some 15-20deg. Bows with angled siyahs, such as Turkish
or Tatar have the siyahs short to minimize this leverage and stabilize the
bows. Angled bows with longer siyahs, such as Manchu or Mongolian rely on
overall stiffness to stabilize - this translates to more massive knees and
siyahs.
The Khotan bow
with the relatively slender and long siyahs required wide bending sections of
limbs for stability plus it was braced high for non-contact. The longer and
lighter the siyahs and the shorter the bending limb, the more efficient the bow
will be, so the Khotan bow would not be a "dead end" as you suggest.
The width of the limbs could be less, I believe, with no loss of stability, on
the other hand such a wide limb distributes the stresses over more material,
making the limbs less likely to creep (string follow) and allowing the bow to
be strung for longer time with little loss of performance, definitely an
advantage.
Posted by:
Dale Yessak 07/19/2002, 09:19:34
I understand
the mechanics (wide limb advantages, etc.), but I'm just curious why this
design does not seem to appear elsewhere. It seems like it would be much more
efficient and a lot quicker than what we term "conventional"
composite designs. Yet it seems as though this particular bow design is a
rarity among horn bows. Why would that be?
I wonder if the Khotan design really was a rarity?
Posted by:
Stephen Selby 07/19/2002, 22:37:42
I wonder if
the Khotan design really was a rarity? The period of the Khotan bow was around
300 AD. All the bows I have seen preserved from that time (four in number), as
well as the 'Gansu' bow which came from another region, was of that design.
Looking back at my translation of the 'Rites of Zhou' about making horn bows
(which may pre-date the Khotan bow by 500 years), there is a lot of stress
placed on limiting the amount of horn used to the minimum required to achieve
the effect.
Seeing Asian
bows of that age from anywhere is unusual; and nearly all the depictions I have
seen of these bows in art take a sideways view onto the bow, so you can't
really guess how wide the limbs were. I have seen just one Tang bow (around
800AD), and that did indeed have much narrower limbs.
Posted by:
Adam Karpowicz 07/19/2002, 12:02:36
As to the bow:
I think the Khotan bow was not perhaps so common, because it requires too much
work to make. The wide limbs complicate the construction considerably, core is
made of many pieces, also horn, not to forget about the bone overlays.
I think the
extreme width of limbs is an exaggeration, unless the bowyer did not have good
horn and wood to use.
Posted by:
Stephen Selby 07/18/2002, 21:34:43
The only
significant difference (at least in surface appearance) between the Khotan bows
and other bows that I have examined used up until the end of the Yuan Dynasty
(to 1368) was (a) that the very wide limbs were later reduced to a size that,
to us now, would seem more conventional, and (b) the siyahs became more
massive.
Posted by: Bede Dwyer 07/18/2002, 20:43:29
I don't think
you can say a design "petered out" if is was the dominant style for
at least 1200 years. The development of thin, stiff siyahs obviously offered
some advantages that prevented the reappearance of recurves for a long time.
I have spoken
to some bowyers who have made this type of bow, though not so extreme as this
one, and they were all very happy with the performance. I think Tim Baker's
comments about stability and economy of materials are right on the mark.
Posted by:
Dale Yessak 07/19/2002, 09:44:57
Bede, it just
seems to me that if the wide-limb-thin-siyah concept had a significant
advantage that it would be seen in many more forms and in many more locations.
But my primary area of inquiry is the siyah angle of bows like this one and the
various Magyar-Hun-Avar reconstructions ... what performance advantage, if any,
would such low angle siyahs provide? Adam has stated that these might be more
stable as opposed to siyahs with TOO MUCH angle due to the latter's propensity
to twist. Perhaps any loss of performance due to a lesser string angle is
offset by the thinner tip's lighter weight. I don't know, it makes one wonder.
The main
reason I'm curious about this is because I made an osage-sinew recurve a few
months ago which I had problems with getting the string to track properly. As a
last resort, I added Asiatic style string bridges along with longer loops on
the string, and the bow became not only more stable, but also faster. I
realized that the speed increase was due to the string bridges effectively
increasing the amount of "string contact" with the limbs and also
because the bow, being braced about an inch lower, gained a little more power
stroke. But the whole experience got me curious about the mechanics of
recurve/siyah angle and the effect it has on the performance of a bow. The
Khotan and Magyar bows are examples of non-contact siyahs, and so my curiosity
was piqued to see if anyone knew if these bows actually represented less, more,
or equal performance advantages over other designs.
Posted by:
Bede Dwyer 07/20/2002, 23:26:34
You wrote
"it just seems to me that if the wide-limb-thin-siyah concept had a
significant advantage that it would be seen in many more forms and in many more
locations" which is pretty much what happened. Due to the use of bone and
antler laths for strengthening the siyahs, quite a few sets of these fittings
have been found in archaeological contexts from China to Roman Britain.
The ratio of
bending limb length to siyah length varies over time and space. The Roman laths
are very similar in size (some quite long pieces have survived in Europe and
China that can be compared). However, over time, siyahs became shorter and
solider. The angle at the base of the siyah changed from almost none in the
Yrzi bow to the pronounced curves in Avar bows from Hungary.
Adam comments
somewhere in this correspondence that the width of the limb was probably
related to availability of suitable timber and lengths of horn. Old Chinese
texts refer to the Northern Barbarians needing several strips of horn to make
their bows because of the lack of horns of suitable length. The
"Traditional Bowyer's Bible" points out how to make good bows out of
inferior timber by making the limbs flatter and broader than usual.
Since the late
1800s, people in the West have been interested in constructing composite bows
and now we have Internet forums like this on the subject. In the past on the
steppe and among its neighbours, bows were important military weapons, but
there was little thought of research and development (Dionysios of Syracuse was
an exception in the West). Detailed testing was not really developed. The ad
hoc testing method was flight shooting which is very old, but that soon turned
into a sport. Target competitions tested the bow and the archer together, but
that also developed into a sport.
Tribal groups
probably each thought their method of bow-making was the best and only changed
when forced to by external forces. In relatively modern times this happened in
Mongolia when it was absorbed into the Qing Empire of China. Mongol Banner
troops were made to use the Manchu bow and eventually stopped making their own.
Modern Mongol bows are a variation of the Manchu bow.
Large empires
like Rome and China could shop around for ideas. The Romans seem to have
adopted the most effective bow in use on their borders. The Chinese were in
such large scale production that military production probably had a life of its
own separate from regional styles.
When a bow
maker today makes a bow, he or she may easily incorporate ideas from different
style of bows. Sometimes I think you start making one type of bow and end up
with another. I don't think any of us knows all the answers so we are still
being surprised. Also I don't think there is a "perfect" bow. Instead
there are bows for different purposes: flight, target, hunting, war, and
practice.
Am I the only
one or is this discussion thread getting too large to keep track of all the
information?
Posted by:
Dale Yessak 07/21/2002, 23:10:46
Point well
taken, Bede. I'm not nearly so familiar as you with the development of
composites, and perhaps I've made an incorrect assumption.
There are
obviously many differing design factors in composites, just as there are in
simpler self and backed bows. I'm just trying to get a feel for them at this
stage.
The Poisson
Effect
Posted by:
Adam Karpowicz 07/12/2002, 08:00:26
Excellent
review of the bow, thanks. It is tempting to make a replica!
As to the
"spoon" shape of limbs, I understand you mean the working sections
cross-sectional shape with the concavity on the back side. This concavity (as I
think wrote to you before at one time) will occur naturally, once a flat-made
limb is flexed.
The
explanation is in the so called "Poisson effect", where the materials
stressed under tension will contract laterally, and stressed under compression
expand laterally (easy to demonstrate with a piece of rubber). This is exactly
what happens in this bow: the back side is under tension, so it contracts, and
the belly is under compression, so it expands. The net effect is the
cross-sectional concavity on the back, as we see.
The wider the
limbs, the more it will show. I made a bow at one time with limbs about 5cm
wide and only 6-7m thick (the proportion of width to thickness not too far from
this bow). The limbs were of course flat when made. Once strung and shot, the
bow acquired this feature immediately, and due to the natural creep in
materials, the limbs stayed like this in the unstrung bow. This distortion of
limbs is permanent.
There is no
doubt in my mind the Niya bow shows the Poisson effect very well.
Posted by:
Stephen Selby 07/12/2002, 08:43:19
That answers
one question, but raises another.
The 'Gansu
bow' is obviously a model. Unless it was once a working bow that was
subsequently stripped of its horn and sinew (which would be a bit bizarre),
then the poisson effect is present in the Gansu bow although that bow has never
actually been stressed in a composite form.
Might a bowyer
anticipate the poisson effect when carving the wooden core?
Posted by:
Adam Karpowicz 07/12/2002, 11:19:20
A strung (not
shot)bow will assume this profile too. The concavity will be more pronounced if
the materials are allowed to creep more, for example in higher humidity and
temperature. The Gansu bow could acquire the profile when left strung in the
grave.
This profile
is not normally noticed at the usual width of bows. Since you mentioned Tim
Baker, I recall having a similar discussion with him not long ago - he had not
been aware of this before. He then found such concavity in a 2" wide
self-bow with the help of a straight edge placed at the back across the limbs.
Posted by:
Pat 07/12/2002, 16:37:37
I have noticed
this effect in reverse while sinewing an Elm bow with pronounced reflex. I
strung the bow backwards and applied sinew. The belly of the bow took on a
concave profile temporarily. I'm sure the moisture from the hide glue aided
this. It was very pronounced. As the sinew matrix dried it no doubt underwent
shrinking across the limb which drew the belly back to a flat profile. After
shooting the limb was basically flat again. The limb was about two inches wide.
Pat
Posted by: Bede Dwyer 07/12/2002, 22:08:33
Thanks for the
explanation of the limb concavity. I have two questions:
Would a model
maker naturally just copy the concavity because that is what all the bows,
which he had seen, possessed? The artificial addition of the concavity in the
Gansu bow could be seen by examining the grain near the uplift, if it is
visible. The concavity on the Gansu bow seems more of a hollowing to me rather
than the lateral flexure apparent in the Khotan bow. Of course, I am relying on
photographs. Stephen is in the best position to check this.
Is the
convexity of the back in many later bows an over-correction for the Khotan
bow's concavity? I wonder if there is a connection, though I admit is very
farfetched at this stage. When you consider how close the Khotan bow is to the
Kum Darya bow and the Hungarian bows on one hand and the Yrzi bow on the other,
we may be looking at a missing link in the evolution of bow design.
Posted by:
Adam Karpowicz 07/13/2002, 06:19:10
It is indeed a
possibility that they copied the profile in wood, although I would very much
doubt it. There can be grain swirls in wood giving an appearance of such work,
proving this would be very difficult.
I do not see
any advantages from the mechanical point of view in the upturned edges of the
bending sections. The edges are under more stress this way. The over-correction
is quite probable, this would be done to compensate, as Pat said, for shrinkage
of sinew as well.
Why the broad
limbs?
Posted by:
Tim Baker 07/13/2002, 18:29:40
Adam, thanks
for the message.
Stephen, Adam:
Yes, level-3 deja-vu. Part of the thinking behind the TBB design was that more
reflex would store more energy, but that conventional-width highly reflexed
bows exceed the elastic limit of materials, take much set, and end up with less
efficient just-unbraced profiles, the solution being more horn-sinew to
properly hold that extra energy, and that this horn-sinew would have to be
added in width not length or thickness. The idea was that if thin/wide enough
the limb could hold a more than tips-touching just-unbraced reflex. But I
wonder if the Khotan bow was highly reflexed. It appears not to have had
string-liftoff siyah action, a low energy storage design. So possibly the wide
limb solved the problem of normal energy storage in low-elasticity materials—most
likely belly material. On the other hand, a no lift-off design would be more
stable, quite a valuable feature in the field, and if reflexed as per above,
stored energy could be very high, and stack still low. Of course there are
other possible explanation for the wide limbs. As for the back’s concavity: if
the limb was originally rectangular, which such wide limb must largely be, then
some portion of the spooning must be P-effect. Adam made me aware of this a few
years ago, and the most simple of tests demonstrate it exists on all near
rectangular limbs. But I imagine that most of the spooning here is due to sinew
contraction. I think the P effect can be overcome by appropriately convexing
the limb during construction, and/or possibly by inhibiting the effect with
bands of sinew running across the belly. Both ideas I believe Adam introduced
on another bow site. But even without such correction ultra-wide limbs work
well and safely--Pyramid shaped limbs four feet wide [plywood] for example.
I sure would
like to see someone make a more than tips-touching-when-just-unbraced composite
as per TBBlll, possible needing four or so inch wide limbs. I think it could be
the fastest natural-materials bow ever made.
Please set me
straight on any incorrect assumption or conclusion I might have made here.
Posted by:
Stephen Selby 07/19/2002, 22:42:02
Following up
on the comments made about the Khotan bow design reducing string-follow: at
Khotan, the maximum temperature range over a whole year would be -20 to +40
Degrees Celsius.
Posted by: Pat 07/13/2002, 22:34:38
If the bow
tended to hollow towards the back due to the P effect would the linear joints
in the horn strips not be prone to separating? Perhaps this would be more of a
problem if the limbs were severely reflexed. It is interesting that bows with
multiple horn strips do seem to exhibit a net reflex(handle setback and Siyah
angle) but the working section of the limb may have(deliberate?) "string
follow". This bow and the Persian bows in a past letter and Volume 2 of
TTBB being notable examples. Any thoughts if this may be the reason for the
lack of reflex in the working limb? I also wonder if the low reflex in the
siyah (likely no lift-off during draw depending on brace height) may be because
a very wide flat limb is quite prone to twisting. A highly reflexed siyah might
make the limb very unstable. I know the reflexed wood/sinew bows I have made
with a thin wide limb and siyahs are very "wobbly" (although I'm sure
horn added would make for a more stable limb). Still, wide and thin in any
material usually means it can be twisted and warped very easily. The bow shown
looks like it would be quite stable due to subtle modifications.
Posted by:
Adam Karpowicz 07/14/2002, 06:27:35
It is true the
horn strips are indeed prone to separating in bows with multiple strips on the
belly. Persian bows of this design were covered with sinew all around to
alleviate this problem. On the other hand, if the bond between the wood and
horn is good, the separations between the horn strips do not affect
performance.
I made only a
couple of bows with the strips so far and I am now sure the strips, as opposed
to solid horn, make the limbs more flexible (at the same thickness), so less
efficient. I also think the Khotan bow is overbuild, narrower limbs would be
much better for a bow this length, given the resiliency of sinew/horn
combination.
I believe the
deflex in the limbs is not deliberate in the Khotan bow. The limbs had to be
build in full reflex, possibly the bending sections were straight in this case.
The deflex came with use.
From my
observations, the "wobbly" problems with wide and thin limbs happens
only if the bending sections are very long. In this bow, they are relatively
short and I would not expect any such problems here. The short bending sections
plus the non-contact siyahs make the bow very stable.
Limb-to-Siyah
Transition Questions
Posted by:
Bede Dwyer 07/17/2002, 09:48:09
I have
attached a rough sketch made from the x-ray photograph of the siyah. It covers
the area from the small fracture in the core of the siyah to the point after
the siyah has joined the main limb.
I have guessed
that the belly side of the wooden core of the siyah was reinforced with another
wooden strip. I assume that it is broken up and has shrunken due to drying. In
the Qum Darya bow, the horn that covers the belly-side of part of the siyah
appears to be a separate, overlapped piece (but that is based only on a
drawing).
I have some
trouble interpreting the x-ray, but it was of immeasurable help in
understanding the photographs of the siyah. From reading the Traditional
Bowyer's Bible, I know that bows were made in America in the 20th century with
static recurves reinforced on the belly.
Stephen, you
are in the best position to comment on this as you have seen and handled the
bow. What do you think?
Posted by:
Adam Karpowicz 07/18/2002, 06:22:02
Bede, if I
understand you correctly, you believe the wooden reinforcement in the siyah is
in one piece (orange on the drawing). I think the broken belly fragment of the
piece is actually another, short piece of wood, glued onto the belly of the
core at the knee. It would be easier to manufacture, by gluing the first (long)
piece on the siyah, then filing it smooth and gluing the next to build up the
thickness there. Otherwise the bowyer would need to pre-bend a thicker piece of
wood, not an always an easy task, depending on the length etc. It is quite
possible both methods were used as well, maybe even on the same bow.
Posted by:
Bede Dwyer 07/18/2002, 20:36:34
I agree with
you, but I wasn't sure. I coloured the siyah reinforcements orange to
distinguish them from the core. I did not think they were pieces split from the
core by drying out of the wood, but I was not certain whether they were
individual parts because of the degree of break up of the pieces.
Posted by:
Stephen Selby 07/17/2002, 10:38:06
The bow is at
the museum undergoing further tests. This picture is all I have to hand. The
picture is a bit misleading: the darker brown wood has shrunk somewhat. The
extension of the horn in the limb, visible in the X-ray, is not apparent in
this view at all.
ENDS