Selecting Sport Kite Spars
Spar Comparison Table,
charts were updated on 26-Aug-1998.
This is a (pretty technical) article about sport kite carbon spars.
It was first published in Drachen Magazin 2/93. This is an update article,
which was published in DRAma 4/1997.
- Mark Cottrell, "Swept Wing Stunt Kites", 1990.
- David Lord, "Selecting spars for a new kite design", SKQ Vol. 3 No. 4, 1992,.
- Simo Salanne, "Mass-stab", Drachen Magazin 2/1993.
- Michael Graves, "Spars, Making your best picks...", KiteLines Vol 11.
No. 2, Winter-Spring 1995.
- Simo Salanne, "Die Qual der wahl", Drachen Magazin 4/1997.
Selecting Spars for a Sport Kite
DRAma published my spar article first time in 2/93 issue. It
was followed by one updated spar table. Since that a lot
of new spar types have been introduced to the market - and
many has disappeared. It's time for a refresh. We also
publish a revised text, explaining the concepts and the use
My experiments, described in this article, were inspired by
David Lord's article in Stunt Kite Quarterly (not published
any more) Vol. 3 No. 4, 1992, "Selecting spars for a new kite
design". Dave had measured spar deflections and calculated
the relative stiffness of several kite spar types used in
U.S. I used similar setup for my measuments of common
European spar types and kept the results compatible. Later
Michael Graves continued spar studies and published the
results in KiteLines, Vol 11. No. 2, Winter-Spring 1995.
In my original setup two spars were placed on a table, under
a weight, and a normal 330 cl (360 g) beer can was hanged on
the other end of a spar in my interest at 0.6 m distance
from the edge of the table. The deflection was measured
between the spars. I measured at least five samples of every
spar type and calculated the average deflection.
In the present setup the spar is supported on two points and
the load hangs in the middle, where the deflection can be
read from a millimeter scale. Distance between the
supporters is adjustable 60-80 cm and the weights in use
are 500-2000 g. This variable setup gives more accurate
results as each spar type can be loaded to deflect similar
to one in a flying kite. The measurement setup parameters,
spar names, deflections, weight and diameter information are
entered in a computer, which calculates the final comparison
Originally Dave Lord selected Classforms K75 (glassfibre) as
the refence spar material. K75 was then used e.g. by Top of The Line
to frame Spin-Off and Hawaiian Team kites. The refence spar is
used as the base to which the other spars are compared, that's
why we speak about "relative" stiffness and weight. Actually the
refence spar could any spar type, but it makes things more
concrete if it's widely used and known. In 1992 Spin-Off and
Hawaiian were THE refence kites, others were compared to.
I have not seen or heard about a new sport kite using K75
frame for many years. It's time to change the refence to a
more contemporary one!
I selected the new 6 mm Exel Extreme as the new refence. Because:
- it has the same relative stiffness as the K75, stiffness
numbers of other spar type don't change.
- I'll predict it will find it's way to number of kites
made by many manufacturers.
- as 6 mm pultruded carbon, it represents the class of
spars most widely used to frame sport kites today.
- relative weight values will change, but new ones reflect
better a typical carbon tube instead of glassfibre.
Exel Extreme is a new product coming to the market in spring
1997. It has improved strenght against impact forces. Thus it
is well suited for trick kites. Unfortunately my laboratory
methods don't measure the robustness of the spars.
Stiffness vs. Flexibility
The relative stiffness is the deflection of the reference
spar divided by the deflection of target spar. E.g. relative
stiffness of Beman Carbon Strong 14 is 0.75. It means it
bends 25% more than Exel Extreme 6. Usually a stiff spar is
more desirable than flexible. It is easy to interpret the
relative stiffness number as "more" means "better". This
works particularly well in the bar diagram, where relative
stiffness is combined with relative weight. Weight is
naturally considered a "less" means "better" matter in a
kite. In the diagram the difference in the height of relative
stiffness and relative weight bars gives an overall
indication of spars characteristics with respect to these
Selecting a replacement spar.
Let's take an example: you break an AFC2300 spar, you do not
have any spare stick around and your kite shop can't keep
all spar types in stock. In the table you can see that both
Exel RCF-6 and Avia 2200 have almost the same stiffness,
they weight about the same. Exel is 0.1 mm larger in
diameter and Avia 0.2 mm smaller. If Exel doesn't fit in the
connector you sand it a little bit. Avia needs some tape
around to fit snug.
Dave Lord's Scale Factor.
Dave has developed a scale factor, which helps you to scale
kite designs. Let's suppose you have a Speedwing which have
RCF-6 frame. You decide to build 25% larger Speedwing having
similar charasteristics. 25% means the leading edge will
1.25 times longer. From the table you will find that RCF-6's
scale factor is 0.96. Calculate 1.25 x 0.96 = 1.2, which is
the scale factor of the spar you need for the larger
Speedwing. From the table you will find that SkyShark 5P,
Exel RCF-8 and Beman Ultra Light 21 have scale factor
practically close to 1.2. Depending on the availability,
price and compatibility with the connectors you have, you
can select any of them for your larger Speedwing. They all
will result to a frame with similar bending charasteristics
as you have in your current Speedwing.
Other way to work it out, is to study the table and then
size your new kite based on particular spar. Example: you
decide to use 5 mm Exel RCF-5 to build a smaller Speedwing.
How large should it be? You take RCF-5's scale factor
divide it by the scale factor of RCF-6: 0.84 / 0.96 =
0.87. This means the small one should have a leading edge
0.87 times the lenght of your current Speedwing.
The scale factor can be
from the formulas used to
calculate deflections of loaded beams. I bypass the theory
and just give the relation of scale factor S and relative
S = R to the 1/4 power
R = S x S x S x S = S = S to the 4th power
Both the bar and radar chart have same spars but due to
scaling and space required, only the middle section of the
table is included in the charts. Spars are identified in the
diagram by abreviated names, for the full names see the
The Radar-chart diagram represents relative stiffness and weight in
graphical (and very compact) format. The spars are sorted by increasing
Relative stiffness does not tell anything about the "strength",
"durability" or "robustness" of a spar. A spar with a good relative
stiffness might break in use more often than another with a smaller
relative stiffness. In general wrapped spars stand better
axial hit loads, e.g. wing tip and globe interaction, than
pultruded spars of similar weight.
One important characteristic of a spar is the price. Wrapped
spars are in general more expensive than pultruded.
Pultruded spars are available in more variety of diameters
and lenghts. A single piece pultruded leading edge can be
more durable than wrapped two piece and ferrule
I have done hundreds of measurements and entered the values
into a computer. I have checked things out, but there might
still be some mistakes. Another possible source of deviation
from similar charts or tables is that manufacturers have
made changes in their products or product names.
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