| Materials
Polyesters
Examples:
Dacron, Fortson
Although there
were experiments in the 1940's and early 1950's with a polyester
called Fortson, and even with nylon (which must have produced
something more like a catapult than a bow!), Dacron was the
first artificial fiber to be widely used for bowstrings.
It was introduced in the late 1950's, and became the standard
almost immediately, as it was such a huge improvement over
anything else available at the time. It is still widely
used today on training bows and older compounds. Newer versions
such as B75 Dacron are available, but have little to offer over
and above the original.
Advantages:
- Cheap
- Safe on any
equipment
Disadvantages
- High creep
- High
elasticity
- Very
variable with changes of temperature and humidity
- Low breaking
strain
Breaking strain
per strand: B50 - 50lb; B75 - 55lb.
Liquid
Crystal Polymers (LCP's)
Aramids
Examples:
Kevlar, Twaron, Technora
These LCP's
were important in their day, especially Kevlar. They still
have important uses outside of archery, but have been replaced
for our purposes by newer, more reliable fibres. There are
still plenty of spools of Kevlar and other aramids knocking
around in cupboards and tackle boxes, but they should not be
used. Even when new they have a short life - often as low
as 1000 shots - and tendency to break without warning.
After a few years storage, especially in sunlight, they could be
positively dangerous.
Vectran
Like Kevlar,
the LCP called Vectran is prone to sudden failure and should
never be used on its own, especially on compounds.
However, Vectran is an important component of most Composite
Fibres, such as the BCY 450 family (see below). It is
heavy and therefore comparatively slow, but it has the lowest
creep of any fibre yet in production - virtually zero under
archery conditions.
Incidentally,
it is often stated that Vectran and Vectran-containing
composites need a special wax to protect them, but according to
BCY, the manufacturer, although there are advantages in using a
wax containing silicon, this is not the case - 'any wax, natural
or artificial will do equally well'.
High
Modulus Polyethylene (HMPE)
Examples: Fast
Flight, Dyneema, Spectra, Dynaflight 97, BCY 8125)
Occasionally
also called High Performance Polyethylene (HPPE) or High
Tenacity Polyethylene (HTPE), this is currently the most
important family of bowstring fibres. HMPE fibres are the
standard for high performance recurves and are used on most
compounds either alone, or as part of composites.
As far as I can
discover there are only two basic fibres: Spectra and Dyneema.
These are manufactured under license in various diameters and
with slightly different finishes to produce a number of
different brands of string material. Probably the best
known HMPE material, at least in the UK, is Fast Flight, made by
Brownells, out of Spectra. The Dynaflight family and,
logically enough, the various types of Dyneema are made out of
Dyneema, principally by BCY in the US and Angel in Japan.
Angel Dyneema has something of a cult following, especially
among recurve archers, but I have no personal experience of it.
Advantages
- Extreme
strength and durability
- The lightest
(ie fastest) materials of all
- Very low
elasticity
- Moderate
price
Disadvantages
- Higher creep
than Composites (but much lower than Dacron).
Breaking strain
per strand: Fast Flight - 95lb; Dyna Flight/Dyna Flight 97
- 120lb; Angel Dyneema; 110lb
Composite
Fibres
Examples: BCY
450 Premium, BCY 450 Plus, BCY 452, Brownells S4,
Ultracam
The definitive
modern fibres for compound bows. They are all mixtures of
Vectran for creep resistance and HMPE for strength and
durability. For example, BCY 450 Plus is two-thirds
Dyneema SK75 and one third Vectran. For practical
purposes, these materials do not creep at all. A new
string, or one that has been off the bow for a while, will
usually creep slightly as tension between the strands evens out,
but should quickly settle in. None of the creep is due to
the materials themselves - see the section on settling
in, below.
Advantages
- At archery
tensions (and far beyond) zero creep
Disadvantages
- Reportedly
slightly slower than the best HMPE
- Expensive
Breaking strain
per strand: All are in excess of 150lb (but note the new,
thinner strands now becoming popular).
The
Future
At the moment
(May 2003), HMPE and composites continue to rule the roost, as
far as compounds are concerned, and I have not located any radically
new fibres on the horizon. If anything, single fibre
HMPE's are making a comeback with materials like BCY's 8125.
It is always a
dangerous thing to say, but I think that, unless a whole new
technology comes along, the development of new fibres for
bowstrings is getting close to its end: strength, durability,
creep and elasticity have all been brought to a point where they
are simply not an issue any more, and only price remains.
As a reaction, it seems to me that the manufacturers are now
going in for tweaks and cosmetic changes. For example,
there is a trend towards ever thinner strands, which is
interesting, because a couple of years ago it was for ever
thicker strands, culminating, as far as I know, in a proprietary
string from Martin with only 4 strands. The justification
for thinner strands is that it is easier to get an even tension
throughout the string, which is interesting, because that was
exactly the justification for thicker strands - ho hum!
Serving
Threads
Nylon
Not long ago,
practically all serving threads were nylon, and it is still
widely used. Nylon serving threads have one advantage over
anything else yet produced: they grip the string well and
are much less prone to slip or open out than the more modern
fibres. Quite a few compound archers, even those shooting
high performance bows, stick to nylon for this reason and accept
that they must check the condition of the servings frequently
and replace them when necessary.
Soft-twist
nylon is the cheapest. It is perfectly adequate for any
end-serving that is not subject to heavy wear, which means it is
fine for recurves and could be used on some end servings for a
compound. It is not recommended for centre servings, as it
wears through too easily. Braided nylon is similar to
soft-twist, but it is a little easier to work with and makes
what some people consider to be better looking servings.
Its main practical advantage is that it is thinner than standard
soft-twist and fits better into cam grooves.
Monofilament
Nylon
Monofilament
nylon is quite different from the twisted or braided varieties.
It is useless for end-servings as it would slip too much, but it
is ideal, almost perfect, for centre servings. It is
smooth, giving a clean, fast release for finger shooters, it
forms a screw thread that can be used for accurate nocking point
adjustment and, above all, it is available in many different
sizes, to give the best possible nock fit. The standard
sizes available from archery suppliers are 0.015",
0.018" and 0.021", but many more sizes are available
from fishing tackle shops as fishing line, usually in the 15 to
25 lb breaking strain range. I have often been told, and
seen it written in several books and articles, that fishing line
is not suitable for archery purposes, but I have used nothing
else, on recurves and compounds, for over 15 years and have had
no problems whatever. One important point to watch is that
monofil weakens and becomes brittle with age. If you find
that servings are breaking inexplicably, often within 24 hours
of making them, this is probably the reason.
HMPE
Serving Threads
Pretty well all
the HMPE brands are available as unwaxed serving threads as well
as bowstring threads. Most are twisted and are intended
primarily for end servings. Their wear resistance makes
them particularly useful where the serving goes over a cam.
Some are braided and intended specifically for centre serving.
Angel Dyneema has a strong reputation as a smooth, strong centre
serving, as does Halo, which is braided Spectra. However,
both are very expensive compared to monofilament nylon which I
consider to be unbeatable in this role. The main
disadvantage of all these materials is that they tend to slip
and open out on the string. In general, the thinner the
thread, the better it will grip, and some, such as BCY black 2D
thread are covered with a light adhesive coating to reduce this
problem. In practice, it does not matter too much if the
serving opens out a little as the string or cable strands are
still well protected and HMPE fibres are so strong that I am not
even sure that serving is necessary for protection, although it
plays a vital role in preventing the string from coming apart!
It is worth remembering that servings can open out because they
are too tight as well as too loose. As with many things in
archery, this is a matter of experiment and experience.
String
Making - Miscellaneous Jottings
The general
method of making strings and cables is too well documented to go
into in detail here, but there are a few points of interest (and
debate) that are worth considering.
NOTE for
sailors and others used to handling ropes: You might find that
some of the discussion and advice sounds back-to-front (eg
serving and twisting directions). The reason is that
conventional bowstrings are not laid-in, they are simply
twisted. In particular, the age-old rule to serve against
the lay is not infringed: when you serve against the lay you are
serving with the twist and that is what is recommended
here.
How many
Strands?
The most
important consideration when deciding how many strands to use in
your strings and cables is safety. The second
consideration, a long way behind, is nock and/or cam groove fit.
However, as we shall see, with conventional equipment and
materials, safety is not an issue.
Why is safety
not an issue? To illustrate, calculate the minimum number
of stands you can safely use. To be very safe, use a
safety factor of 10 x Peak Weight as a minimum. This is
more than adequate to take into account things like the 'jerk'
of the string at the end of the power stroke, and the inevitable
weakening of the fibres with age and use (remember - you
must always change your strings and cables at least once a
year). For example, for my own bow set to 52 lb peak
weight, using 450 Plus, gives a minimum of 52 x 10 / 150 »
4 strands. For a very heavy hunting bow, with a 100 lb
peak (!!) the answer is still only 7, and it is virtually
impossible to get any sort of decent nock fit with less than 12.
In fact I use 14 - a safety factor of 3500%.
So, in
practice, provided you are using HMPE or composite fibres, the
only thing to worry about for strings is nock and groove fit.
For cables, I simply half the number of stands in the string and
round up if necessary - eg 8 strands for my own bow. This
is probably over-kill, but it is very safe and has no unwanted
side-effects.
If you are
using Dacron for your strings, you are still very safe indeed if
you let nock fit dictate. I presume that no-one uses
Dacron for cables.
To
Serve Or Not To Serve...
It is not
necessary to serve end loops on any modern material, even
Dacron. You may like the look of served loops or feel
uncomfortable about leaving them bare. If so, go ahead and
serve them, but it is not necessary. I have never served
any loops on any of my compound strings or cables and have never
had any problems with wear or fraying.
Measured
Length Versus Final Length
It is important
that your string or cable is made to the correct length, and the
only way to achieve this is to make careful measurement when you
set up your jig. But there is a problem: whatever jig I have
used, whether metal or wood, home-made or factory-made, the
length of the finished string is always different from the
measured length of the jig. In my case, it is always
shorter, but I know of experienced string makers who find it to
be longer. I presume this has something to do with the
strands stretching or the jig bending, but whatever the reason
this factor must be taken into account whenever you are making
strings or cables. For me, a good rule of thumb is to add an
eighth of an inch, but it is something you'll have to work out
for yourself. If in doubt, err on the long side, as you
can shorten strings by twisting, but there is no way to lengthen
them.
When
Should I Put On My Centre Serving?
Put on the
centre serving when the string is stretched on the bow but before
it is twisted. It is fairly obvious that if you put on the
centre serving when the string is loosely stretched on the jig,
the serving will also be far too loose. It is also a good
idea to put on the serving before the string is twisted. This
way, when the twists are put in, the serving will be jammed in
place, as the twisted part of the string has a greater diameter
than the un-twisted part. I have always used this method and I
can honestly say that in nearly 20 years I have never had a
centre serving slip or open up. Some people seem to worry that,
by serving before the string is twisted, the twists will be in
some way uneven above and below the serving. Well, they will be,
of course - they will be in proportion to the length of the
upper and lower part of the string, which is precisely what you
want. Incidentally, make sure the serving is put on in the
correct direction - see below.
How
Many Twists Should I Put In My String?
There are three
reasons for putting twists in a string or cable. Firstly twists
stop the string opening up and thrashing around during the power
stroke. Both Hoyt and Beiter produce videos that show this
effect clearly. Secondly, they help even-out of tension between
the individual strands. No string is ever perfectly tensioned
and this function is important. Finally, as everyone knows ,
twisting a string or cable allows fine adjustment of length. So,
it is clear that you must have some twists whatever material
you're using.
The general,
manufacturers recommend around one twist every four to six
inches - say 12 to 15 in an average compound string.
However a number of top archers recommend that you use far more
twists than this - usually around 1 twist-per-inch. This
is said to increase speed and help consistency, and is the
policy I have used for several years now: I have approximately
50 twists in the 54 inch string on my Max 2000 at the moment.
The upper limit is determined by the point at which the string
starts to twist and kink. This sounds a bit vague, but in
practice, like so many things in archery, you will know it when
you see it. The upper limit for my Max seems to be about
80 twists. Not that I would choose to shoot with this many
twists - positioning the peep sight would be quite a challenge,
for a start - but I could if I had to.
Does
It Matter Which Way The String Is Twisted?
Yes, I believe
that it does. All threads are twisted during their manufacture.
The terminology confuses me and I can never quite sort out
right- and left- handed twists and, of course, clockwise and
anti-clockwise depend on which direction you are looking.
So I will stick to referring to the 'standard' direction, as
shown below. As far as I know, all archery threads, no
matter what they are made from, or who they are manufactured by,
are twisted in the 'standard' direction* and any bowstring made
from them must be twisted in the same direction. If it is
twisted in the other direction its threads will open out and be
weakened. To be honest, I am not sure how much this matters with
modern materials, but it is a simple rule to follow and one that
I never break.
*
The only threads that I know of which are twisted in the other
direction are those which form one half of the 'Zebra' strings
made by Matthews.
Does
it Matter Which Way the Serving is Turned?
Again, I think
it does, but there are two opposite schools of thought, three if
you count the one that says its not important at all. One
view is that you should serve the string in the opposite
direction to the way the twists will be put in. So,
assuming that you will twist the string in the correct direction
in the first place, the serving tool would spin clockwise,
looking in the direction of its travel. The logic of this
is that when the twisted string is put on the bow, the twists
tend to undo, and the clockwise serving will be tightened.
I understand
this argument, but I don't agree with it. I have found in
practice that servings put on in the same direction as the
twists (ie spinning the serving tool anti-clockwise, looking in
the direction of its travel) stay tighter and are less prone to
opening up or slipping than the clockwise version. This is
particularly true of centre servings.
So,
to summarise, my personal recommendation is: looking down the
length of the string, twist the near end of the string clockwise
and serve it anticlockwise so that the strands, the string and
the serving all tighten in the same direction.
Tying-off
the String
This apparently
simple task is a matter of heated debate among the archery
fraternity and I know otherwise sane people who have almost come
to blows over it! Some people recommend tying the strands
in a knot, others do some kind of over-wrapping or pleating,
while others yet simply rely on the serving to hold the string
together and stop the strands slipping. Personally, I
suspect that the serving alone is good enough, provided that the
length trapped under it is long enough (say 3 inches or more,
but that's just a guess). However, I prefer the
belt-and-braces approach. I don't tie any knots, because
it is very easy to see that simple knots do not hold in waxed
bowstring threads: if they hold firm, then they weren't needed
in the first place. I criss-cross the loose ends over and
under themselves and the string at least three times and then
serve over the resulting trellis-work (see diagram). This
may not be strictly necessary, but it definitely works.
For cables, I start the over-and-under work straight away, just
below the loop. For strings, where the served end goes
over a cam I don't start the until I reach the part where
the serving is beyond the cam. This is because the
over-wrapped part of the string can be too thick for the cam
groove and be subjected to excessive wear.
How
long does it take for my string or cables to settle in?
Another area of
hot debate! With modern materials most, if not all the
initial creep of a new string is caused by uneven tension on the
jig, or by slippage caused by poor finishing, not by elongation
of the strands themselves. Go to as much trouble as it
takes to ensure that the strands are evenly tensioned and
properly finished and you might be pleasantly surprised at how
little creep there is. My experience is that if a
composite string or cable creeps after the first 100 or so
shots, maybe 200 for pure HMPE, it is because of the way it is
made, not because of the materials it is made from.
I don't expect any detectable creep on my own BCY 450+
cables or strings after the first dozen or so shots, and am
rarely disappointed. I have seen too many archers moaning
that their strings and cables 'still settling in' after half a
season when in reality, they are not settling in at all, they
are coming apart! And, I might add, some of the worst
culprits are commercially made strings and cables.
Apparently one major manufacturer advises that it takes
1000 shots or 1 dry fire(!) to settle in a new string or cable.
This is nonsense, driven more, I suspect, by the litigious
nature of the American legal system than by technical reality:
if your string or cables are still creeping after a couple of
hundred shots, replace them with some properly made ones before
they come apart and do serious damage.
Bibliography
The most useful
guides to bowstrings are :
-
Tekmitchov,
George, String Materials: More than You Wanted to Know,
Archery Focus, July/Aug 1998.
-
Serralhiero,
Pedro, Making Bowstrings Parts 1, 2 & 3, Archery Focus,
Nov/Dec, Jan/Feb and Mar/Apr 1999.c
-
Stamp,
Don, Field Archery - now out of print and I've lent my copy
to someone, but this is the book I learned string making
from twenty years ago, and I still more or less stick to its
advice. If you want a copy - and it is a very good
introduction especially for recurve shooters - try http://www.abebooks.com
-
The
BCY website - http://www.bcyfibers.com
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