Weak Links - 70 Pack
ALL OF OUR WEAKLINK LINE IS NOW SOLD PRECUT IN 20" LENGTHS AND MUST BE TIED INTO A DOUBLE GRAPEVINE KNOT BEFORE USE BY THE END USER.
150 pound test (white with a blue striker)
200 pound test (white with an purple striker)
250 pound test (white with a red striker)
300 pound test (white with a black striker)
Any sane tow operation will
tell you that you ALWAYS should have the following items when you elect to go
A suitable weak link.
A hook Knife.
Glasses (preferably with unbreakable lenses).
The most important of
course is the last item, and hopefully the winch operator will possess one as
Weak links are one of the most important safety features that can be
applied to tow launched flight. There are arguments for and against their use,
but the most compelling arguments are for their obvious safety benefit.
A weak link acts primarily
as a safety fuse to prevent overloads from being applied to the towed pilot.
Under ideal circumstances, the tow operator could foresee every possible
development that could occur and react instantaneously to prevent any type of
overload from occurring. The real world doesn't work that way. Lines jam, skip
off pulleys, get run over by other vehicles or animals, tension control systems
fail, operators and pilots alike make mistakes, and meteorological conditions
conspire to occasionally overload the system. A properly designed weak link
will blow out before any excessive forces can be transmitted to the pilot or
their glider. One weak link does not work for all pilots. Most reliable sources
believe that a weak link should be sized so that it breaks at 75% to 100% of
the in-flight load. Light pilots need weak links that break with a lower force
than heavy solo or tandem pilots. Wrapping a shoelace and tying a knot is
perhaps better than nothing, but very few pilots actually have any idea at what
force their links will break. Tying a weak link has always been the way most
people get by. But when you take some 200 pound test line and tie a knot, at
what force does the link break? Different knots, how the knot is tied, and even
the length of the line used to tie the knot greatly affect how the link will
If you want the absolute
most consistent link possible, you get that by sewing eyes in the ends of the
weak links. Even for an anal type like myself, this is a wee bit too much. We
gave up after sewing a couple thousand weak links. It’s just not worth the
extra effort required.
If you look at the failure
mode for ANY type of line you will note the following. The line will stretch
elastically – It will begin to elongate as the load is applied. If you remove
the load, the line will recoil to its original length (with the exception that
it will be a wee bit longer since you have settled the fibers into the weave) .
If you further increase the load it will deform plastically, meaning it will
stretch and when the load is removed it will stay stretched to a great extend.
If you continue to increase the load, the line will break at its weakest point.
This will ALWAYS be at the knot unless you have another defect in the system.
The line is restrained in one direction so it can only stretch one way, and as
the fibers pull the knot tighter the friction at the knot is increased, heat is
generated, which further weakens the line material and the fibers break/melt at
the knot to line transition.
If you load the line to the
point of failure, you will see the link elongate 60- 100% before it ultimately
fails. On our systems, we use that knowledge to our advantage. We use a small
1/8” diameter S/S quicklink to attach the weaklinks to our leader line. We
typically mount 2 weaklinks on the link simply for comparative analysis. When
you hook up to the line, you check the links. They should both be the same
length. If one is appreciably longer, it means that it has been overloaded
during the last tow to the point where it plastically deformed and has already
been weakened. We cut it off, use the other link, and it’s replaced when the
line is handed off for the next tow. THE USE OF THE QUICKLINK IS REQUIRED to
achieve a consistent breaking strength. Some operators simply girth hitch the
weakling k to the end of the towline. This essentially creates 2 load paths,
and doesn’t allow the link to slip and equalize the load over the entire link.
It also means any jerking motion from the tow is absorbed by the weaklink
sawing through your tow bridle closure loop rather than simply rolling around
the smoother quicklink.
I frankly could care less
where the knotted part of the link goes, but generally stick it so it’s halfway
between the quicklink and the bridal release loop. I suppose placing it
elsewhere might generate a differential breaking force equivalent to
47,638.2586 gnat farts (roughly) but You would have to perform your own
comparative analysis to verify this.
Tied Weak Links - If you are unsure how to tie your own weak links, we have
provided a pictorial set of instructions below on how to tie a double grapevine
THERE IS A HUGE
MISCONCEPTION IN THE PILOT COMMUNITY RELATED TO HOW STRONG A TIED WEAKLINK
ACTUALLY IS IN PRACTICE! KNOWING WHAT LOAD YOUR WEAKLINKS WILL WITHSTAND BEFORE
THEY BLOW OUT IS CRITICAL TO FLIGHT SAFETY. A WEAKLINK THAT BLOWS OUT
PREMATURELY CAN IN MANY CASES BE WORSE THAN USING NO WEAK LINK AT ALL!
A popular misconception
uses a very common 135 pound test green spot trolling line. In fact the myths
associated with this product are so strong we no longer carry it. If you wish
to obtain some, it can be easily found at any decent fishing store as Dacron
Trolling Line made by the Green Spot company. Here's how the myth goes - Look
at the knot tied link shown in the picture above. Many, many pilots believe
that if you take a 135 pound weak link line and tie it in a loop that it will
break at 270 pounds. The logic is that there are 2 parallel load paths sharing
the load, each one capable of withstanding 135 pounds. They miss the fact that
any properly tied weak link will ALWAYS fail at the knot. The
line is pulled tighter and tighter at the knot and should break right at the
knot every time. Depending on the knot used the line will break at 40-60% of
its rated strength. Typically a weak link tied with a double grapevine will
break at slightly over the rated strength of the line. The only way to tell for
sure is to break test it with a calibrated tester. There have been many
inadvertent weak link breaks caused simply because pilots ASSUMED that
the link was stronger than it was. This has led to the asinine belief that if
one loop is good 2, 3, or 4 is even better.
In use the weak link
material will stretch well over 60% in length before it fails at the knot.
Adding a multitude of loops makes it impossible to spread the load evenly, and
the link will likely bind up causing it to fail in an often unpredictable and
unrepeatable manner. To ensure consistent breaking strengths, stacking equal
length weak links works, using a single appropriate strength link is the best,
and multiple wraps should never be used.
We currently stock pre-cut
and marked weak link line in 150, 200, 250, and 300 pound breaking strengths.
150 pound test (white with
a blue striker)
200 pound test (white with
an purple striker)
250 pound test (white with
a red striker)
300 pound test (white with
a black striker)
If you are a paraglider pilot we are not very fond of anything under the 250# weak link material. In theory, the 150 or 200 # material is adequate for lighter pilots. In practice there are some conditions where this link will blow out prematurely. For example it's not uncommon the launch a pilot behind a boat, and as the pilot clears the tree line they hit a wind shear or a thermal. You can still see the bow in the line and the winch is showing 70 pounds or so of tension. As the pilot hits the shear they actually pull on the line from the pilots end and their weak link can blow before the line comes anywhere near tight, giving the winch a chance to respond. We generally use the 250# (red and white) links for solo pilots, and the 300# (black and white) for tandems. Some purists will say that the 250# link is to big for a solo pilot. My answer after towing thousands of pilots is that even a 90 pound pilot on a tandem weaklink can tow to maximum height where they run out of line and blow out the link flying hands off. There is a minor surge, but nothing dramatic happens to the glider. It typically won't surge far enough to come even close to a collapse. When the same lady trips on takeoff and falls, the tandem link blows out immediately. On the flip side, it's pretty rare to see an inadvertent weaklink break with a solo pilot on a 250# link.
If you happen to tow with
us on one of our towboats, you would notice we almost always have a pair of
links tied from 250 pound test line and another from 300 pound test line. The
smaller links break at around 264 pounds and are used by almost all solo
pilots. The larger link breaks at around 384 pounds and is used for tandems.
The links are all the same length when tied which we use to tell if a link has
had a transient overload applied. Let’s say on the previous tow the pilot had
an overload situation occur that dramatically stretched the link, but didn't
cause it to blow out. This will be obvious when the next pilot is hooked up for
the tow, as the overloaded link will be an inch or 2 longer than the others. We
simply cut it off, use the other link for the tow, and replace the link when
the drogue is rewound for the next tow.
We no longer carry any weak
link material in bulk spools for a couple reasons. The plastic spools used to
hold 100’ of weak link material were prohibitively expensive, and shipping
costs greatly exceeded the cost of the material. Certain well meaning but
clueless operators devised ingenious release mechanisms that threaded 15 - 20
long sections of material through a set of pulleys. In order to function
properly, the entire link had to unthread through a series of pulleys. To add
to the odds of it malfunctioning they added several wraps to the line. I’ll
freely admit that risks of selling the material greatly exceeded any potential
gain. None of our systems can achieve more than 300 pounds of line tension, so
we no longer carry links beyond that rating.
ALL OF OUR WEAKLINK LINE IS NOW SOLD PRECUT IN 20" LENGTHS AND MUST BE TIED INTO A DOUBLE GRAPEVINE KNOT BEFORE USE BY THE END USER.
It’s possible for us to put
up custom calibrated lengths of material with a 6 - 8 week lead time and a
minimum order of 3000 feet. Contact us if you require more information.
Leader Lines are typically short lengths of spectra with sewn eyes on both ends. Typically they are connected between the apex of the drogue chute and the weak link clip in link. Leader lines serve 2 functions. Primarily they are used to add space between the drogue chute and the pilot. In the event that tow tension is reduced in flight, the drogue chute will inflate. The use of a leader line ensures this happens well in front to the pilot, rather than in the pilots face. We typically use a leader line of lower breaking strength than the towline and use it as a "backup" weak link for those applications where a pilot elects not to use any type of safety link. Our sewn leader lines are available in 6 - 8 foot lengths in 825 Lb. breaking strengths.
End of the Towline - How to Hook up your Drogue, Leader and Weak Links
Several people have asked us for a set of drawing or images on how we hook up a pilot to the end of the towline. There are lots of different ways this can be done, and a few of them are really bad ideas. If you look at the end of either of the towlines coming off our towboat, you will see the same setup used. The following text describes how we do it, why we do it, and perhaps, some things that don't work so well.
It all starts by attaching the drogue to the end of the towline. There are lots of drogues that can be used. For simplicitys sake we really like the round vented drogue. It works perfectly with our winch. It provides the appropriate amount of drag to allow us to rewind our line with a tight stack on the spool, and it’s very reliable. We like to splice the line where it attaches through the parachute shroud line loops. Some people like to splice a quick link into the end of the line. You either have, or you will slam the drogue right through the tracking head on a rewind that comes in faster than planned. It’s much less damaging to smash a bunch of line into the tracking head, than a metal link. If it happens to nail you in the head during a rewind, you’ll appreciate NOT having a link there as well.
Here’s a picture of Victoria splicing a new drogue onto the end of the tow line. Thread the line into a fid (We’re using a #5 fid for our standard 1100# towline). Thread the fid and line through all 12 loops, and pull out 3 to 4 feet of line. Insert the fid back into the center of the line and feed it through until the loop is closed. Pull the fid back out of the line and smooth the line out. You should end up with a very neat installation with no knots. To remove the drogue if needed, you simply work the splice loose and you can pull it apart. In use, the harder you pull, the tighter the line grips. You can make a much shorter splice (around 12 inches or so) if you want to stitch the line so it can’t unthread accidentally.
If your drogue was supplied with a tie strap to keep the loops tidy and in order, you can cut it off now, since it’s no longer needed.
That takes care of attaching the drogue to the towline. Now you need to attach the pilot to the end of the drogue. Many pilots simply attach their weak links to the drogue. There’s nothing wrong with that system, except that if the line tension is reduced inadvertently during the tow, the drogue is going to inflate in the pilots face. To an experienced tow pilot it’s no big deal. To a novice tow pilot, it can be a traumatic experience and the potential for a pilot induced loss of control is pretty high.
We prefer to attach a short leader line to the end of the drogue for a couple reasons. First, is the simple fact that it gets the pilot away from the drogue, so an inadvertent opening is no big deal. Secondly, there are a few pilots who steadfastly refuse to use weak links for various reasons. We use a leader line that breaks at 865 pounds, so even if a weak link isn’t used, in the event of a serious over tow situation, the leader line will break, preventing a line break that might pose other, and significant issues.
To attach the leader to the drogue, we use a Stainless Steel 3/16” quick link. Loop the link through all the webbing straps at the apex of the drogue, and girth hitch the leader line through the other end.
If you unsure what weak links are for, please refer to our weak links page. You can also refer to that page to learn how to tie the double grapevine knot.
The best way to secure your weak links to the leader line is to girth hitch a small 1/8” Stainless steel quick link to the end of your towline. DO NOT use a plated steel one found in the typical hardware store. Stainless links have twice the tensile strength for the same size, and they don’t corrode or rust causing sharp edges that will damage your weak links. Some people like to skip the quick link altogether and simply girth hitch the weak links to the end of the line. THIS IS A MISTAKE for a couple reasons. When girth hitched, the link will never stretch evenly, so it will blow out with inconsistent results. Since the link isn’t free to rotate around anything, any pulses, or surges during the tow will actually occur at the interference point between the weak link and the closing loop of the tow bridle. In just a few tows, you will saw through the closure loops of your tow bridle, especially if you are using something like a friction style payout winch that isn’t know for the smooth tows you experience on a TowMeUp.com hydraulic system.
The quick link makes it simple to attach weak links to the leader line, and they are readily replaced. A keen observer will note that we almost always have a pair of solo links, and a tandem link attached to our quick link at all times and have asked why we do this. The reason is simple, and easily understood if you understand how weak links typically fail.