Introduction:
This paper by Andy Wardley describes the technique required for constructing an Active Bridle.
Elements of an Active Bridle:
The Active Bridle can be considered to be a static bridle with two extra elements added. These are known as the "Stabiliser" and the "Activator" legs.
The Stabliser makes a connection between the upper leg and the lower legs and is shown highlighted in red on Diagram 1. The function of this element is to introduce a lateral instability in the tow point that allows it to move in towards the spine or out towards the leading edge, depending on differences in tension in the flying lines. In effect, the tow point is pulled in when the kite is flown in a straight line and thus increases the tracking precision of the kite. When one flying line is pulled, the tow point under influence moves out and thus improves the kite's turning characteristics.
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| Stabiliser Leg Diagram 1: | Activator Leg Diagram 2: |
The second additional leg, known as the "Activator" and shown in red on Diagram 2, connects the inner and upper legs together, pulling them in slightly. This serves to give the kite "Pitch Control" and allows the nose to move forwards or backwards by a small amount, depending on the wind, relative position of the kite in the window and tension, or lack of it (in a stall, for example) in the flying lines. This gives the kite better speed control and a smooth, responsive feel in flight.
The two legs when applied in combination in a standard Active Bridle have a synergy that imparts a number of desirable features on the flight characteristic of the kite. Either leg, when used in isolation, has a noticeable benefit, but it is the combination and inter-working of the two that produces the final result.
General Construction Tips:
Bridle Stick
The easiest way by far to make any bridle from a set of measurements, is to first construct a "Bridle Stick". By measuring and marking lengths on a piece of wood, one has a simple ruler against which to quickly measure, cut and mark bridles.
To make a bridle stick, take a solid length of wood, at least a few centimetres wide on each side, and about a metre long (a little more than a metre is recommended). Mark a line (the "0-Line") at right angles across the wood and a few centimetres down from one end. Bang a thin nail into the wood at a point on the 0-Line roughly half way across the stick. Leave part of the nail sticking out of the wood so that loops of bridle line can be held around it. Measure down a metre from the 0-Line and mark another line across the stick (the "100-Line"). Bang a nail in here in a similar way to before. If your stick is less than a metre long, you might want to choose another convenient distance (75cm, 85cm, etc.).
Pre-Marking
When we come to build our bridles, we will pre-mark the lengths directly onto the piece of wood by measuring down from the 0-Line with a ruler or tape measure. Lengths greater than a metre long (or however far down your second nail is) will be measured down from the 0-Line, around the nail and then back towards the 0-Line. Here lies the convenience of having a metre between the nails: a length of 1.45m, for example, can simply be calculated to be 45cm (1.45m - 1.00m) from the 100-Line back up towards the 0-Line.
A bridle stick can be marked with measurements for many different kites, but be warned how easy it is to get confused when you have 11 versions of 3 different bridles all on the same stick. You may find it useful to mark measurements for different kites in different colour pens or on different sides of the stick (if you don't mind banging in more nails and marking more lines). Mark all measurements with the name of the kite and the name of the section or mark that it indicates (e.g. "OSpace Upper Leg Activator Mark") and don't be afraid to scribble reminders or makes notes on the stick. You'll be glad of them when you come back to it a month later and can't remember what any of the markings mean.
Bridle Line
Use a good quality bridle line that contains a Spectra (a.k.a. "Dyneema") core. This prevents the line from stretching in use. For ultralight and indoor kites, you may wish to use unsheathed "raw" Spectra to cut down on weight. Line rated at around 70kg/150lb breaking strain proves adequate for the job.
The only problem with using unsheathed line is that knots are more prone to slippage. You may wish to add small overhand stopper knots in the end of the lines to prevent this, although you should remember to tie the knot at the required distance and then cut the line, rather than cutting the line and then tying a knot. The latter will shorten the overall length of the line. Where continuous lengths of line are joined, mark the correct position on the line with a marker pen, holding the nib in place to allow the ink to sink into the fibres of the line. The ink from the pen should provide enough friction in the bridle line to hold position as well as serving as a clear visual marker when assembling the bridle.
Measuring Single Pieces
When measuring a single piece of bride line, I find it convenient to prepare one end of the line (which usually includes creating a flange - see following section), hold it up against the 0-Line and then pull the line down taut along the bridle stick. I can then read off the exact length required from the markings I've made onto the stick itself. If the end requires a flange then I cut it a little long, or if it's a bridle marking required, I simply dab a marker pen or metallic pen on the bridle line above the underlying marking.
Making Flanges
One
elegant alternative to tying stopper knots in bridle lines is to melt
"flanges" onto the ends of the lines. This technique was brought to my
attention by Tim Benson who has used it successfully on his range of kites
for many years.
When measuring the line exactly (preparing the first end does not require any such precision), cut the line about 5mm longer than required. Take the line and gently tap the end with a finger to open out the braid. Next, take a cigarette lighter and quickly melt the end of the line. As the strands shrink back and melt, tap the end of the line against a hard, fireproof surface. The metal surround at the top of a cigarette lighter is ideal for this. Your finger is not.
This process should create a visible flange of hardened plastic which opens out at the end of the line. This is usually all that is required to stop the end of the line slipping through a knot. A little further melting and pushing can be used if required to shorten the line further to the exact length.
We have so far been unable to solve problem of what to do when the line has been melted too short.
Tying Loops
The connection of a bridle at the centre T-Piece of the kite is usually
best achieved by doubling over a length of bridle line and tying a simple
overhand knot around the doubled line. This creates a loop which will
slip around the spine of the kite (usually below the T-Piece). When tightening
this knot, it is useful to first slip the knot over a spar from the kite
in question (or one of the same diameter) to ensure that the loop is a
reasonably snug fit.
The two ends of the loop section will become the inner legs of the bridle on both sides of the kite. In the Active Bridle design, one continuous piece of line runs across the bottom of the bridle, from lower leading edge one one side, around the spine at the centre T-Piece and then out across the other side to the opposite lower leading edge.
Measuring Loop Sections
When measuring a loop section, take a length of line a little longer (15cm, for example) than twice the required length. Fold the line in half and tie an overhand knot, as described above, using a suitable spar to gauge the correct diameter of the loop. Slip this loop over the nail at the 0-Line, pulls the ends of the line taut along the stick, around the 100-Line and back if necessary, and hold the line up against the required marking. When cutting lines, don't forget to add 5mm or so to add a flange, if required. When marking points on a line, simply dab a marker pen or a metallic pen across the line on the relevant mark.
Tying a Sheet Bend
The
Sheet Bend knot is used to join together several sections of the Active
Bridle. The knot holds fast in even extreme conditions, yet remains relatively
easy to loosen and accurately adjust.
When tying a Sheet Bend for the Active Bridle, you will be joining a loose end of line, complete with a flange, onto a continuous section at a given mark (which you should have pre-marked on your bridle line). First, fold over the continuous section at the required mark to create a loop. Feed the loose end up through the loop from behind, and down back around the outside of the loop. Bring the end right around the loop and then tuck it back under itself. The loose end should be running under the part of the line pushing up through the loop, but over the edge of the loop itself.
To tighten the knot, gently pull on the long end of the single section of line, holding the loop folded over at the required point with the other hand. Nudge the loose, flanged end in towards the knot and tighten until it lies snug up against it. The flange should prevent the end from slipping through the knot.
Making Tracers
The Active Bridle requires 2 separate "Tracer" sections, one for each side of the kite. These are simply small loops that connect to the bridle with a Prussik knot and form the sections onto which the flying lines are attached. The length of the tracer section is not particularly important, within reason, as long as both Tracers are of the same length. A length of 10cm is adequate and can be marked on the bridle stick along with the other bridle dimensions.
Take a section of bridle line a little more than double the required Tracer length (e.g. ~30cm for a 10cm Tracer) and fold it in half. Loosely tie an overhand knot around the loop, tightening it towards the free ends. Slip the loop over the nail at the 0-Line, and make the final adjustment to the position of the knot so that it lies directly over the specific mark (at 10cm, for example), and then pull tight. Cut the bridle line 5mm or so past the knot and flange the ends, in situ, with a cigarette lighter.
The loop end will be attached to the bridle with a Prussik knot (next section) and the knotted free ends will form the point to which the flying lines attach.
Tying a Prussik Knot
The
Tracer section is attached to the bridle with a Prussik. This can be thought
of as a "Double Lark's Head". The Prussik is advantageous because it can
be rolled over itself to lock it tight in position, or opened back out
to allow it to slide along a line for adjustment.
Take a Tracer loop and pass the knotted end behind the continuous length to which you're attaching it. Bring the knot up and over the line and then feed it through it's own loop end. Bring the knot back around and through the loop once more and then pull the knotted end tight.
When the continuous length is pulled tight so that it straightens out when going the knot, it will allow the Tracer to slide up and down it. By pulling on the Tracer and holding the Prussik knot between one's fingers, the knot can be rolled back on itself (looking much more like a Reef Knot) to lock it into position.
Tying a Running Knot
The
final knot to learn is the Running Knot, used to attach the bridle to
the frame of the kite. There are many methods employed for attaching bridles
to kites; one can tie the line around a leading edge connector, tie it
around the spar, under or over the fixing, perhaps also with a stopper,
use a loop, use a loose end, or configure it in any other way you see
fit. I've learned from the experience, and adopted the practice of Tim
Benson who bonds a small C-clip onto the spar with super-glue, sitting
directly beneath the leading edge connector. The bridle line is tied around
the spar between the connector and C-clip. Pushing the connector up away
from the C-clip stopper makes tying and tightening the knot easier. The
connector can be pushed down again once the knot has been tightened.
The knot employed is a basic Running Knot with a flange in the line serving to stop the tied end from slipping through the knot. The loose end is passed around the back of the spar and then up and around itself. The loose end is then tucked through the loop it has just created by doubling back on itself. The overhand knot that has effectively been tied around the long end should be tightened so that the flange on the short end lies snug against the knot. The long end can then be pulled to tighten the slip knot up around the spar.
Attaching the Bridle to the Kite
When creating an Active Bridle for a kite, you may wish the leave the existing bridle on the kite. Attach the new bridle to the kite, leaving the existing one in place and then simply switch the flying lines between the different bridles to compare the flight characteristics of one against the other.
This method is strongly recommended when designing an Active Bridle for a kite from scratch. By leaving the original bridle on the kite, you have something to gauge your new bridle measurements against. The bridles can be held taut alongside each other to give an approximation of where the required connecting points should be on the Active Bridle. See the section below on "Designing and Tuning an Active Bridle" for more information.
When the Active Bridle has been properly configured and gives satisfactory results, the original bridle can be untied and removed from the kite.
Construction of an Active Bridle:
Lower Leg Loop (x1)
The Active Bridle uses a single lower leg piece running all the way across the bottom section of the bridle on both sides. This section is created by tying a loop (see above) and measuring down the required length (typically around 1.50m) and cutting both ends with enough excess to make flanges. The distance down the line is measured from the back of the loop rather than the knot. When using a bridle stick, the loop can simply be hooked over the nail at the 0-Line to achieve the correct starting point for the measurement.
Two marks are required on the bridle to indicate the connection points for the Activator and Stabiliser section. These are known as the Activator Mark and Stabiliser Mark and are both measured down from the end of the loop. The lengths from the loop to these marks are known as the Lower Activator Mark (LAM) and Lower Stabiliser Mark (LSM) lengths. The overall length of the legs from loop to end is known as the Lower Leg Length (LLL).
The lower leg section is attached to the kite by passing the loop over the lower end of the spine and pulling it up to the T-Piece. The loose end are then fed through the T-Piece cutout from the back. The loose ends are tied, one to each lower leading edge spar, directly below the connector and above the C-clip, using the Frame Knot technique described earlier.
Upper Legs (x2)
One
upper leg section is required for each side of the bridle, making two
in total. These sections are of a fixed length and require flanges at
each end. Remember to cut a little surplus length in the line and then
melt the flange down until the line is of the correct length.
Two marks are made on the upper leg sections, both measured down from one end (the upper end). These marks are known as the Activator Mark and the Tow Point. The Activator mark will be roughly half way down the line. The Tow Point will be close to the lower end of the line. These lengths from top end to mark are known as the Upper Activator Mark (UAM) and the Upper Tow Point (UPT) lengths.
Attach the upper end of the line (furthest from the Tow Point) to the upper leading edge spar of the kite, directly below the connector and above the C-clip, using a Frame Knot.
Attach the lower end of the upper line to the lower bridle leg at the Stabiliser Point using a Sheet Bend.
Activator Legs (x2)
Two
Activator legs are required, one for each side of the kite. These are
short lengths (about 15cm) that require flanges in both ends. No markings
are required on the lines. The length of the line is the Activator Leg
Length (ALL).
Attach one end of the Activator to the lower leg on the Activator mark using a Sheet Bend. Attach the other end to the upper leg on its Activator mark, also using a Sheet Bend. The Activator leg should now connect the upper and lower legs, pulling them slightly in towards each other.
Tracers (x2)
Two tracers are required, one for each side of the kite, made as described in the earlier section. The tracers are attached to the upper legs using a Prussik. The knot should be moved onto the Tow Point mark and then rolled over to lock it tightly in place.
The section of the upper leg below the Tow Point has now become the Stabiliser leg.
Your kite has been Activated.
Copyright, Usage and Distribution Policy:
The Active Bridle concept and design is © Copyright 1997-98 Andy Wardley. All Rights Reserved.
Permission is hereby granted for any person to use or modify the Active Bridle for any purpose, under the following conditions:
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The Active Bridle, and any technology or design based on the Active Bridle, must remain free for everyone to use, forever, under these same terms.
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The Active Bridle must not be renamed. Significantly different derivatives may be renamed, but should indicate that they are based on the original Active Bridle by Andy Wardley, where that is the case.
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Original credit for the bridle should be given, where appropriate, to Andy Wardley. It is not necessary for this to be stated anywhere on the kite or accessories. Credit should be given if the Active Bridle is used or mentioned in accompanying documentation, sales or advertising literature.
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No royalties or fees are due for using the bridle on commercial kites. Please feel free to donate money to a charity of your choice if you feel indebted in some way for the Active Bridle. The proceeds of sale from a single kite would probably be an appropriate amount, but it is entirely at your discretion.
This document is © Copyright 1998 Andy Wardley. All Rights Reserved.
You may freely copy, mirror, print or distribute unaltered copies of this document, provided that no charge is made for it. Please contact the author if you wish to reproduce the document in a commercial publication or in an altered format.
Version: 0.3
Copyright © 1997-98 Andy Wardley. All Rights Reserved.
Document Last Updated: 13-Dec-98
It's A Breeze Kites thanks the author of this paper for openly sharing information that encourages innovative enhancement and speeds the growth of our sport. For the full report on the Active, Dihedral Active and Trihedral Active Bridle, check out the aurthor's site.
