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Issue 12 (Nov/Dec 1999): BORK - Bridles

archive rec.kites bridles

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#1 Mike Gillard (RIP)

Mike Gillard (RIP)


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Posted 01 December 1999 - 04:00 AM

This promises to be the biggest BORK issue ever. You guys keep contributing such useful material. These past two months have seen a noticeable decline in flammable postings. This month we are also including some material from the KiteBuggy email list. We have a tremendous resource here and now you can search through all of the issues together. Keep up the good work!

The multiple issue keyword search can be found here - Search all issues

Once again I'm indebted to the faithful BORK staff:
  • Craig Rodgerson
  • Peter C. Hugger Editor
This page is a summary of postings on the rec.kites Usenet news group that our editors believe to be interesting and useful. Opinions expressed in these postings are not those of KiteLife Magazine or its staff.

Bridles Content:
  • GFAK Adjustment
  • Matrix Active
  • Bridle Design
  • Bridle Dimensions
  • Tim Dihedral
GFAK Adjustment

I purchased a Bandit made by Go Fly A Kite, a few months ago but cannot get it to stay up very long. The kite starts rocking side to side more and more then noses dives to the ground. I attempted to adjuster the bridle but now I'm afraid it's way off. Is there a measurement that would be a good starting point to level out the kite? Any other ideas that might correct this problem.

I also have a Thunderfoil from New Tech that I like very much. But I would like to use the Bandit and try some tricks. I'm starting to learn that perhaps the Bandit is not the best kite for this. It also seems to require alot of wind.

Lastly, I might want to pick up a better stunt kite that requires less wind but would like to stay under $60.00 US. Any suggestions?


There's usually some type of mark at the starting point for the bridle -- look for some type of black mark on the bridle string. If it doesn't exist, you can try adjusting the bridle attachments so they're about 1/3 of the way from the top of the bridle string, and then work downward (in 1/4 inch steps) until the kite seems stable in an 8-10mph wind.

I highly recommend the Beetle by Flying Wing. Most kite stores and on-line kite shops sell it for about $46 or so. Very durable, and with upgraded lines quite the trickster.

One place to look is http://www.intothewind.com; another is http://www.windpowersports.com (and no, I'm not an employee of either -- just a happy customer of both).


Hi Todd, hi folks.

To complement on Joseph post I would say that, on some kites, the bridle is black and the reference mark, when there is one, is usually of ligth color.

If you see no mark then try the following:

1 - Adjust the bridle as per Joseph suggestion.

2 - Hold the kite by the two tow points (the small loops of line or clips where you attach your control lines).

3 - The kite shall make an angle with the floor (or ground) such that the wing tips are a little lower than the nose.

4 - If the nose seems to low, your bridle adjustment is probably too heavy (or radical as some say it). Move the tow points up.

5 - If the nose seems to high, your bridle adjustment is too ligth. Move the tow points down.

Remember to adjust the two sides so they are identical. Use small increment while testing. Try to find the upper and lower limits and mark these on the bridle with a suitable pen. Someone, in this group, suggested using a sewing thread of constrasting color to mark the bridle. Then another mark in between can indicate the optimum setting.

Another point to check is that no bridle leg is wrapped around a spar. Generally, upper bridle legs goes on top of upper spreader (if there is one) and bottom bridle legs oes on bottom of lower spreaders.

I have observed that on most of the kites in my bags (yes I have two bags full, kites can become an addiction) the tow points are adjusted in such a way that if I pull down (toward trailing edge) on the bridle so as to flatten it against the frame and with the upper leg parallel to the spine, the junction of the bridle legs will be slightly lower than the lower spreader. This can also serves as an indication that your bridle is not too bad or in real need of some adjustment.

If your kite is relatively small (3 to 4 feet in wingspan), it may require some higher wind to fly than a full size kite (around 8 feet). I use the word may, because, today, with the craze for small indoor/low wind kites, you can find very small kites (2 feet wingspan or even less) that will fly in a puff of air.

A smaller kite will also require very small hand movements for control. And also, try to position your hands as if you were controling a bike (in front of you, with elbows at your side and sligthly in front of your chest. That way you can pull a little to steer and push back to resume straigth fligth.

Wind or no wind, fly for fun :-)
Jean (Johnny) Lemire

Matrix Active

>has anyone got experience making a turbo bridle for a stranger?

Yes, I used a method similar to Andy Wardley's turbo mod for the BoT, where you keep the same bridle but add a knot. It does leave the bridle a little shorter so the frame might get more stress in higher winds. This hasn't been an issue so far on my UL, it has a 5P frame, but if you try it on a standard Stranger you might want to keep eye out for the frame deforming - RCF6 isn't the worlds strongest carbon.

Anyhow heres my old post:

Yes I have on my UL, and it works very nicely. It made the kite generally snappier but it especially improved flic flacs, flip flop/fountains, flat spins from a horizontal pass and multiple flat spins. On the, only slightly, downside fades take a little more care to hold and those killer stops don't back up quite as nicely as with the static bridle. Because the turbo bridle pulls the nose in when driving, it's better behaved in very low wind without lightening the bridle too.

The dimensions I used are

Upper LE
	\  185  
	 /	 \			  All dim's in mm, measured between knots
	/	   \
   / 565	 \
  /		   \
Lower LE	  T Piece		keep the in-haul as standard.

I still like the laid back feel of the original bridle so I came up with the following knotting scheme which allows me to adjust the bridle without unknotting the yoke, and even to convert from turbo to static bridle just by loosening the larks head at the left and sliding it over the stopper knot up to the pickup point. I've since used it on the BoT and Fusion too, it's never slipped yet.

Top LE Conn							  Pig-tail
	 \									|
	  \								   X
	   \								 _X_
Larks  \  _____________________________|_|_|
Head	_|_|_						   | |	  Larkshead
	 / |_| |_|  ^					   |_|_|
	/		  Fig8 Stop Knot			 \
   /									   \
  /										 \
Bottom LE Conn							   T-Piece

-- Ian Newham

Bridle Design

Eeeagh! Why does my kite fly like a piece of crap?


Adventures in powerkite bridle design.

An usenet posting by Simon Stapleton

Well, here goes again. Another long and technical posting on powerkite design, probably full of errors and useless to most people, but I'm in a braindumping mood having been away from the 'net for 10 days. If you're not up for nasty vector maths, or at least the concepts of vector maths, moments and forces etc, it's probably best to tune out now.

And for those mathematicians out there, I've probably messed up my terms in all this - it has, after all, been in excess of 15 years since I studied any of this stuff. Apologies in advance.

Are we seated comfortably? Then I'll begin.

Once upon a time, there was a young and enthusiatic kite designer, who we'll call Simon. Although well versed in the black arts of sewing ripstop and shaping the skins of kites, he was still, shall we say, _rubbish_ at designing bridles. Hence, out of the bag, his kites tended to look pretty bad, and fly worse. And it took him a lot of iterative shortening and lengthening of lines to get the worst of the problems resolved, let alone fine-tune the things to fly as they were supposed to.

His primary bridle design had improved since the early days, mainly through conversations with other designers, but his secondary bridles were generally made by hanging the kite from the ceiling of his castle^H^H^H^H^H^Hflat and adjusting until it looked 'right'. Which sort of worked, but required a lot of fine tuning later on. So he decided to look into what actually makes the shape of the kite, and how adding line here makes the kite sag there, and a whole lot of other stuff, with the intention of automating the process. And in the process of doing this, he found a lot of things he'd never even thought about. And his head hurt. A lot.

OK. I'll be honest. That designer was me. I was trying to optimise the bridle for the blue spot kite, and I wanted to bung some parameters into my spreadsheet, hit the 'go' button, and feed the results directly into a CAD package for a nice graphical display. Problem was, I was away from home, mainly on French trains, and had no access to computers. So I decided to try and derive this stuff from basics. I've done higher level mechanics, so this should be no problem, right? Hah!

First thing I did was make some assumptions, as follows.

1: A cellular powerkite can be considered spanwise as a series of hinged girders with point loads acting upon them. This (point loads) is fine for the bridle attachment points, but less than optimal for the lift forces, but I wanted to simplify this as much as possible.

2: The lift at any point across the span of a kite is directly proportional to the chord at that point, assuming the same profile across the entire span, and the same AoA for all profiles.

From the second of these two assumptions, it can be seen that at a given point x on the span of a kite, the lifting force applied is a vector L perpendicular to the surface with a magnitude |L| = f(Cx), where f is the function describing the amount of lift generated for the chord C at point x.

This gives us assumption 3... 3: The lift generated over a section of the span is given by the sum of the lift vectors acting across it.

So, if we split the kite into rigid, straight sections, we can deduce a point load acting at the geometric mean point in terms of chord which has magnitude |L| = Sf(Cm), where S is the span of the section and Cm is the mean chord over that span, and acting in a direction perpendicular to the angle of the surface. If you see what I mean.

So, we now have a kite, split into a number of straight sections, and we know what the loads are acting on those sections. So how the hell do we go about bridling it?

Well. Let's get simplistic. We have a one lined kite, rigid and flat. We can work out what the lift forces are, and find out by summing the vectors (ass. 3) where the total lifting force is applied. So we can attach a line to that point and voila! we're bridled. If we attach the line elsewhere, there will be a turning moment generated and the kite will spin around.

However, in the real world, you can't do that. Air is not uniform, and lift changes as we move and airspeed changes, and a kite bridled as described above would most likely fall out of the sky, or at least flap around a bit. What we need to do is bring the bridling point below the kite, which will reduce the sensitivity to twitchy changes in airspeed.

So, we attach a line directly to either end of the kite, and bring them together. But where do we bring them together? Well, from the above example, we can intuitively see that the point must be on the line of the combined lift vector, or the kite will rotate around it. So we can move the point up and down the line of the lift vector, and the only thing we change is the sensitivity to gusts, right? Well, actually, no. As the point moves towards the kite, the angle A of the 2 bridling lines moves further from the perpendicular, and a compressive force C of magnitude |L|tan(A) is applied at both bridling points, across the span of the kite.

Time for a diagram, methinks.

	 --->C	   |	  C<---
	:\					  /:
	: \					/ :
	:_/\				  /\_:
	: A \				/ A :
		 \			  /
		  \			/
		   \		  /
			\		/
			 \	  /
		  T1 _\|  |/_ T2
			   \  /
				\|/ T

It can be seen that vector T, the tension in the flying line, is equal to the sum of the two vectors T1 and T2, the tensions in the bridling lines, as well as the lift vector L. T1 and T2 are both equal to L/2 + C.

Combined with the turning moments which are generated by having the bridle lines offset from the centre of lift, the compressive force C will tend to make the kite bend. New, we're assuming that the kite is rigid, so the point on the kite where these forces are maximised will be at the centre (assuming a constant chord). So, as the span increases, the kite becomes more and more likely to bend, until finally it does.

Once the kite has bent (and we're still assuming a rigid surface, so we'll assume it bends at the centre point only), we now have 2 lifting surfaces of equal area, but inclined from the horizontal by an angle. By summing the two lift vectors L1 and L2 from the two surfaces, we still get the lift vector L, but as part of the lift is acting sideways, we have reduced the efficiency of the kite. Some of the lift is now being used to tension the bridle lines, rather than being transmitted down the flying line and generating pull.

Now, the angle that the 2 sections assume will be roughly perpendicular to the two bridling lines - if we move the bridling point for the 2 sections to the centre of lift of each section, the sections will be exactly perpendicular to the bridle lines. So, as the point where the bridles come together approaches the surface of the kite, more and more of the kite's lift will be wasted on the bridling, rather than generating pull.

We can now see that in order to get the most pull from the kite, as much of the kite as possible should be perpendicular to the combined lift vector L of the entire kite. Note that this does _not_ necessarily mean that the kite should be bridled 'flat'. Why's that? Let's see.

We will now consider a 2 lined kite, which is basically what quads are as well. This 2 lined kite is split into two basically independent wings, and the lift from each of those wings is transmitted by a single bridling line directly to the flying lines. For a small enough span, and a rigid enough kite, this is an acceptable model, and the bridle lines will be attached directly at the centre of lift. The lifting force L for each wing must act directly down the flying line and through the handle. For a small enough span, we can assume parallel flying lines, and the kite will be bridled 'flat'. However, as the span increases beyond a couple of metres or so, the median points on the surface get further and further apart until, if the kite was bridled flat, the flyer would need to hold his/her arms spread as wide as possible from the body.

So, for a (relatively) fixed handle point, as the span of the kite increases, the lines will move further and further away from the parallel. And the force applied down each line must be parallel to the line it is being transmitted down, so the combined lifting force for each wing must move away from being parallel to the combined force for the whole kite.

What does all this mean? It means that the optimum shape for the kite is, at least in part, determined by the length of the flying lines. When I realised that, I used what my daughter would probably term "bad words". Repeatedly.

And then I realised that as the shape of the canopy changes in order to make the lift vector run down the flying line, the median point moves and the angle of the flying line changes. Which will tend to make your head head hurt. Because the shape of the kite has to change again. It was at this point that I went away and got myself a beer.

OK. So it was more than just _a_ beer. But I'm back now. Let's have a little recap.

1: For maximum power transmission to the flying lines, we need as much of the surface perpendicular to the combined lift vector as possible.

2: The lift vector for each wing must pass through both the median point of the wing, and the flying line itself, back to the handle. This will not be parallel to the one for the other wing for any useful span kite.

3: In order to minimise stresses within the kite and bridling, as many as possible of the bridle lines should be perpendicular to the lift vector for the supported area.

Now, combining these, we can see that it's going to be difficult to get a bridle that maximises all three of these. Because of (2), we need at least _some_ bend in the kite to bring the lift vector for each wing through the flying lines, so we compromise (1) above. And (3) above means that it is difficult to bring the bridle lines together, so that will probably be compromised as well.

Thus I propose a hybrid between a standard bridle, and the "arch" bridle used on some of the Sputniks.

How's that going to work? Something like this, I hope.

Take a flat kite, and work out the median points for each wing.

Work out the angle of the lift vectors for each wing such that they pass through handles and median points for a chosen line length.

Pick a line attachment point, on that line. This will be a fair way from the kite, as will be seen later.

Work out how much curve to put into the _tips only_ to bend the lift vector for each wing to that angle. The tips will bend along a circle with a centre point of the attachment point.

Bridle the centre sections vertically down to an arch between the two attachment points.

Diagram time again...

========================= M
	  |		   |	  =:====
	  |		   |	   :	====
	  |		   |	  :	/	===
	  |		   |	  :   /	   ==
------+		   |	 :   /		  =
c	 \---	   |	 :  /		__/	
		  b\---   |	:  /	  __/	  
			   \--+	: /	__/			
				   \  : /  __/			
				   a\ :/__/  

The line described by the colons indicates the lift vector, passing through the median point M, the line attachment point A and the handle H.

We must make sure that the angles of the arch part of the bridle correspond to the angles of the combined vectors passing through them (lift and side pull) and therefore section a will be very steep, b less so, and c flat.

It is left as an exercise to the reader to try and actually calculate this stuff - I'm half way through doing it, and it's hard[1].

Comments, as ever, are welcomed.


[1] My brains are trickling out of my ears.


I like reading this stuff, it's excellent. Thankyou for taking the trouble to post it. I have a few comments.

For a Peter Lynn cross bridle (this is what you mean by a standard bridle?) the tow points are bridles across to the opposite wing for some distance, thus the wings that you have modelled are effectively larger, but overlap and so the tow points move closer together.

The non-parallel forces from each wing, in addition to tensioning the bridle lines, must add tension (or at least reduce the compression) in the centre of the kite where the wings join. Therefore the effect on the bridle lines is not as great as the geometry of a single wing would imply. It would be interesting to know how great this additional tension on the centre of the kite is.

Although a flat kite is best for lifting efficiency, it does not necessarily fly well due to stability and controlability issues.

The cross bridle design was (as I understand it) developed to give the best control of two line foils. Since two line control is not so important for four line kites, (particularly hybrids?) then there may be advantages (drag) in going to an arch or your hybrid bridle for the main bridle.

Keep up the good work!



Erm, no, I meant a standard cascaded bridle running to each side of the kite. The crossbridle is (from observation) slack at the 'crossed' bit - i.e. on the 'other' side of the kite and only comes into action when turning. I thought I'd ignore turning at the moment, it's all quite complex enough anyway. But when it's flying flat, the bridle on a peel is absolutely standard.

I _can't wait_ to start modelling what happens in turns. Not.

> The non-parallel forces from each wing, in addition to tensioning the > bridle lines, must add tension (or at least reduce the compression) in > the centre of the kite where the wings join. Therefore the effect on > the bridle lines is not as great as the geometry of a single wing would > imply. It would be interesting to know how great this additional > tension on the centre of the kite is.

This is true, and I'd be interested to know as well. I'm still building spreadsheets to try and model all this.

> > Although a flat kite is best for lifting efficiency, it does not > necessarily fly well due to stability and controlability issues.

Yep. Bang on. I'm trying to find the best mix of the flat and the curve.

> The cross bridle design was (as I understand it) developed to give the > best control of two line foils. Since two line control is not so > important for four line kites, (particularly hybrids?) then there may be > advantages (drag) in going to an arch or your hybrid bridle for the main > bridle.

I hope so.


Hi Alan,

Allen Stroh has a very detailed page on bridles. Have a look at:


Andy Wardley's site has loads of good information too.


There is a lot to absorb with these sites, bookmark them and refer to them often -I do! ;-]

~strings attached~

Bridle Dimensions

Hi All, Maybe an odd question but is there a formula out there somewhere that can be used to calculate inhaul and outhaul bridle lengths? I realise fully that there are many variables involved but before I start to make a new kite (with no bridle dimensions available or a completed kite to peruse!) I would like to have some "rule of thumb" to work to. Any ideas? (except become a commis chef and leave kites alone :-) )

Phil H.


Hmmm... now let me think - Ah! I know! Is it a dual? If so, http://surf.to/a-edwards and peek at the bridle calculator.


Tim Dihedral


I have build a trick kite Tim and now I want to replace the static bridle with a dihedral active bridle (see Andy Wardley's website).

Is there anyone who has done this before and who is able to give me the right dimensions for a dihedral active bridle for Tim? In what way did it change the flying attributes of Tim?

Thanks in advance,



I have built three TIM kites. One has the standard bridle, one has a turbo (dynamic) bridle, and the last has an Andy Wardley Dihedral Active Bridle. I prefer the way that the activated kite behaves. I like how it's easier to stall (for me) which makes all the cool tricks easier. At the request of a few other TIM builders, I have thrown together a simple web site detailing some of the construction methods I used and some of the suggestions I have for building a TIM. Included will be the measurements of the dihedral active bridle on my most recent TIM. I have about 2/3 of the site finished and posted, so you can check it out if you like. I will be adding the remaining information including the bridle stuff in the next few days, so check back. The URL is http://www.relia.net...y/TIM/index.htm I know it's not a fancy, graphical, web site, but there's hopefully something you can use there.

-Kent : Utah Kite Nerd
Mike Gillard

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