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I have recently set up wire rope to support my installation of retractable sail shades.

My setup is as follows:

  • I connected 2 ropes in parallel that run from the back of my house to fence posts. They are approximately 8m apart and are not tensioned.
  • Onto each rope I connected 3 rope grips and 3 steel rings approximately 1m apart
  • I then hook 3 ropes approx 7.5ish metres in parallel to the rings on the first 2 ropes.
  • A turnbuckle is on each of the 3 ropes which is then tightened up to bring the whole system into tension
  • I then attach 2 sail shades approx 3x5m using snap hooks
  • I then further tension the 3 ropes

I am finding that after a few days/a week the tension on the wire ropes reduces and everything begins to sag.

I have taken off the shade sales reduced the length of the 3 ropes, re-tensioned the rope again and re-hung the shade sails. However, this does not solve the issue for very long.

Can anyone suggest a way to stop this set up losing tension?

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  • 4
    Are you sure it is the wire rope and not the anchors(fence posts)?
    – crip659
    Jun 10 at 17:12
  • 1
    Thanks for getting back to me. The fence posts are concreted into the ground. I don't think that it has moved.
    – James
    Jun 10 at 17:27
  • 4
    You will be surprise with how much non braced posts can move/tilt. The wire clamps might be slipping also. The wire rope will stretch a bit at first, but the turnbuckles should be enough to remove that.
    – crip659
    Jun 10 at 17:38
  • 3
    I would absolutely bet on the posts moving. Check their plumbness. Typical concrete post setting is nowhere near immobile, and if it is, wood posts can be bent.
    – Ecnerwal
    Jun 10 at 17:45
  • 1
    How do I stop 25' of wire rope from sagging, but still be able to side something along it, on posts that can't take [X] pounds of tension? Nope, +1. - The Q is what's the tension required, for the weight of the sail to not sag the wire. A, @25': too much for that fence? Also, what hardware and how would you get it that tight? - The wire, cabled stayed to the house, provided it's ok that it's two sails that retract to their either sides.
    – Mazura
    Jun 11 at 20:21

2 Answers 2


Assuming everything is rigid...

Then one thing to provide constant tension is to run a loop towards the ground and hang a mass from that loop, gravity works.

Another example is the mass used as a door closer, sometimes the mass is "hidden" in a tube to stop fingers getting trapped or pinched. Often the masses were adjustable especially for heavy barn doors so there was a long rod that the masses could be added to easily.

You might need to increase the mechanical advantage by using pulleys.

Be careful choosing the amount of mass and also if kids can use it as a swing.

Also to consider is a spring either in tension or compression. But for this use I think the mass is more visible in terms of safety.



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  • That's only redirecting, not multiplying. Try this lesics.com/what-is-an-automatic-tensioning-device.html (note the counterweight here is 1/3 the desired wire tension but has 3x the movement) Jun 10 at 20:12
  • @Harper-ReinstateMonica just need the equation for the catenary curve :) - not sure the OP is up for that... But The OP wanted an example of how it works - multiplication is easy.
    – Solar Mike
    Jun 10 at 20:14

Here's the thing...

To keep a non-rigid member flat it has to be under extreme tension. In fact, the tension required increases exponentially as you approach zero sag. Therefore, it's not a reasonable goal to expect flatness when attached to fairly flexible supports. (The comments are correct that your fence posts are not as stout as you first believed.)

Therefore, embrace the arc. Treat it as a suspension bridge, which you'll notice all have substantial sag for the reason stated above. Your tilting fence will thank you.

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