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I recently got a slackline which has a ratchet (here's an example of the type I mean) for each end rather than a single ratchet. When I first heard of this type of setup, I'd thought it wouldn't help, since the limiting factor on how much tension you can apply to the slackline is the amount of force you're able to apply to the ratchet lever. However, after you've pulled some slack out of the line so that it's spooled around the ratchet, it is pulling further away from the axis of rotation and so applies more torque. This means that if you have a second ratchet which doesn't have as much line spooled on it it, the line will be applying less torque to it.

There are two strategies you could do:

  1. Maximally tighten one ratchet, then the other, leading to an unequal amount of line being spooled on each ratchet
  2. Tighten them simultaneously so that it's symmetric

Do these strategies result in the same final tension on the line or is one better than the other? If the answer depends on things like the material the line is made out of or the dimensions of the ratchet, when is one better than the other?

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Your strategy number 1 should work out better to attain the maximum tension you can apply by hand. Functionality should not be impaired by differing amounts spooled onto the ratchets as long as you do not exceed working load limits for the ratchets or the line.

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  • $\begingroup$ Can you show a derivation of this fact or give some intuition about why it's true? $\endgroup$ Commented Jun 23, 2021 at 4:25
  • $\begingroup$ The ratchet acts similarly to a class 2 lever, as the spool radius gets larger it moves the load further from the axis giving less mechanical advantage. $\endgroup$ Commented Jun 23, 2021 at 6:31
  • $\begingroup$ I can definitely see why you get less mechanical advantage as you spool more line, but what I don't see is what makes the "one ratchet all the way, then the other" strategy better than the simultaneous strategy. $\endgroup$ Commented Jun 23, 2021 at 6:37
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    $\begingroup$ Strategy 1 will have one ratchet with a smaller spool radius than any other configuration. $\endgroup$ Commented Jun 23, 2021 at 6:44
  • $\begingroup$ It's worth putting some physical numbers into this. I suspect that, unless the spooling leads to a change in the effective lever length of at least 25%, you won't see much change in the effort required of your muscles. $\endgroup$ Commented Jun 23, 2021 at 13:35

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