Support Line Handling [Level 3]
Notes and Resources:
- Mechanical Advantage
- Neatness Counts
- Maintain Control
- Dealing with a Jam
- Always Be Able to Reach the Ground
The common way of running support lines is to create a "pulley" system between the harness and the ring, albeit without any actual pulleys. A pulley works by decreasing the force you have to apply to move an object, in exchange for your pulling the rope father than the object actually moves; this tradeoff is called mechanical advantage, and is commonly written as a ratio; for example 2:1 mechanical advantage means a system where an object is pulled with twice as much force as you apply to the rope, but only moves half as far.
The most frequent configuration for suspension is where you start at the harness, go through the ring, back through the harness again, and then (optionally) through the ring a second time. This produces 3:1 mechanical advantage; the easy way to know what ratio you've got is to just count how many lines go up from the harness. Going through the ring the final time doesn't change the mechanical advantage (still 3 lines going up from the harness either way) -- it just changes the direction of force so that you can pull down instead of up.
In a frictionless universe, 3:1 mechanical advantage would mean that you only need to apply 33 lb of force to lift a 100 lb load. But even actual pulleys produce some friction; passing the rope over a ring or carabiner creates quite a bit of friction, and if you don't use a carabiner at the bottom, rope running against rope produces a huge amount of friction. Altogether this means that once friction is factored in, despite your 3:1 advantage, you might need to apply something like 80 lb to lift a 100 lb load; almost all the nominal 3x gain in force is lost to friction.
However, these friction losses don't mean the 3:1 system is useless; although it may only help slightly with lifting, once you are holding your bottom in place, and when you are lowering them, the friction works to your advantage. In the above example you lost a little more than half your force to friction; that means now you only need about half of the nominal 33 lb of force to hold the load in place; so even though it took 80 lb of force to lift your 100 lb load, you only need 15 lb of force to control the line as you're tying it off or lowering it. That control, which comes from the combined benefits of mechanical advantage and friction, is what makes this system so useful.
Of course, most bottoms weigh more than 100 lb; I just use that example to make the math easy.
Choosing the Right Ratio
For an in-depth treatment of the forces involved in different lifting setups, see this article.
The short version is: a 3:1 ratio is always best if you have rope-on-rope friction at the bottom. If using carabiners at the bottom, some lifting advantage can be gained by using a 4:1 lift (which means starting at the ring) or a 5:1 lift (which works best if you use two separate carabiners at the top), but it's not a huge amount more than in a 3:1 setup -- although it does provide better control if your bottom significantly outweighs you.
When lowering, be careful that you don't have too much friction; when the amount of load on the line you are holding starts to approach 0, the likelihood of jams increases and the motion of the rope becomes jerky and unpredictable. If you perform extra wraps up and down between ring and harness after lifting, as part of securing your support line, always remove those wraps before lowering to avoid jams; if anything you want to lower with less mechanical advantage than you had when lifting, never more (but also never less than 3:1). When removing extra wraps, keep firm tension on the line to avoid any slippage before you are ready to lower.
Direction of Force
Because going through the ring at the top doesn't add any mechanical advantage, but does add friction, it takes less force to lift your bottom by pulling up on the line from the harness than down on the line from the ring. However, it's also frequently much harder to pull at that angle, which tends to balance out the benefits; all things considered, most people prefer to pull down. You can get the best of both worlds by using one hand to pull up and the other to pull down:
Treat your support lines as you would a column tie encompassing harness and ring; go in a continuous circle, with no twists or overlapping of lines. Going at diagonals can pull one part of the line under another, creating a jam; twists can cause one rope to pop up over another when they go through the ring, also creating jams.
You want to be in complete control when handling a support line. My preferred way of doing this is to always have at least one hand maintaining firm tension on the working end of the rope.
Whenever you have at least a 3:1 setup, you can stop the line from moving by firmly grabbing around the whole bundle. I think of this as the emergency brake -- it is there for when things go wrong, or for special maneuvers. Don't rely on it for everyday driving. If you are afraid to let go of the bundle and use the working end of the rope, you're not really in control; more practice or a different setup is called for.
The trickiest place to work without over-reliance on the e-brake is when you're unwrapping a support line through a ring or biner; you want to learn to transfer tension on the line from a hand on one side to the opposite hand on the other side. Always make sure you can feel the motion/tension of the rope between your two hands before letting go with the source hand; if you've grabbed the wrong rope on the other side of the bundle, you may have insufficient mechanical advantage to control your load!
If you don't feel confident, practice this procedure with an inanimate load. Routinely grabbing the bundle so that you can pull a loose rope end through your ring builds bad habits that will slow you down later.
All that said, in a live suspension, any time you feel uncertain or like you're losing control, do grab the bundle! It's there for when you need it; use it, don't abuse it.
Dealing with a Jam
Note that that last circumstance is very unlikely to come up; normally you get jams where multiple passes of your support line go through the same space.
Always Be Able to Reach the Ground
For main support lines, you always want to have enough length that you could lower your bottom with control all the way to the floor if they lost consciousness. Avoid the temptation to tie off a harness with line leftover from tying it! Always attach a new support rope that you know is long enough.