Support Line Selection [Level 3]
If a main support line breaks during a suspension, the consequences could be catastrophic -- someone who was inverted could potentially break their neck, although I've thankfully never heard of such a case. However, rope breaks do happen; while most incidents likely go unreported, there have been several published stories of rope breaks during suspension in the past few years, often involving skilled and experienced practitioners.
My preference is to tie with rope sufficiently strong that I can't imagine any plausible scenario where it would break during normal use, even if I screw up. This page focuses on how I approach calculating how strong that should be.
It's pretty standard in other, much better-studied, industries involving lofting humans to use a safety factor of at least 10 for any lifeline equipment. This means that the breaking strength of the equipment is at least 10x how much stress would normally be expected to be placed on it in use. That might sound like a really big safety margin, but it gets used up pretty quickly. Here's how I'd tally my safety budget when using natural fiber rope:
- 50% derating due to inefficient knots and/or small bend radius
- 20% derating due to manufacturing variation
- 30% derating due to treatment and wear
- 30% derating due to dynamic loading
If you multiply those together, my starting safety factor of 10x is now just a 1.96x safety factor to account for all other unknowns. Of course, those numbers are all just guesses, which is part of the point -- natural rope is so variable, and there's so little hard data on the forces involved in rope suspension, that we want a big margin of error in case some of our guesses aren't very accurate. So with natural rope, 10x is just barely enough to make me comfortable. If I were using it in an application where death in the case of failure was assured rather than just remotely possible, I'd want more like 20x or 30x.
Calculating Your Load
Hopefully I've convinced you that you want rope at least 10x as strong as the forces you expect to put on it. So then what are those forces? Well, there are two ways of approaching it.
In a static suspension, that is, one already tied off to the ring, where the bottom isn't changing position, there should generally be at least 6 individual lines attaching the bottom to the ring. So if you multiply the weight of your bottom by the 10x safety factor, then divide by the 6 lines sharing the load, you want your rope to be about 1.7x as strong as the weight of your bottom; for a 150 lb bottom, you'd want rope with a breaking strength of at least 250 lb.
However, most suspensions involve using the support lines to lift the bottom at some point; and when you're lifting, while the force supporting the bottom is spread over 6 or more lines, the force you are applying to the rope is only shared by the single pair of ropes you're grabbing -- that force you are applying with your hands is the highest force in the system, and the rope break incidents which have been published in detail seem to support that most rope breaks occur along the line between the rigger's hands and the bottom's harness, during a lift.
This means that the right number to be multiplying by our 10x safety factor is how hard you can pull. That may not be easy to determine, so a convenient way to guess at it is your own weight; I know if I pull significantly harder than my own weight, I'll be going off the ground myself, in a typical lift situation (something I have had happen). Since that force is shared between two lines, that gives us a simple rule of thumb: support lines should have a breaking strength at least 5x the rigger's body weight. Thus a 150 lb rigger would want to use support lines rated to 750 lb.
What to Use
The only natural fiber rope I've ever routinely used for main support lines is 8mm hemp, with a breaking strength of roughly 800 lb -- enough to satisfy the above rule for my ~140 lb bodyweight. Interestingly, 8mm hemp is also the only common natural fiber rope which has (to my knowledge) never been reported to break during rope bondage use. There have been a couple reports of 6mm hemp breaking, and numerous reports of breaks in 5mm and 6mm jute (which is as little as half the strength of hemp).
If you are tying with hemp, using 6mm for tying and 8mm for support lines may be a good option.
Otherwise, I'd strongly urge you to consider use of synthetics for main support lines. Spun polyester seems to be the most popular option for this, because it has a soft hand, holds knots fairly well, and can be bought in natural-rope color (for example POSH).
HempEx is another hemp-lookalike rope made from spun polypropylene instead of spun polyester. It is also suitable for support lines, but be aware it tends to be a bit fuzzy in imitation of natural hemp -- except that unlike natural hemp, you can not singe off the fuzzies, because they are plastic and will melt.
The synthetic rope recommended for support lines by Suspended Animation, a group which has performed many hundreds of suspensions in harsh environments, is New England Ropes Regatta Braid. It is exceptionally strong due to blending filament and spun polyester, and very supple. I'm not a fan of it aesthetically, but one advantage is that it is frequently available at local boating stores.
If you like to tie with jute, another option may be custom made dyneema-reinforced jute designed specifically for rope suspension. The rope at that link, made by TwistedView, is to my knowledge the only reinforced jute rope that has actually been shown by break-testing to be stronger than regular jute. Watch out for other "reinforced" jutes that contain just a small amount of synthetic; some have been shown to not be any stronger than the best pure jute ropes.
Nylon rope of a twisted or solid braid construction -- as can frequently be found at hardware stores -- is also a perfectly workable option. Just be aware that there's a great deal of variation in quality in nylon rope, and some of it is not pure nylon; so be sure to check the strength listed on the packaging. Some people say not to use nylon for suspension because of its elasticity, but I've never found that to be a significant problem in practice. Nylon does have poor abrasion resistance, so you need to be careful not to pull it against itself or rough surfaces at high speed, and it may wear out quickly. The only real advantages of nylon over other options are availability, and that it can be dyed at home. MFP behaves essentially the same as Nylon, except that it can't easily be dyed.
Regardless of which synthetic option you choose, I recommend the closest diameter to 6mm (or 1/4") that has the level of strength you need. Smaller than 6mm tends to cut into your hands when lifting; larger than 6mm can take up an inconvenient amount of space on your ring. The only time I use thicker support lines is with hemp.
After trying lots of different lengths, and teaching suspension to students of a wide variety of heights, my recommendation is to always use at least 30' support lines, even if you normally tie with shorter ropes (e.g. my preferred length for tying with is 27'). Anything less than 30' will not reliably reach all the way to the floor for all common combinations of ring height and hanger configuration. If you are more than 6' tall and like to place your ring high, you may want even longer support lines.
It's extremely important for safety reasons that your primary support lines are long enough to lower an unconscious bottom all the way to the floor with control. You'll also sometimes want to lift directly from the floor, which if anything is even more awkward with slightly-too-short ropes.