Rigging, Load Ratings, and Suspension

Posted: September 13, 2019 by Isaac Cross in Uncategorized

I am not a rope person. I don’t do rope suspensions.

In kink, however, we often have to draw from sources of information that are designed for similar applications. I think that theater is one of the best analogies for a rope suspension scene.

It just so happens that I worked professionally as a theater technician for a number of years. The information below comes primarily from the Backstage Handbook and the Stage Rigging Handbook, two excellent sources of information for anyone interest in the subject of rigging. However, I have links to online resources along the way as well.

In other words, while I am not a “rope person”, I did dangle very heavy things (and even sometimes people) over other people’s heads for a living, so I have some idea of what I’m talking about.

Here is the question being addressed (Paraphrased):

Many people in the rope community say that every part of a suspension rig requires a load capacity (rating) of at least 10x the weight of the heaviest person that will be suspended from it. But I can’t find a formula that supports that.

Short answer.

There isn’t one.

The formulas tell us that roughly 3x is the safe load capacity. And since manufacturers also buffer their numbers, the load rating on a piece of equipment will likely be well below the weight at which it will actually fail. So if you set up your rig at 3x or 4x the max static weight that will be hung from the system, you will probably be ok.

But if you want to be extra safe, at very little cost, kicking that up to 10x shows that the safety of your bottom matters to you.

Resultant Force

If you are hoisting your bottom (Pulling on the loose end of the rope to lift them up off the ground) then the formulas will tell you that you need a load capacity of, at minimum, 2x the weight of the bottom for a hoist at direct downward pull or 1.907x at a 35 degree angle. This is called Resultant Force.

Variable Affecting Resultant Force

This formula does not include the friction coefficient or account for other factors which can increase the force necessary to lift the object, and can increase the weight capacity to well above 2x the weight of the person.

Weight of Suspension Equipment

Also necessary to consider is the fact that the rigging itself has weight, which is applied upward (here referring to direction of systemic force, not necessarily in the opposite direction of gravity) in the system, so if your suspension ring and pulleys and clips and rope all weigh 10-20 lbs, that is all added to the calculation as well.

Dynamic Weight and Shock Force

That formula also assumes that the load is completely static, and that the force on the line is equal to their weight, this is rarely true. A drop of as little as 3 inches (called shock force) for an average weight person (80kg, 175 pounds) will add about 134 pounds (approximately 600 newtons) of force to your line. (Formulas for these calculations are gravitational potential energy and work-energy principle. That three inch drop can easily occur if the bottom is squirming, and you would do well to calculate for 6 inch (+263 pounds) or 12 inch (+526 pounds) drops to be safe. This calculator works well, but you will have to convert from pounds and inches to kilograms and meters and then convert back from newtons to force pounds.

This means that an acceptable safe minimum for a suspension point under just normal conditions, with average or below-average weighted individuals, with nothing unexpected happening, is roughly 3-4x, or about 1000lbs.

The Give-a-Shit Buffer

Increasing the load capacity of your suspension rig and it’s various components is relatively easy and inexpensive. So if you care about your bottoms (and I am assuming that you do), why not go ahead and increase the rating to 4000lbs or more, just to be safe?

What If I’m Wrong?

Some rope folks call bullshit and say that 2x the weight of the bottom is more than enough. Some even go so far as to say that the weight of the bottom plus 50-60 pounds is plenty.

I think the important question to ask is what happens if I’m wrong.

The bottom line is that if I am wrong and I use a 4000 pound capacity when a 300 pound capacity would have been enough, it means that I are being more safe than I need to be and everyone walks away happy, though it cost me a few bucks more than it needed to.

If I go with the lower capacity, though, and I am wrong, it means that I am putting partners at risk of serious injury or death because I’m too lazy and/or cheap to use something better.

Given the relative consequences of the two, I prefer to err on the side of being too safe.

Note on Intention:

I want to clarify that I am not trying to be the safety police of the scene. In fact, I have been actively and aggressively criticized over the years for being willing to teach both a safer method for something and a less-safe method side-by-side, one of many things that earned me the moniker of Unsafe Asshole in certain groups on Fetlife. I tend to draw from the philosophy applied in comprehensive sex-ed that giving more information and letting people decide is the best way to go, and has the best results in the long run.

When you treat people like children by withholding information from them and simply telling them what the “right” thing to do is, they tend not to trust you and make worse decisions for it. That’s why areas with “abstinence only” sex-ed also tend to have the highest teen pregnancy rates.

While I don’t do rope suspensions, I am hoping to begin doing hook suspensions soon, and so these numbers were important to me as I began my research. This post is, more or less, the thought process that I used to arrive at what I felt was a comfortable safety margin (10x), but I also wanted to be clear that someone using 3-5x was not necessarily being unsafe, since I have seen that accusation being made in some circles.

More than anything, I want people to have information from good, credible sources so they can make informed decisions about their own safety and the safety of their partners, instead of dogma passed from others that may or may not be reliable.

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