All About Pulley Systems - Part 4 - Rope Characteristics

After looking at the characteristics for our main pulleys, as well as the attributes for our brake, it's time to delve into the topic of what size rope and how long it needs to be for our pulley system.

First I would like to look at the different sizes of rope that we have to choose from. In the Balance Community Shop, there are two options for the kind of rope that you can get: 9mm PMI Classic Professional and 11mm PMI Access PRO, both of which are very high quality ropes that work perfectly for all slackline pulley systems. Now, you are probably wondering what the difference between these two ropes is. Well, there are a few differences that can help you decide which rope is best for you.

The first, and most important difference between these two ropes is the diameter. A rope with a smaller diameter is lighter, more supple, and also not quite as strong (if made from the same fibers as the larger rope). What this means for pulley systems is that the smaller diameter rope is very usefull when weight is a serious issue. When you are rigging larger lines with bigger pulleys, the larger diameter rope's added strength becomes a necessity. For alpine highlines and shorter lines in the park where you are using your lightweight pulley system (i.e. your Rock Exotica Mini Machined Double Pulleys), the smaller diameter rope works perfectly. Also, when using smaller pulleys, the sheave width is typically a lot smaller than larger pulleys, so a smaller diameter rope will glide through the rolls a lot better.

Another difference between these two types of rope is how they are constructed. The 9mm rope has what is called a 16 carrier construction compared to the 32 carrier construction of the 11mm. What this means is how many bundles of fibers are used on the sheath of the rope when weaving. The smaller the number of carrier's, the less bundles of fibers are used. For instance, a 16 carrier rope will have 16 bundles of fibers on the sheath (the outer protective layer of the rope). This usually means that the rope has a much higher abrasion resistance because the fiber bundles that provide the abrasion protection are much larger, which makes them harder to abrade through. A higher carrier number usually means the rope has a softer feel since the fiber bundles that make up the sheath are much smaller.

The 9mm rope above is built with a 16-carrier design which is entirely made from nylon fibers, which are known to have lower abrasion resistance compared to polyester. This rope will be significantly more stiff than the 11mm because of the lower carrier construction. However, the 11mm has a sheath made from polyester, which is known for having much higher abrasion resistance compared to nylon. This means the hand will be a lot softer and you will still achieve the high abrasion resistance inherent in the lower-carrier construction ropes.

Here's a nice table showing the differences between the two types of ropes above:

The next thing I would like to look at is what length of rope you should use. This aspect totally depends on the kind of webbing you have, how long of lines you want to rig, and what type of pulley system you have.

The kind of webbing you have greatly affects how long your rope needs to be for your pulley system. This is because every type of webbing has a different stretch and the higher your webbings stretch, the more slack you will have to pull out of it to get it tight. The more slack you have to pull out of your webbing, the longer piece of rope you will need for your pulley system. Article to come on the stretch characteristics of most popular webbings out there. For now, please refer to the Slackline Webbing section of the shop and navigate to the product page of your webbing.

The length of the line you are rigging also greatly affects how long your pulley system rope needs to be. A longer line typically needs more tension to be able to walk. This means that you will need to pull slack out of your webbing (both percentage-wise and distance-wise), which means that your pulley system rope will need to be longer. Stay tuned for articles on tension for various lengths and sags. For now, please stick to the 50:1 ratio (for every 50 ft. of slackline, you should have no less than 1 ft. of sag).

The last thing that will determine how much rope you need to get is the kind of pulley system you have. If you have a typical 5:1 (five-to-one) pulley system (two double pulleys and a brake), you will need five times as much rope than you will need to pull out of your line to get it tight. For instance, let's say you have a 300 ft. piece of webbing that stretches 10% at the tension that you like to walk it at. That means you will need to pull 30 ft. out of your 300 ft. line to get it to this tension. Also, let's say you have a simple 5:1 pulley system. Now, since the pulley system will need to be 30 ft. long, this means that you will need at least 150 ft. of rope to get this particular line tight.

That concludes this article on rope characteristics. Stay tuned for the next article in the series, all about rigging plates!