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Have you ever been in the situation where you have pulled all that you can out of your pulley system, but the line is still not tight enough? When you start doing longer and longer lines, this problem can be extremely frustrating. There have been times in the past where I have spent over 2 hours tensioning a line, just to have it not be tight enough in the end. The technique below is a good way to prevent such a thing from happening.
The method is very simple, but it requires that you first know the elongation of your webbing. To find this, you can look on the BC Product page for your specific line. If you line is from another manufacturer, sometimes they will provide this data and sometimes they will not. You can always measure this yourself, but I will not go into that now.
Here is a chart of the current BC webbings and their stretch values and working load limits:
|Webbing||Working Load Limit||Stretch @ WLL|
|1" Threaded Slack-Spec Tubular||6.7 kN (1,500 lbf)||11.1%|
|Type 18||6.7 kN (1,500 lbf)||11.9%|
|Type 18 MKII||7.1 kN (1,600 lbf)||10.2%|
|Aero||6.7 kN (1,500 lbf)||6.3%|
|Mantra MKI||8.4 kN (1,890 lbf)||3.1%|
|Mantra MKII||8.4 kN (1,890 lbf)||4.4%|
|Mantra MKIII||8.4 kN (1,890 lbf)||7.5%|
|Spider Silk MKI||12 kN (2,700 lbf)||2.5%|
|Spider Silk MKII||13.4 kN (3,000 lbf)||2.5%|
|PowrLine||12 kN (2,700 lbf)||12.0%|
|RAGEline||12 kN (2,700 lbf)||8.3%|
Now for the how-to. You are going to want to start off at the static anchor, which is where you should begin your rigging anyway (more coming soon). In order to be more efficient, we will do a few things at once here. We will flake our line out of our BC LineBag, making sure the line is flat, and measure the distance of the line. This will help speed up rigging a bit, which we discuss soon.
The first thing you are going to want to do is to anchor your line on the static side with your static side Webbing Anchor. Once anchored, take a hold of the line between your fingers and lean against the static tree.
Now, start to take uniform steps directly towards the tensioning tree. Make absolutely sure that every step is roughly the same size! Count every single step along the way. You should walk all the way to the tensioning tree and note how many steps it took to get there. If you ended in the middle of a step, always round up. For example, if a gap is 55 and a half steps, you should mark it as 56 steps.
Now that we know how big our gap is in steps, we need to calculate how big our pulley system needs to be. Using the chart above and the following formula, calculate how many steps your pulley system needs to be:
Line Length (in steps) x % Stretch @ WLL (in decimal form) = Pulley System Length (in steps)
For example, let's say that we have a line that is 50 steps long that we are rigging with Type 18 MKII. Using the above percentage chart and formula, we have the following:
50 steps x .102 = 5.1 steps
Now, when the result is a decimal, always round up. So, in the case of our example, we should assume the result is 6 steps.
Okay, now that we have how many steps long our pulley system should be, we should start from the tensioning anchor and walk this many steps towards the static anchor. Once measured, you should mark the position with some sort of marker. I usually put a stick into the ground at this location, which I can also lay on top of the webbing to ensure I remember what flat is.
The last step is to extend your pulley system out to this point and install your tensioning side Webbing Anchor.
This is an extremely effective way to determine the correct amount of distance to use for tensioning your slackline. It can prevent having both too much pulley system length as well as not enough. Using this method, you can pretty much eliminate having to retension your lines.
One last little tip: if you want to add more tension to your line then the suggested WLL, you should give yourself more room in the pulley system. This is not recommended though.