This is difficult to answer without any other info as this extender length depends completely on the mainline elasticity, the static tension of a line, your practice (only walking, surfing, bouncing, amplitude…).

Bigger loops of backup are more confortable to walk as they will stabilise the line, but if the loops are too big, the line becomes easily a big mess but more important, the shock in case of a backupfall can be extremely violent. Sometimes the slacker can even fall all the way down to the ground before being caught by the backup… !

Shorter loops are a pain to slack on, if the backup gets tightened during slacking on it, you cannot enjoy the full behaviour of the mainline. In my opinion, the best length for a backup is the shortest one that still allows to bounce like a machine without tightening the backup while doing so.

I so decided to develop a simple calculator based onto physical laws and experience that you can find HERE.

We noticed that usually, the ratio between static and maximum dynamic load on a highline is almost fixed and not depending on the webbing properties or weight of the slacker. With the biggest amplitudes it goes down to 0.6.

The calculator is based onto this ratio and the laws of physics. You will only need to fill :

• The length of your splits (50m, 100m or any special length)
• The elasticity value of your mainline
• The static tension that you use to rig, with you seated in the middle of the line. If you don’t have a dynamometer, just use the formula T=L*P/4/s with : T= Tension (kg) | L= length of the line (m) | P= your weight (kg) | s= sag (m) . And the calculator will give you the best suitable length for your extender.

Enjoy!