Highlines: Mainline and Backup
A true highline consist of a mainline for walking on and a backup line to protect against mainline failure. Leash rings slide along both to create the attachment point for your safety system. The backup line can be either rope or another piece of webbing. Over the next two articles, we will discuss what webbing is acceptable to use for the mainline, the differences in using rope or webbing as a backup, how to anchor the highline, what to use as a leash and how to tape the mainline and backup line together for highlining.
Generally, webbing used for highlining has a breaking strength of at least 6,500 lbs. In addition to strength, there are a few other things to be considered when considering a mainline. Webbing can have a high degree of influence on the shockload amplitude generated during leashfalls. A low stretch webbing in a short line will be very harsh on your body and on the anchors if you take a whip on it. In fact, it is probably a good idea to avoid very low stretch webbing up to lengths of 150 ft. For very short lines it is advisable to use a stretchy piece of nylon webbing like type-18 or threaded tubular that has better energy absorption capabilities.
Rope backups come as either static rope, dynamic rope or dyneema rope. Each have their pros and cons. Dynamic rope is widely available through climbing stores. The downside is the amount of stretch. If you end up taking a fall onto the backup you’ll be going for a gnarly long ride. Be sure that there is nothing close by that you could hit if you were to ride the backup. Dynamic rope is best used way up high. Static rope is the preferred rope to use for a backup. It is less stretchy than a climbing rope and also widely available through retailers. Dyneema rope, like Amsteel, are close to no-stretch as you can get. These ropes have fairly recently come onto the scene and people are still experimenting with the best way to use and anchor them. Some people are experimenting with screamers to limit the load transferred to the anchor if one were to take a leashfall. One thing we know for sure is that these ropes perform fairly poor when tied in knots, so other anchoring methods must be used. The main advantage of rope over webbing is its abrasion resistance. Ropes with a mantle and core separate the load bearing characteristics from the outer protective layer. Amsteel does not have a mantle to protect the load bearing fibers.
Another advantage of rope over webbing is the ease of anchoring. You can use a belay device like the Edelrid Eddy or Petzl Grigri on one side and tie the rope to the other side using a knot such as a figure 8. Furthermore, rope backups act as a wind dampener so you generally do not have to bother with putting those on your highline. When rigging midlines (i.e. highlines where the stretch of the backup becomes a concern because they are close to the ground) you have to be careful of the backup’s stretch, especially with dynamic rope and webbing. Always be looking for what you could hit if you were to take a leashfall.
A webbing backups has the same properties as the mainline, which in theory makes the entire system more susceptible to modes of failure, especially abrasion. Best practice is to fix the backup webbing in another set of weblocks, otherwise you end up with a weaker backup than your mainline. The main concern with webbing backup is going to be abrasion. If there is any chance of the line touching rock or other abrasive surfaces, pad like you’re mad. There are many forms of padding: carpet, towels, yoga mats or similar, and tree padding. Make sure the padding is sufficiently secured and there is no chance whatsoever of it slipping or blowing away and becoming litter. It is also imperative to check on the condition of your padding regularly during longer highline sessions, especially when returning to the highline if you left it up overnight or any other extended period of time, speaking of which: wind dampeners are always a good idea on double webbing highlines, especially when you leave them up longer or if they’re rigged fairly tight.
Anchoring the System
Webbing in a highline system should be anchored in a weblock in order to retain maximum breaking strength of the webbing, as the shock loads during leash falls are very high. The mainline always consists of webbing whereas the backup can consist of either rope or webbing, both have their advantages and drawbacks. To anchor the mainline use a weblock as described in previous articles. Methods for anchoring the backup line depend on the material used, but generally webbing anchors and knots for rope are used.