SELECTING THE ROPE
Selecting a rope involves evaluating a combination of factors. Some of these factors are straight forward like comparing rope specifications. Others are less quantitative like a preference for a specific color or how a rope feels in your hand.
Fiber and construction being equal, a larger rope will outlast a smaller rope, because of the greater surface wear distribution. By the same token, a stronger rope will outlast a weaker one, because it will be used at a lower percentage of its break strength with less chance of overstressing.
Consider the opinion of professional climbers who may have more experience as to how well a rope performs. Consider also the reputation of the rope manufacturer. Are they involved with and supportive of the arborist industry? Do they stand behind their products with consistent quality and reliable service? Buying unproven ropes because they are a little less expensive is false economy and can lead to disaster.
Some things to consider when looking at ropes are:
Construction
Kernmantle rope consists of a central core (kern) of fibers that support most of the load on the rope (about 80 percent). A woven sheath (mantle) covers this core and supports less of the load (about 20 percent). The sheath’s tight weave protects the core fibers from abrasion, dirt, and sunlight. The resulting rope is strong and resists damage, yet is easy to handle.
In laid construction, small fiber bundles are twisted together and combined into larger bundles that are twisted around one another. The “lay” of the rope is the direction in which the strands are twisted. Most ropes are right-laid (strands spiral upward to the right when the rope is held vertically). The lay may be either hard or soft. Hard-lay construction creates a stiff rope in which knots are difficult to set and hard to untie after use. When under a load, these ropes resist abrasion and hold their shape better than soft-laid ropes. Soft-lay construction results in a flexible, easy-to-use rope, but one that unwinds easily and is not recommended for rappelling. All ropes of laid construction tend to untwist when loaded, causing spin and rope kinking. The amount laid ropes untwist when loaded depends on the strength of the lay. Because each fiber may appear at the rope’s surface in several places along its length, the load-bearing fibers are more susceptible to abrasion damage.
Braided ropes have gained popularity among tree climbers because of their excellent knot holding and handling characteristics. Common types include solid braid 12-strand and double braid mantle and core 16-strand. Both types of construction are used in life support climbing lines and in rigging ropes.
Strength
When given a choice between ropes, select the strongest of any given size. A load of 200 pounds represents 2% of the strength of a rope with a breaking strength of 10,000 pounds. The same load represents 4% of the strength of a rope that has a breaking strength of 5,000 pounds. The weaker rope is having to work harder and as a result will have to be retired sooner. Braided ropes are stronger than twisted ropes that are the same size and fiber type.
Ropes used for safety lines, safety straps, lanyards, and climbing lines should have an abrasion resistance and a melting point that is equal to or greater than nylon rope and a minimum breaking strength of 5,400 pounds (24kN). Climbing lines that are UIAA approved and meet the standard for single-rope use are suitable. Smaller diameter rope of less than 5,400-pounds breaking strength may be used to construct slings and prusik loops if the finished product meets or exceeds 5,400-pounds breaking strength. However, the margin of safety will be lower because the smaller diameter rope will have less resistance to friction wear for these purposes.
Elongation
It is well accepted that ropes with lower elongation under load will give you better load control, a big help at complicated job sites. However, ropes with lower elongation that are shock loaded, like a lowering line, can fail without warning even though it appears to be in good shape. Low elongating ropes should be selected with the highest possible strength. Both twisted ropes and braided ropes are suitable for rigging. Twisted rope has lower strength and more stretch. Braided rope has higher strength and lower stretch.
A static rope has less than 20-percent elongation at the breaking strength and less than 2 percent elongation at a working load of 500 pounds. As a rule, these ropes are stiffer than dynamic ropes, which can make knot tying more difficult. Generally, static ropes are more resistant to abrasion and dirt penetration. Principal uses for static ropes include haul lines, lanyards, safety straps or slings, SRT ropes, and rappel ropes. Never use static ropes in situations where a fall could occur, such as with the 4-inch tie-in system, or as a safety line for a belayed ascent.
A dynamic rope has an elongation of 40 to 60 percent at the breaking strength and less than 10 percent elongation at a working load of 176 pounds. A dynamic rope absorbs the shock of a fall, giving the climber added protection. Principal uses for dynamic ropes include safety lines and 4-inch tie-in systems. Because of their versatility, dynamic ropes are suitable for lanyards and rappelling. They are NOT recommended for haul lines, SRT, and DRT because of their tendency to stretch.
Semi-static/dynamic ropes fall into the elasticity range between fully static and fully dynamic. Ropes designed for use in DRT climbing systems are commonly in this elasticity range, i.e. from 3 to 4 percent elongation at working loads of 500 pounds.
Firmness
Select ropes that are firm and round and hold their shape during use. Soft or mushy ropes will snag easily and abrade quickly causing accelerated strength loss. Because the fibers are in a straighter line, which improves strength but compromises durability, loose or mushy rope will almost always have higher break strengths than a similar rope that is firm and holds its shape.
Stiffer ropes generally resist abrasion better than more flexible ropes. All ropes may be abraded from within by dirt particles rubbing against the fibers or from the outside by contact with rough or sharp surfaces.
