Rope elasticity
The use of static or , ideally , ultra-static ropes can make tree guying operations significantly easier . The preferred cordage types are Aramids such as Kevlar and Technora , or High Modulus Polyethylenes ( HMPE ) or Ultra-High- Molecular-Weight Polyethylenes ( UHMWPE ) such as Dyneema or Spectra .
HMPE fibres are low stretch , lighter than Aramids ( relative density of HMPE 0.97 ) and do not suffer from the same degree of strength loss when used around small bend ratios . HMPE cordage has high abrasion resistance ,
very low elongation , the highest strength-toweight ratios of any fibre and approximately twice the strength of common-grade steel wire of the same diameter .
The use of more elastic fibres such as polyester or nylon present some challenges for guying . Because the force applied to the guy is a function of the tree ’ s weight ( which cannot easily be altered ) and the degree of lean , any guy which allows the tree to lean further and pick up momentum must experience greater load to bring that movement to a stop .
The image above shows the risks with using more elastic ropes for guying hung-up trees . Image A shows the pine tree used in the previous example , which has failed and become hung-up on another tree ( not shown in illustration ). A single polyester rigging line has been attached under hand tension to secure the tree whilst a rescue takes place . As can be seen , the angle between the guy and the stem is approximately 30 °.
Image B shows the movement of the tree when it becomes dislodged and begins to move . Polyester ropes frequently have up to 10 % elastic elongation under working loads . If the original rope length was approximately 50m , this elastic elongation represents 5m of movement before the rope reaches its working load limit . It will continue to stretch as the tree continues to move .
The tree gains mechanical advantage as it rotates . If the rope was at 90 ° to the stem , a metre of elastic extension would be required for every metre of lateral tree movement . As the included angle becomes more acute , each metre of rope extension translates into a significantly greater rotation of the tree . As the angle drops below 30 ° the whole situation becomes rapidly untenable .
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