Applying a 10:1 safety factor to a rope's NEW breaking strength is fine. However, it is my experience in the field and the lab that single braid ropes with heavy use can have a 40% drop in strength after only an 18 month period (not a problem for climbing because of the safety factors involved. However, I always replace a well used single braid after 2 yrs). Therefore, the safety factor needs to be increased this amount after this period of time. Localised damage that may not be evident externally can weaken the rope further, especially on double braid where the interior cannot be seen.
I suspect double braid would weaken in a similar manner because of its loose weave. This is in contrast to highly abrasion resistant caving ropes that have been shown to retain almost full strength after a period of 15 years regular use. The difference is the outer protective sheath is very effective at shielding dirty water from the core - the load bearing core. Tree ropes get a lot of internal and external abrasion because of their supple nature (allows grit in) and external load bearing fibres (suffer abrasion even if run through pulleys).
Also, avoid where possible long sections on big wood - the longer the section, the greater the fall force no matter how close to the block. The fall distance is measured from the center of mass - roughly half the height of the section. The fall distance on a fifteen foot section would have been at least fifteen feet. 15 X 700lbs = 10,500lbs. Even if we cut the guesstimated force in half for dissipating the energy by letting it run somewhat, and then put the previously mentioned factors in to play, its not too difficult to see why a two year old double braid might break.
However, I too feel the need to take bigger sections sometimes - I just keep a seperate kit for this and record the falls and retire accordingly. On smaller diameter wood, I just upgrade the rigging.
Thanks for the example Mark, I often struggle to get people to believe that they actually need a rope bigger than 5/8" double braid!
I suspect double braid would weaken in a similar manner because of its loose weave. This is in contrast to highly abrasion resistant caving ropes that have been shown to retain almost full strength after a period of 15 years regular use. The difference is the outer protective sheath is very effective at shielding dirty water from the core - the load bearing core. Tree ropes get a lot of internal and external abrasion because of their supple nature (allows grit in) and external load bearing fibres (suffer abrasion even if run through pulleys).
Also, avoid where possible long sections on big wood - the longer the section, the greater the fall force no matter how close to the block. The fall distance is measured from the center of mass - roughly half the height of the section. The fall distance on a fifteen foot section would have been at least fifteen feet. 15 X 700lbs = 10,500lbs. Even if we cut the guesstimated force in half for dissipating the energy by letting it run somewhat, and then put the previously mentioned factors in to play, its not too difficult to see why a two year old double braid might break.
However, I too feel the need to take bigger sections sometimes - I just keep a seperate kit for this and record the falls and retire accordingly. On smaller diameter wood, I just upgrade the rigging.
Thanks for the example Mark, I often struggle to get people to believe that they actually need a rope bigger than 5/8" double braid!
