Skinny rope on base anchor leg?

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Webfoot

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Oregon
On a base tied SRS the back/anchor leg of the rope only sees a portion of the load, and none of the direct wear of ascending and abseiling. To save weight I would like to use a skinnier rope in this section for higher climbs. How thin would you go and what dangers are foreseen? Already considered:
  • low stretch needed to avoid sawing
  • a bend appropriate for unequal rope diameter
  • bend to remain below TIP on the anchor side
  • basal lower-off probably impossible
Conceptually I don't see why even something dramatically thin like Petzl Pur Line 6mm could not work.
 
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I'd stick with arborist ropes and wouldn't go below 9 mm. That rope sounds stiff and slippery. Maybe I'd use it for a retrieval line for a canopy anchor.
 
Muggs said:
I wouldn't go too far down that rabbit hole if it was me. Rope has to be fully rated.
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I don't plan on being a test dummy but for the sake of argument why does it need to be fully rated? In my hypothetical extreme Pur Line has a breaking strength of 15 kN.* The 7/16" Sterling HTP I climb on has a MBS of 30.5 kN. Since the back strand only sees about half the load of the front strand this appears to be strong enough. It would never see ascender teeth or the heat and abrasion of ascend/descent. What hazards are a thicker or stronger anchor strand protecting against?

*I don't know if that is minimum or average, but let's treat it as MBS for simplicity.
 
Webfoot said:
... To save weight I would like to use a skinnier rope in this section for higher climbs...
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A better solution would be a canopy anchor with a light weight retrieval line. You could use as light a line as you want without comprising safety, and save even more weight.

You need to use a base anchored climbing line for a while to recognize how vulnerable it can be to damage.
 
DSMc said:
You need to use a base anchored climbing line for a while to recognize how vulnerable it can be to damage.
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Can you give examples that apply to recreational climbing? “Only a fool learns from his own mistakes. The wise man learns from the mistakes of others.”
 
Webfoot said:
Can you give examples that apply to recreational climbing? “Only a fool learns from his own mistakes. The wise man learns from the mistakes of others.”
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Keep climbing, your arms will get stronger and the weight won't bother you any more.

DSMc makes a very good recommendation if that's really, really, really, for some really weird reason, a really weird thing you care about still.

Using a rope rated for climbing to climb is just a good idea, if you're going to use it to... climb.
 
Webfoot said:
I don't plan on being a test dummy but for the sake of argument why does it need to be fully rated? In my hypothetical extreme Pur Line has a breaking strength of 15 kN.* The 7/16" Sterling HTP I climb on has a MBS of 30.5 kN. Since the back strand only sees about half the load of the front strand this appears to be strong enough.
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Am I missing something here? How does the “back strand” only see half the load? Also, just looked at the 6mm petzl line: that is NOT a life supporting line/rope from anything I gathered. More of a retrieval line/gear haul line like mentioned above.
 
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Stumpsprouts said:
Keep climbing, your arms will get stronger and the weight won't bother you any more.
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It's not the arms, it's the back. I wish weight didn't matter but it does. Not every tree I want to climb is right by a road.

Even within "rope rated for climbing" there is a lot of range. I am trying to understand the conditions this strand may be exposed to. I know that I am overly analytical to a lot of people but I am not ashamed of it.
 
Webfoot said:
Can you give examples that apply to recreational climbing? “Only a fool learns from his own mistakes. The wise man learns from the mistakes of others.”
Click to expand...

A good part of our safety is based on capacity of our equipment to withstand the abrasive nature of a tree climbing environment. The base leg is often out of sight and basically unattended. It needs to be as strong as the rest of your system.
 
Also, just looked at the 6mm petzl line: that is NOT a life supporting line/rope from anything I gathered. More of a retrieval line/gear haul line like mentioned above.
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I chose it as an extreme example that works out mathematically (2:1 strength) and is low elongation, not a declaration of intent to employ it.
 
I have no personal experience with climbing lines that thin.

How bad is your back? You are not going to save a significant amount of weight per 100' of high strength static rope.
 
It is a lifelong problem but I have better and worse days. I really noticed the additional weight of the Big Shot and two four foot poles walking a mere quarter mile carrying all my gear, and that was with only a 90 foot 7/16" rope. I don't think I can add another 100+ feet of the same rope to that so I am looking for places to cut weight. Or it's time to get a cart.
 
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