Forums: Climbing Information: The Lab: Spanish Burton conversion to 7:1 & other musings: Edit Log


Nov 20, 2011, 7:52 PM

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Spanish Burton conversion to 7:1 & other musings
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I’ve known about the Z pulley system and a few other self-rescue basics for some time but never went much beyond that. Recently I got into the Tyson & Loomis "Climbing Self-Rescue" and was quite taken with the Spanish Burton pulley system: same 3:1 ratio as the Z system, requires one more prusik but allows pulling down instead of up. I also liked the easy conversion of a Z 3:1 to a 5:1 but was disappointed to find no equivalent conversion option for the SB.

Of course there is the option to have a 2:1 pulling on the SB’s pull cord for a 6:1 system but that requires adding an upward pull anchor. If instead of the anchor, you attach the second pull cord to the primary prusik, you convert the SB to a 7:1 as shown here. Searching for this system on this website, elsewhere on the web and in various other self-rescue manuals, I keep finding either nothing (in most manuals), or references to a 7:1 system with no description (in many posts), or the description of a 7:1 based on a modified Z that, of course, requires pulling up. I should mention that, of Fasulo’s “Self-Rescue” book, which is one of the places where that description is given, I’ve only seen the pages shown on GoogleBooks. Anyway I figure that the SB-to-7:1 conversion is too obvious and simple not to have been described before so pointers to prior descriptions would be appreciated.

Working out the ratios also gives the proportions that go to the anchor and I became concerned with the forces that the SB and its 7:1 conversion apply to the anchor. In the “Mountaineering Handbook”, Connally dismisses concerns about high rates of transmission of load to anchor, but both he and Tyson&Loomis warn about ending up pulling hard on a high ratio system as a result of high friction and/or snags at the load end. Working out actual ratios using a 1:0.70 transmission factor over biners (ie in a self-rescue situation with no pulleys available), you get the following estimates of the ratios (along with the proportion transmitted to the anchor point in brackets).

- Z = 2.2:1 (1.2)
- Z redirected downward = 1.5:1 (2.5)
- Z conversion to 5:1 = 3.4 :1 (2.4)
- SB = 1.9:1 (2.9)
- SB conversion to 7:1 = 3.9 :1 (4.9)

After staring at these numbers for a bit, their obvious practical significance finally hit me. If you've got your feet planted on a ledge and are pulling up, the load on the anchor will be that needed to hold/raise the hanging load (weight of the fallen climber, plus any friction, snags, etc.) minus what you're applying by pulling on the system (eg Z system: anchor load 1.2 = hanging load 2.2 minus pulling load 1). Standing on the ledge pulling down, the load on the anchor will be the hanging load plus what you're applying pulling down on the system (eg SB converted to 7:1: anchor load 4.9 = 3.9 + 1). If you're on a hanging belay on the same anchor you're hauling from, whether you're pulling up or down and regardless of the system, the overall load on the anchor will be the same: your own weight plus the full hanging/dragging load. If you're pulling down, you apply more tension on the system but lighten your own weight and if you're pulling up, it's simply the opposite.

Based on this, it would seem that the choice of which system to use should be much more dependent on the specific situation than I've seen suggested anywhere. The SB is ideal for hanging belays. In that particular situation, it puts no more load on the anchor than the Z and is easier to operate, not just because of the movements involved but also because pulling down will ease the discomfort of sitting in the harness whereas pulling up would greatly increase it. One other advantage of the SB is that, if putting your whole weight on the pull cord by stepping into a foot loop is barely enough to raise the load, you can increase your own weight further (and somewhat decrease the hanging load at the same time) by pulling up on a handle (sling) clipped into the biner of the primary prusik. If that's still not enough, it's doubtful whether the Z converted to 5:1, with its less effective pull-up movement, would have been a very significant improvement whereas the straightforward conversion of the SB to 7:1 should definitely make a difference. Of course, it’s essential that the anchor be bomber and that care is taken not to end up pulling hard on this high ratio system because of serious snags or way too much friction.

In the opposite situation: belaying from a ledge (or even a half-decent stance) but from a not-so-bombproof anchor, it’s probably best to avoid the SB. Before deciding on hauling, all other rescue solutions should have been seriously looked at but, depending on how “not-so-bombproof” the anchor is, another hard look may be indicated. If hauling cannot be avoided and, despite your best efforts, you’re still not 100% sure of the bombproofness of the anchor, the go-to system should be the Z. However, converting it to higher ratios should be avoided as much as possible. In addition to masking the true load being applied to the anchor, these systems also make it too easy to generate spikes in the forces through acceleration if one is not careful to pull progressively. In this situation, in addition to making every effort to minimize friction and avoid snags, pulling up harder on a lower ratio system (adding a prusik on the rope at the tail end for better purchase) would be the way to go to raise the hanging load while further decreasing the overall load on the anchor.

In intermediate situations, it would be a matter of weighing all the factors, selecting the best possible combination of stance and pro options within the limited range of options handed to you by the circumstances, and installing the system that provides the highest possible safety (anchor loading, raising rate, etc.), as well as reasonable ease of handling, for that combination.

I’ve not seen a discussion of the choice of systems based on the circumstances in any of the manuals I've checked. I feel like I'm reinventing the wheel while being unable to find a description of the wheel anywhere. So, do these ramblings make any sense and could someone please point me in the direction of the bloody wheel?

(This post was edited by jktinst on Dec 10, 2011, 7:58 AM)

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Post edited by jktinst () on Dec 10, 2011, 7:58 AM

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