NZcaver wrote:fuzzy-hair-man wrote:This has been a good topic, it's confirmed some of the stuff I intuitively thought about redirects, ie they can see some very high forces if you aren't careful and are basicly pulleys and act in much the same way.
Ah, but those "very high" forces you mention can never (theoretically) exceed twice the load. And even then, any higher-than-normal loads are on the deviation anchor only. Tell me - how many anchors and anchor systems do you use that do fine with a single-person load, but would be in danger of failing with twice that load? (Please think regular SRT, and not agency-response technical rescue situations.) This would mean a safety margin of only 2:1 - not very likely.
Bottom line - pick good anchors and you should be fine. Don't use a flimsy piece of cord to rig the deviation (which I've seen before). Be aware of the higher forces involved - but don't worry about them too much. After all, we're not talking tensioned highlines here...
Hmmm
But.... we said before in this thread the consequenses of a redirect/deviation failure aren't generally considered as severe as the consequences if a rebelay or main anchor were to fail this is why as discussed before in the example we could do away with the backup to the deviation/redirect. The exception I'd put is what is the rope going to come in contact with if the redirect fails? if it is a sharp edge it may end up cutting the rope .
For the same reason anchors that cannot be used as rebelays are able to be used as redirects/deviations. If we are going to be placing forces equal to or greater than that of the mainline doesn't this suggest as you said the that the anchors and rigging components need to be at least as strong as a rebelay?
The way I see it is we probably need to avoid redirects which put an angle of greater than say 45 degrees on the rope, if we do put them in we need to do so with the knowledge that they are taking a load greater than or approaching that of the main line, and select anchors appropriately.
I'd also say that any thing over 90 degrees would probably be simpler with a rebelay anyway, correct me if I'm wrong but I think you'd need 2 redirects in a Z or rather N pattern to achieve an angle of 90 degrees or over any way then you may as well make the second redirect a rebelay (its seeing > 100 percent load anyway) and do away with the upper redirect. (maybe that needs a picture, tomorrow it's dinner time )
thoughts?