In reply to The Ex-Engineer:
> They are generally hanging belays. The statement is not necessarily at odds with my previous post as I did specifically say they were an exception.
Libby does not make that distinction, and what she says is pretty much in line with what I said. I quote from Libby's article:
"Just occasionally you may be happy to use only one totally bombproof anchor"
"Most of the time you'll want at least two bombproof anchors in your belay. If either of these are anything less than perfect add a third and so on"
This is in contraindication with what you said - "Suggesting otherwise (even implicitly) by equating more gear with a BETTER belay is not something I would consider particularly sensible"
> On a good VDiff type stance, if you have doubts about a belay anchor it is trivially easy to inspect and adjust it. Also you will generally be able load it or even bounce test it fairly safely and see what happens.
Your original posts were not limited only to large VDiff ledges, in any case do you really go about bounce testing belay anchors?
> As Jim points out the climbers weight of perhaps 2kN explains the difference between my 2-3kN figure and the 6kN he mentions.
Not really The weight of the fallen climber is accounted for already in the slipping of the belay plate and most climbers weigh around 80kg or less so the belaying climbing is unlikely to add much more than 1kn to the system if the attachment to the anchors is fairly tight. So using your model we are still looking at the belay only needing to hold 3-4 kn.
I wonder what forces would be exerted on the belay if climbers are 100 metres up, the leader factor 2 falls, the belayer can't hold the fall and let's go of the rope and the leader than falls with the rope free slipping through the device until the rope has completely slipped through the device. Or indeed factor 2 falls with all the rope paid out (which is possibly however unlikely - perhaps some gear rips but isn't good enough to slow the fall much). In this case the force on the belay could be much higher than 2-3 kn!
Yeah I have read that before.
> He has actually test result for an ATC XP. With a really firm grip (400N) on a single rope the highest force he obtained was 2.8kN.
Sure but I did mention the ATC XP guide which I have found (from owning both devices and abseiling) has quite alot more friction on narrow ropes than the normal ATC XP and has different shape cleats.
> Doubles ropes will not double the forces and as the same grip force is just split between the two ropes. In fact, going from Jim test results for varying rope diameters, with 8.5mm ropes the force is more likely around 1.2kN (4x power ratio at an average 300N grip strength) and certainly no more thn 2kN (5x power ratio at 400N grip strength).
OK if belaying on a single 8.5mm rope you can see from his test results that the holding force before the ropes slips through the device on a normal ATC XP in high friction mode is around 70% of a 10mm. So actually we'd be looking at roughly 1.96 kn for 400n hand force.
I also question your assumption that this figure would be the same for two half ropes as the hand force is split. I can see your logic but the ropes are both going over a breaking cleat so I suspect the actual result is more complex than that, certainly you would expect the ropes to slip less through the device when holding heavy falls than using a single strand - but I do take your point that my original assumption was incorrect.
Post edited at 11:25