In reply to henwardian:
> Turns out I'm a little behind the curve here. I had assumed (wrongly!) that the megajul and microjul were similar to an atc or a reverso. After watching the video from Edelrid, I'm guessing the trade off for all that functionality and failsafeness is that the tight angle the rope goes through on autolocking (compared to a gri-gri) is what you suggest will damage ropes?
> I am suprised that this sort of hair trigger locking (if I can't say "autolock" ) is at much risk of damaging a standard rope though. The locked position has the rope doing the same thing as a standard atc and I'm not aware that under the normal run of things I let rope slide through the device as I'm catching someone (if a dynamic belay is being employed I'm reasonably sure I do it just by moving my body).
> Am I missing something?
> I'm afraid I'm ignorant of the Sliding X, can you link it? (Googling just gives me sliding X anchors)
> What do you mean when you say the Megajul is "weak"?
> In terms of tests, belay devices have been around for decades, there are hundreds of designs, millions of people have used them and they must have held tens/hundreds of millions of falls. I think you would have to get pretty exotic to have any chance of thinking up a new way of loading them that hasn't happened at least hundreds of times before. Perhaps I'm being disingenuous but my take would not be that eventualities are not known but that they are not tested for one reason or another. It may be that situations like the 20m 1.9 fall factor fall isn't tested because the extreme stresses on equipment and a climbers body would preclude a manufacturer from risking a claim about this sort of use because any fall like that is almost certain to result in serious injury.
All this type of belay device work by using the force of the faller to pinch the rope between the karabiner and the body which puts considerable stress on a small area of rope. To save the rope from being actually chopped through the karabiner is prevented from contacting the body and a small gap is left which allows some rope movement at higher loads. The manufacturer has either the choice of making the gap small so braking performance is good with thin ropes in which case thick ropes can get into the zone where damage may occur OR they leave a bigger gap and braking performance with thin ropes is marginal. The various devices on the market cover both of these possibilities.
With conventional belay plates the amount of braking force is reasonably proportional to the gripping force of the belayer on the rope, the assisted braking devices such as the MegaJul are confusing because initially they provide a substantial amount of braking force with little or no braking effort from the belayer and it was always assumed this extra braking effect was also proportional, the reality is that initially they outperform conventional plates but at higher loads an increase in belayer gripping force is not rewarded with the same benefits as it would be with a conventional plates. At higher fall forces the MegaJul displays weak braking performance compared to many conventional plates. Whether this is of concern is another debate regarding the likelyhood of rope burns and injury due to excessive fall distance.
The sliding X reference was regarding anchors, the original development and promotion of the idea was accompanied by some rather weak testing and failure to analyse what was actually happening and it was left to others to do the work more carefully and show the drawbacks involved. 30 years on we know a lot more but this could and shoulkd have been done before launching the concept onto the climbing world.
The test requirements to be awarded the UIAA safety label and achieve prEN certification require the device is tested with the thinnest allowed new rope and the rope is prevented from moving in the plate, with an older (but still satisfactory) rope of larger diameter and with no restraint on movement apart from the normal simulated hand force some of the devices are exhibiting the potential for rope damage . The concept behind climbing safety equipment requires all the equipment survives considerably worse than a 20m FF1.9 so devices which fail to do this are a problem, in particular when other well established but less "trendy" devices are capable of better performance with no negatives. That the manufacturers do not test their equipment to the extremes due to laziness is also a problem.