/ Lowering off pair of bolts without chains
It is more normal tho thread both. What you are guarding against is a faulty bolt placement, not trying too hard to equalise the load. The resolution of forces means that once you are hanging even a little below the bolts the extra force isn't significant for a correctly placed bolt.
Completely agree with GrahamD. You need to be more concerned about lowering / abseiling off a single bolt than about the forces equalised or otherwise on 2 bolts.
Rings like that are usually arranged one above the other. Either way, thread both unless there's a very good reason not to (bolt in loose block, sharp edge etc) in which case, especially if concerned about loose I'd consider changing to a different lower-off or clipping the bolt below as a back-up.
Bolts don't care about 'weird forces' that aren't aligned 'down', they're all basically shear forces.
Correct - bolts in good rock will take the same load in any direction parallel to the face of the rock. If anything, threading two rings which are set apart horizontally reduces the net force on a single bolt, compared to simply threading one ring.
Read this article http://www.chetwynd.info/other/anchors.htm It provides a mathematical description of why you don't want to thread two anchors arranged in the manner you describe. It seems very clear that it is much safer to use one only lower off in this situation than to use both.
Did you read the same article you linked?
The point of 2-bolt lower-offs is redundancy.
You just beat me to it. I think that there have also been studies that show that achieving 100% equalisation is nigh on impossible especially when knotting a sling.
I agree its normal to thread both bolts. However I would say its far from 'normal' that they are arranged one above the other.
I did read it yes....I agree that if the bolts are equalised with a chain or a knotted sling then redundancy is the key advantage here. But the OP stated that the bolts in question had rings but these were not joined so they would be threading the rope through both rings, not through a single point that equalises the forces back to each ring.
If the bolts are vertically positioned then this is not an issue and would offer some redundancy if the higher bolt failed, but if they are horizontally aligned then this would create an angle approaching 180 between the two anchors. This would have the effect of creating a 'near infinite' force pulling these anchors together horizontally.
>> If anything, threading two rings which are set apart horizontally reduces the net force on a single bolt
I suggest you read up on the death triangle, as what you've said is clearly incorrect.
" http://en.wikipedia.org/wiki/American_death_triangle "
That said I wouldn't generally worry about the death triangle in this situation ( when lowering / abbing ) from bolts placed at a similar horizontal level, as the force that is potentially getting multiplied is very low anyway.
It's fairly normal here in France. Especially on multi pitch stuff where you often find two glue-ins, one above the other, slightly offset and one at 90 degrees so the rope doesn't twist when you pull through.
Happy to be wrong on this but I don't think it's possible to generate a 'near infinite' force from 2 bodies pulling on two bolts regardless of the vector directions.
Thread both, and if you are really worried that the added weight of the belayer might cause too much force on the anchors, abseil off as this will reduce the force on the anchor by upto half.
I think the force in question is ~1.4*climber's weight (when abseiling - double it if there's a belayer on the other end(?)), which is not very big compared to a leader fall.
Ok we are arguing semantics here however in my experience the norm for alignment of belay bolts is there is no norm ie you can't basically expect one alignment.
I'd say in dorset the most common alignment appears to be having them set at the same horizontal level which is also my memory of Scotland, I can't remember noticing a pattern in France, Italy or Switzerland but there may well have been one.
We're talking about a different situation to the one in that article. The rope is threaded through the two rings and back down:
i . . . . . i
i . . . . . i
i . . . . . i
i . . . . . i
i . . . . . i
B . . . . C
(Belayer and Climber)
The tension in the rope is equal to the weight of the climber. On each bolt, the forces are one going downwards and one going towards the other bolt, both equal to the weight of the climber. The infinite force effect described in the article does not occur.
No it wouldn't, not even close. You've misunderstood either the scenario or the maths but either way the generalised advice is the same: Thread both.
If you attached the two bolts by a tight non stretchy piece of wire and then pulled on the middle of the wire the force to the two bolts would be very highly multiplied (up to the point the bolts or wire breaks). Of course in the real world the wire would give a bit and so would no longer be completely straight so would not be infinitely magnifying the force on to the bolts.
I actually meant a single abseiler, but I appreciate it was ambiguous, and presumably if belayer and climber are the same weight then the forces on rope and bolts are all simply doubled.
The ADT effect clearly doesn't apply, because the bottom angle is effectively zero. Look at the diagram I just posted.
American Death Oblong?
Your diagram shows a belayer lowering the climber off, the OP has mentioned abseiling and lowering off in their opening question - clearly scope for some confusion here.
The OP has later clarified they meant a single abseiler so the ADT would come into play.
Thread them both, that's why there are two rings there. They are only taking body weight if you are abbing (or a bit more than that if you are lowering) so all the discussion about forces/triangles etc. etc. etc. is pointless,
But this isn't the same as the climber pulling down on either end of the rope via abseiling i.e. you are not pulling the middle of the strand. Hope this makes sense?
stewieatb's diagram looks pretty good to me, and also symmetrical (if you assume belayer is the same weight as climber). Slightly confused about your interpretation of it.
Earlier diagram was different, it was showing two ropes going up to a single ring then across the the second ring. Formatting error in the post text I expect. Edited diagram is much clearer - edited my no longer relevant post.
What? I don't think you've correctly understood the setup, because if one ring fails the load swings across onto the other ring, it doesn't somehow remove the entire belay system. Its also a symmetrical system, so if it works with one bolt failing it works with the other failing.
But in general, as Chris says, its a pointless discussion. You won't generate enough force on a bolt by lowering off it to break it with inward forces and death triangle loads and stuff. You're using two anchors in case one of them is placed badly, rusty or otherwise dodgy, basically. You don't need both of them in order to safely lower off if they are both well placed. So use both in case one of them is bad.
Nope, the tension in the rope is still simply the weight of the climber, provided the belayer is still standing on the ground. Assuming the tension is the same all the way through the rope (ie. no friction at the anchors), examine the situation where the climber is hanging freely on the rope. Equilibrium of forces tells you that the tension in the rope is equal to his weight.
At the other end of the rope, the belayer is standing on the ground (or hanging from a set of anchors). His weight is held up by a combination of the rope tension and a reaction force from the ground (which may reduce to zero, in which case he will take off). The tension in the rope is the same as if the rope were simply tied to an anchor.
Of course, for this simplistic model to work, the belayer must be heavier than the climber. In a real system, there is friction at the anchors, which allows the belayer to be lighter than the climber but still hold his weight. However, the principle is the same, and the average tension in the rope will not exceed the mean of the two climbers' weights.
Yes, sorry about that - the UKC autoformat ruined my ASCII diagram by ripping out all the spaces required to put the second rope in the right place. Now fixed.
Fair point, but only at the very top of the abseil. The bottom angle would be less than 5 degrees once you're a metre or so away from the anchors (depending on how far apart they are), so the force gain is not in the dangerous range. Assuming you're not hanging from the anchors while sorting the abseil out, you can probably start your abseil far enough from the anchors to stop it being a problem. If you are hanging from the anchors, then you may be better ditching a longish sling or some tat and making a narrow-angled v-anchor to thread the rope over.
When abbing the scenario quite clearly is a death triangle however as I'll explain there is also little difference when lowering quite often too.
Ok for ease of explanation let's imagine climbers B and C weigh the same and Climber A weighs B + C. Also lets call the lowers off LO1 (the left one) and LO2 the right one, and also assume climber A B and C are free hanging on the rope
Do you think it makes much difference if climbers B and C hang on the rope as per your diagram (call this scenario X) or only climber A does (at the same hanging height) making a death triangle (Call this scenario Y?
In fact it makes very little difference as long as the bolts are placed fairly closely together and the lengths of rope to the climbers are long. Neither scenario is going to magnify the loads on the bolts and in fact both bolts will take less load then either one of them being loaded in isolation.
This is because of the way the death triangle works to act as if the anchors were independently attached to the Centroid of the (death) triangle and pulled down from there. So much the same as with belay anchors the load will get magnified once the angle between two anchors exceeds 120 degreesand the greater this angle the greater the exponential magnification. Or in the case of a death triangle once the strands leaving climber A to LO1 and LO2 exceed an angle of 60 degrees between them / equilateral triangle. When the angle is smaller than this the anchors share the load when its greater the load is magnified (but there is some redundancy at least).
With the case of lowering off the angles at the centroid of the triangle will be 90 degrees so will be sharing the load, however there is nothing forcing B and C being in vertical alignment with each other (other than them free hanging in this example but you get what I mean) or indeed preventing ropes crossing over which could create exactly the same (or worse) scenario as a death triangle.
In the real world because belay bolts are placed fairly close together with fairly low loads and because the angles to the anchors are generally not multiplying forces the death triangle is not something we need to consider in terms of safety. However for setting up a belay it would be quite easy to equalise the bolts to create a scenario where the forces would be multiplied and this coupled with the potentially higher forces involved its worth making sure the anchors are equalised to minimise the force applied to them.
The tension in the rope is the weight of the climber, but the force on the anchors is double it, surely, since there's tension from either side as it goes over the anchor, both belayer and climber are getting an upwards force from the rope tension and the only place a downward force can balance than in a simplified system is at the anchor. That's why its safer to abseil than be lowered off truly marginal gear.
Still makes no odds of bolts mind you.
"It seems very clear that it is much safer to use one only lower off in this situation than to use both."
This advice is plain wrong and also potentially dangerous advice to give out to a newbie sports climber.
Actually, thinking about it, what I wrote is true for lowering/general climbing, but when you're abseiling on a halved rope, the tension is half your weight (your weight is shared between the two strands). So actually you were right, and the forces are doubled for lowering - is that what you meant in the first place? Apologies if so.
So are you trying to reduce the force on the bolt, or back it up in case it was placed by an idiot/rusted through/whatever?
You're right about it being safer to abseil from marginal gear. The actual net force on the anchors, once you're far enough away that the bottom angle of the ADT is nearly zero (and the rope turns through 90 degrees at each anchor), is the square root of 2 times the climber's weight for an abseil (and twice that for lowering).
"Nope, the tension in the rope is still simply the weight of the climber, provided the belayer is still standing on the ground. Assuming the tension is the same all the way through the rope (ie. no friction at the anchors), examine the situation where the climber is hanging freely on the rope. Equilibrium of forces tells you that the tension in the rope is equal to his weight. "
You seem to have misunderstood the OP's point, he asked are the forces doubled at the anchors (and in the rope) if you compare your example where a climber is being lowered or if they abseil off.
He was indeed correct. For simplicity assuming no friction, a single attachment point to the lower off and also that the climber being lowered is currently static. In the case where you are abseiling on both half's of a rope through the lower off, the tension in each strand of the rope is half the climbers mass times gravity and the force at the anchor will be mass time gravity. In the case where you are being lowered, the strand leaving the climber has their whole body weight on so is T (mass times gravity) and the other strand must also have the same tension to be in equilibrium. There fore the force at the anchor 2 times mass times gravity.
Your maths is a bit wonky.
If you assume the bolts are at the same height and there's no friction at the ring, lowering off one loads it downwards with 2x the climber's weight wherease lowering off both loads each with 1.4x the climber's weight at a 45 degree angle.
1.4x, 2x or 2.5x the climber's weight should all be trivial for a well-placed bolt though, so it really doesn't matter.
I acknowledged this in my post at 15:25.
For goodness sake clip 'em both it'll be fine!
You're right, I don't know where I got my numbers from! Can't repeat it now :-P
If you go back to the first two responses then you will find the safer climbing answer, use them both
Spot on with your explanation.
Ignoring friction, and using the triangle of forces calculation each bolt will loading about 142 % of body weight if lowering.
This of course can be reduced to 71 % each if you abseil off. This will also reduce the chance of the rope getting twisted.
Using two bolts gives redundancy :-)
Where's Jim Titt when you need him.
Me? Iīm here.
You can lower off one or both bolts and thread them any way you like, the forces are irrelevant in the context of modern bolts. If the bolts are less than trustworthy then use both or do something else.
Bolts placed in pairs horizontally tend to twist the rope horribly, increase friction and wear, common practice on the continent is only to use one for this reason and often only one is provided anyway.
Lower-offs are better arranged in a vertical orientation (or offset vertical) than horizontal, only one bolt then needs to be easily replaceable for wear.
> Thread them both, that's why there are two rings there. They are only taking body weight if you are abbing (or a bit more than that if you are lowering) so all the discussion about forces/triangles etc. etc. etc. is pointless,
100% in agreement.
Quite frankly some of the dangerous misinformation being touted up above is potentially confusing and might actually lead the inexperienced/gullible climber to think twice about what is a very straight forward thing.
Clip both rings.
There is an in-depth discussion about this at the end of this page (section 15):
Ex-engineer and others, please let me know if I've got anything wrong! Thanks.
Well explained jim on all lower offs and for explaining horizontal placed bolts that twist the ropes, when will bolters learn that?
I think it is only an issue when one (and worse when both) of the rings/maillions is flush with the rock. If they are both at 90 degrees to the rock it isn't so much of a problem,
Twisting can still be a problem if the rings arenīt free to hang but touch the rock somewhere but itīs not too common to find a suitable overhanging bit of rock! Vertical is the way to go for sure, canīt remember when I last sold a Vee chainset for an outdoor application (walls are usually different due to the weakness and position of the individual attatchment points).
The lower offs at Tintern Quarry are particularly bad as they comprise of two horizontally adjacent maillons which do a really good job of twisting the rope. I think that the uncertain future of access to the quarry has put people off replacing them. It's a shame, despite the dubious character of the place there are one or two very good climbs down there only spoilt and made more dangerous by this issue.
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