I am starting out and me and my crumbcrunchers are just doing TR's right now. My first taste of climbing back in late 80's included reading the old classic "Mountaineering:Freedom of the Hills" where I learned to use doubled oval biners with gates opposed when a locker was not available.

I just wondered though if doubling the biners which have a strength rating of 18kN is now 36kN. What say you? I'm no engineer but I am guessing its something more than 18kN and less than 36kN.

Doubling the biners doesn't increase the breaking strenght of the system, it simlply makes it redundant.

Assuming you have identical biners and are loaded identically, why wouldn't they double the strength of the system?

I am a structural engineer. Assuming the biners are loaded equally, it does indeed double the strength of the biners.

Ladd,

It doesn't necessarily increase the strength of the "system", since the rope strength stays the same. However, it does double the strength of one component of the system, i.e., the biners.

With regard to strength: Doubled biners increase the strength at that point in the system. Adding a biner for strength purposes may or may not increase the overall strength of the system.

Edit: Redundant post :)

Casting my mind back to my studies of Failure Analysis in aluminum alloys (I hold a Masters Degree in Materials Engineering {metallurgy}, with research into aluminum alloys but not specifically Al6065 of which biners are made);
If each biner were to be loaded equally in tandem then the strain of a given load would be distributed equally also, therefore allowing twice the load to be applied before failure. This is all under perfect conditions and assuming that both biners would fail under the exact same load. In reality biners break within a range forming a bell curve- so when the first biner begins to fail the second would receive greater than half of the load and so would fail also- yielding twice the strength of the weakest biner.

This does not necessarily double the strength of the entire system as the weak point my not have been the biner in the first place, but does, as was mentioned, provide increased redundancy.

In conclusion: Two are better than one.

Probably this was not much help to you but was interesting to me by forcing me to think back to my college days, thanks for that and just go enjoy climbing.

I am also a structural engineer, and I second Mike D's description of what's going on here. The strength of this portion of the system would be at least double the strength of the weaker biner. Important to note that this is assuming the biners are loaded equally, along the spine of each. However, if you're planning on putting a 36 kN load on your biners, you should probably re-evaluate your climbing practices beforehand.

Why don't you retire those heavy old oval biners and buy some new gear?

Pick up some Trango SuperFly Screwlock biners (41 g weight, 24 kN strength):



Or some Black Diamond Positron Screwgate biners (56 g weight, 25 kN strength):



Lighter and stronger than your old ovals, and you won't have to double them up.

On a toprope setup, I'd highly recommend doubled and opposed lockers to run the rope thru off the TR anchor. Why risk using just one?

My mistake; the "won't have to double them up" comment was about locking biners at belay anchors, not toprope anchors.

I use double locking biners to toprope as well, for increased friction, reduced wear on the biners, and redundancy. You can't always inspect what's going on at the anchor when you're on the ground; the second biner is a safeguard against weird things happening to cause a biner gate to unlock or open. I prefer the Petzl Attache biners for toproping:



I use only one locking biner to clip the climbing rope into a belay anchor, however, but usually back it up by clipping in with my Personal Anchor System.

Sweet, so it not only provides redundancy but enhances the strength at that point.

I guess I always thought that I should reduce the sharpness of the bend where the rope runs through by doubling the biners and the ovals being symetrical line up better when opposed. I am certain that I read that somewhere...is the tighter bend in the rope of any importance?

As far as the strength goes though, seeing as 1 inch webbing is rated at about 18kN, any single locker should be equivalent or better (as long as its weighted along the spine that is) and the sling is the weak point.

Just wait till y'all get your grubby little paws on the new DMM Locking revolver. Wahoo!

http://dmmclimbing.com/productsDetails.asp?id=3&id2=83

It should hit the shops this spring. Perfect for top ropes and who knows what else. Enjoy.

Jerome

Jerome,

I sure hope you are kidding. If not you're a fvcking idiot.

This DMM "Revolver" reminds me of a question. If there are rescue pulleys that are as strong as a biner, then why arent they used for toproping instead of biners. It seems that they would put less wear on the rope thru friction.

Some have mentioned the increased friction being a plus, possibly for the lowering part of the climb so you don't come down too fast. But if the belayer has got you on belay and slowly letting out the rope, then its really up to them how fast you go.

I know there must be a reason why pulleys are not used and I thought hmmm maybe it's because they have more moving parts that could break than a biner. But then a carabiner would also be safer for rescue work.

I try to never enter into net discussions where folks clearly don't know what they're talking about because they usually (and quickly) degenerate into name-calling. However, I started to write a response this time because I really don't like being called a fucking idiot, even if its true. I reserve that right to myself. But as I wrote I found myself knee-deep in techno talk, and that bores even me. Here's the quick and dirty:
The friction in a toprope system is supplied by the belay device and the belayer; any other friction in the system is incidental. If you don't believe me, try it your self some time under controlled cicumstances (i.e. not with a climber on belay). In the real world, use 2 locking biners for added safety and redundancy. I'll be very happy with my locking revolvers and less rope wear, less biner wear, and less difficulty taking up rope, thank you very much. And if you wanted to you could indeed use a pulley in a top rope anchor with no problem, but who wants to carry a pulley?

Jerome
(flames and other stupid stuff will be ignored, unless exceptionally witty).

oops - PS - I would only use the locking revolvers in a toprope anchor doubled, just as I would only use doubled biners in any toprope anchor. I was not suggesting replacing 2 lockers with 1 locking revolver.

JS

Additional friction in a toprope situation occurs where the rope is in contact with the rock on less-than-vertical lines. In these situations, I could see where a Revolver pulley-biner might work OK when a lighter belayer lowers a heavier climber.

But for a vertical toprope, with no contact between rope and rock, brenta's analysis applies. A lighter, unanchored belayer would be lifted off the ground with a locked-off belay device and a low-friction pulley at the top. Not something I would want to trust.

At the Boulder Rock Club climbing gym, the top ropes are wrapped twice around the big metal bars at the top of the wall. This is to increase friction in the lowering system, so lighter belayers can lower heavier climbers without getting pulled up in the air. But it still happens. Imagine what would happen with a frictionless pulley at the top of the wall!

I have the acceleration of the belayer, assuming a massless, frictionless pulley and massless rope, which should be a suffecient approximation for our purposes, to be proportional to the ratio of the climber's mass over the belayer's mass. For the mathematically inclined:

a_belayer = -1/2*g + 1/2*(m_climber / m_belayer)*g

If the masses are equal, acceleration is 0, as it should be. If the mass of the belayer is greater than the mass of the climber, the acceleration of the belayer is downward, thus 0 by the constraint that the belayer is standing on the ground and experiences a normal force. If the mass of the climber is greater than the mass of the belayer, the belayer accelerates upward according to the above equation until the other constraint we placed on the system occurs...the climber experiences a normal force. Ouch.