Using single line technique for more than ascent

Daniel,

Rope stretch is often given by manufacturers of the rope. Some use 300 lbs, 600 lbs, 10% of MBS as the test weight to measure stretch so that makes it difficult to make precise statments about rope stretch. For example, here's PMI's specs for 11mm EzBend - I used that because it's handy for me,

Elongation: 1.2%(300lb), 2.8%(600lb)

You can easily see that the elongation (stretch) at 600lb is a wee bit over double what it is at 300lb.

But, stretch characteristics are not gonna vary so much as to come anywhere close to offsetting a doubled rope factor. So what if it's 10% with one strand and 5% on each strand of a doubled rope? The shock loading on the doubled rope is still nearly twice what it would be for a single strand.

Also, the more rope there is to stretch, the more the shock loading is reduced. If a climber is hanging on a limb in a DdRT setting he is connected by two strands of rope sharing his weight. If he's cinched or choked to the same limb on a single strand of rope with the distance from the limb to the climber the same, friction is pretty much out of the picture and there will be approximately twice the shock loading to the TIP and climber for a given fall distance than with the single strand of rope.

If the DdRT hitch slips, that's a different situation.

A ground tie most assuredly puts more force on the TIP than a DdRT or choked setting would. In reality, friction at the TIP will modify the force to be somewhere between 1.5 to near twice the climber's weight. Further, a ground anchor would tend to bow the tree more near the top because there can be near twice the loading on it. In the case of a fall, the TIP has up to twice the shock loading of the climber plus the weight of the climber.
 
At the ITCC in St. Louis the access line for the Work Climb had a recording dynamometer. All of the people who entered the tree were weighed [I think] and then sent up the doubled line. The numbers were crunched to find out what loads are put on a TIP. I read an article but I don't know where...but my recollection is that the loads were roughly doubled from bouncing. Not all of the climbers bounced though. Some were smooth so they didn't double the load. There was a range.

Years ago when I started to learn about rope stats I spent almost an hour with Dick Hildebrand talking about how stretch is measured and designed into a rope. He said that there is no required, standard, test for stretch. This means that from manufacturer to manufacturer there is going to be widely different numbers. During forum talks I've seen the evidence of this. A new rope will come out with a stretch spec of X. Then, when climbers get on the rope they compare how the new rope climbs vrs. another with +/-X amount of stretch and the specs don't make sense.

The EU specs for ropes actually have credence. They have specs for rope applications.

For me, Tachyon meets the strength and stretch specs as well as other performance attributes perfectly. For more specific or limited applications I might choose another rope though.
 
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...The numbers were crunched to find out what loads are put on a TIP....the loads were roughly doubled from bouncing. Not all of the climbers bounced though. Some were smooth so they didn't double the load. There was a range.

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ABSOLUTELY Tom!!! That's exactly why it is unwise to count on friction at the TIP reducing the load on the TIP. Bouncing could overcome all the friction and pre-tension the anchor side.

As for stretch characteristics, it's almost irrelavent. A single strand is gonna stretch approximately twice as much as two strands holding the same load. The stretch factor seems to increase with heavier loads.
 
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Daniel:... I'd still like to see some scicene done using a real tree, and real climbing line... I think there may be some false assumptions presented here.. related to rope elasticity etc.. Does rope really stretch twice as much with twice the weight or half as much with half the weight?...

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Ron:...If he's cinched or choked to the same limb on a single strand of rope with the distance from the limb to the climber the same, friction is pretty much out of the picture and there will be approximately twice the shock loading to the TIP and climber for a given fall distance than with the single strand of rope...

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Ron is certainly right that a doubled rope has twice the stiffness of a single rope, but that does not mean the doubled rope experiences twice the shock load from a falling weight. It also does not mean the doubled rope experiences only half the stretch of the single rope. The following link to the official rope specs for Yale Blaze includes one of Yale's beautiful charts of energy absorption, also known as the stress-strain curve. As you can see, the curve is wonderfully linear, i.e., the rope's stretch is almost directly proportional to the load applied. That answers Daniel's question.

To deal with Ron's assertion that doubled rope experiences twice the shock load, imagine a single line that we shock with a large load that almost breaks the line. That means the rope has absorbed almost all the energy it is capable of absorbing without breaking. Now let's do Ron one better. Take the doubled rope and double it again. Now we should see 4 times the load as the single line. This puts us right on the verge of breaking the rope, as before. But we are still dropping the same old load, thus the same amount of energy that the rope must absorb. If one rope can barely handle the energy, then for 4 ropes it will be a breeze and the rope will be nowhere near breaking.

The correct numbers come right out of Yale's beautiful energy curve (with a little math). You absorb the same amount of energy with two ropes by stretching .7 times as far as the single rope, and the maximum tension goes up by a factor of 1.4 (1/2 the square root of 2 and square root of 2, respectively). The energy curve is not only the goto way to solve problems of rope stretch, it is also a great tool for understanding what is really going on.
 
Moray,

No where did I say a doubled rope experiences twice the shock load. Here's what I said, copied right from your quote of what I said, notice what I put in bold:

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Ron:...If he's cinched or choked to the same limb on a single strand of rope with the distance from the limb to the climber the same, friction is pretty much out of the picture and there will be approximately twice the shock loading to the TIP and climber for a given fall distance than with the single strand of rope...

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That's what I asserted. But yes, the rope is twice as stiff, but the shock to the climber and TIP is 1.4, not twice as I stated. I really should have thought that through a bit more.

The chart for Yale Blaze may be linear, but not all ropes are. The data I quoted from PMI indicate some non-linearity. IIRC correctly KMIII also has some non-linear stretch characteristics.
 
Ron, do you ever leave your keyboard?

I didn't mean to misconstrue what you said. Clearly you didn't intend to say the rope experiences twice the shock, but rather, unless I still have this wrong, the doubled rope. We both seem to agree that the load on each half of the doubled rope is about .7 times the load experienced in the single-rope case.
 
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Ron, do you ever leave your keyboard?

I didn't mean to misconstrue what you said. Clearly you didn't intend to say the rope experiences twice the shock, but rather, unless I still have this wrong, the doubled rope. We both seem to agree that the load on each half of the doubled rope is about .7 times the load experienced in the single-rope case.

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Nope. I was comparing a fall on a double rope to the same fall on a single rope.

I said a doubled rope would generate twice the forces on the TIP and climber that would be generated by a climber on a single rope experiencing the same fall.

But, again, yes we agree on the 0.7 thing - I'm just glad you spotted that.

Edit: Oops, I forgot: I'm at school, if I'm not in class, I'm pretty much sitting in front of my computer working, usually on it. We've gone 'paperless' so most of my grading gets done electronically. I just spot check the boards every now and then - you can't ever tell when somebody might post something interesting!
 
I have taken poetic license more than once when I talk about load multipliers/reducers.

At the beginning of any talks I have a 'disclaimer' portion where I let the audience know that I can't speak in fine accuracy because of the variables that we work with. The differences between ropes, loads and friction are hard to incorporate.

After doing talks where I've gotten down to extreme accuracy and lost people I moved back to using whole numbers. For me, a .7 factor is close enough to 1 to be at peace with the inaccuracy.

If I have time or a small audience I will go to a more accurate set of data. Maybe I should annotate by using 'roughly' when I use multipliers and combining factors.
 
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... Maybe I should annotate by using 'roughly' when I use multipliers and combining factors.

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Yup. There's always some nerd like me waiting to catch you out.
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Treeman 805,

Now has anyone actually done testing on climbing SRT and with what kinds of cordage/ angles/ shock loads, etc.? No matter if you are climbing SRT or Double line, you don't want to fall! I personally think there is less TIP load with a static line and smooth ascent/ climbing technique, not to mention it is backed up by numerous crotches from redirecting.

X-man O.G.
 
O G XMAN
sure i think poeple have done some testing but no one would ever print anything in todays sue happy forest.once again u r scared of one line or u r not

PS why make 2 cuts when u can just make 1
 
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