Rapping Off Different Diameter Ropes

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Rapping Off Different Diameter Ropes

Postby Burley » Wed May 27, 2009 2:31 pm

Below is an email I got from Brent C with some good info to keep in mind:

Hi Erick,

I was checking out the Gripped website and saw that john won the Tour this month. This lead me to check out the UNB website and I notice that you guys are now running an advanced rock school. I had a discussion with some climbers a few weeks ago and a topic that came about was double rope rappels with different diameter ropes. This is a situation that is becoming more common as people are carrying 7mm and even 6mm tag lines for rappeling. The issue with this that most people don't realize is that the different diameters feed through belay devices at different rates. This causes two major problems.

1. The middle moves and one end of the rope is shorter than the other making a situation where one could rappel off the end of the rope.

2. The shift of the middle usually happens all at once. If your anchor is just a piece of chord or webbing you risk buring through the anchor.

I have seen a block and tackle rigging system which works well (at least with two bolt anchors). This system uses a biner and a large knot that blocks the large rope from pulling through the anchor and the tag line is simply used to retrieve the rap line.

This is not a huge problem but definelty worth mentioning to folks.

Cheers,

Brent

___________________________________________________

Thanks for the information Brent.

We also had an off topic discussion about rapping off sling(s). Some were convinced it was fine ("do it all the time on ice"). Norfolk and I did a demo to show how quickly a rope can burn through 1" webbing when weighted - about 20' with speed and body weight and it cut through on new webbing. The instance you outline is what we used for the real world example of the potential situation.

So if you're using different dia. ropes you could be toast if you are the last one to rap off that point on the webbing in a group of three. I thought this could be the case and it is nice to know it is.

In the school we climbed with one leader and two students. Each student climbed on their own rope. Leader dragged up a line that was either the same diameter or very close - we didn't use a skinny light tag line. Attached them with a single EDK for long raps.

Erick
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Re: Rapping Off Different Diameter Ropes

Postby *Chris* » Wed May 27, 2009 3:32 pm

Yep. I don't quite understand the comment about the movement happening all at once though? Seems to me like it would happen gradually over the length of the rap as the two ropes stretch and move through the device at different rates. What is the mechanism that causes the sudden shift?

Personally... I find sudden shifts which happen mid-rap to be unsettling :|
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