belay off anchor or belayoff harness

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Jon Beck

Trad climber

Oceanside

Jun 15, 2013 - 08:51pm PT

Ron - Fall Factor 1.375

Click Here for Fall Force Calculator

rgold

Trad climber

Poughkeepsie, NY

Jun 15, 2013 - 09:00pm PT

Argh, not that "calculator." It is completely bogus!

Ron, when you say "40 feet out with a 30 foot fall," this would normally mean the leader was 15 feet above the last piece, which was 40 feet from the belayer. If this is true, the fall factor was 30/55=0.55.

If you mean the leader was 30 feet above the last piece, (which would be a 60 foot fall), then the fall factor would be 60/70 =0.86.

In neither case is it anywhere near 1.375.

The only fall force calculator I know of at present that is "correct" in the sense that it corresponds to a valid mathematical model is Jay Tanzman's at http://jt512.dyndns.org/impactcalc.

In answer to your other question, the belay device doesn't change the fall factor. But the fall factor is only fully relevant when all the fall energy is absorbed by the rope stretching. To the extent that other mechanisms are involved (tightening of knots, rope slipping through belay device), the fall factor can only describe part of the energy-absorbtion process, and most of the "other mechanisms" depend solely on fall height and not on the ratio of fall height to amount of rope out.

aguacaliente

climber

Jun 16, 2013 - 02:23am PT

I took Ron to mean that the leader was 40 feet up and 15 feet above pro, thus a 30 foot fall on 40 feet of rope, or fall factor 30/40 = 0.75.

From that you can see it's hard to get a fall factor 1 without falling past the anchor.

Good lord that myoan.net calculator is useless, it can't get even the simplest cases right.

rgold

Trad climber

Poughkeepsie, NY

Jun 16, 2013 - 01:20pm PT

I took Ron to mean that the leader was 40 feet up and 15 feet above pro, thus a 30 foot fall on 40 feet of rope, or fall factor 30/40 = 0.75.

I just worked this out in the previous post! There are 55 feet of rope out, not 40 feet (you have to count the rope from the last pro to the leader's waist). So it is 30/55, not 30/40.

rgold

Trad climber

Poughkeepsie, NY

Jun 16, 2013 - 01:35pm PT

I couldnt get that second link to make any sense, or even complete the input.

What to enter in each entry box:

Fall factor: put in 0.75 as aquacaliente indicated above. (I didn't read your description correctly.) It may be less than that, since the fall factor is calculated before rope stretch, not after.

Climber's weight in pounds: Obvious meaning.

Impact force rating: This is the UIAA rating for your rope. Probably around 8 kN.

Friction factor: 0.33333... Leave as is. This represents the mechanical efficiency of the top carabiner as a pulley.

aguacaliente

climber

Jun 16, 2013 - 01:48pm PT

I just worked this out in the previous post! There are 55 feet of rope out, not 40 feet (you have to count the rope from the last pro to the leader's waist). So it is 30/55, not 30/40.

I understand, we just have different ideas about what he meant by 40 feet up - was the leader 40 feet up, or was the last piece 40 feet up? Either way I think we agree that the fall factor is moderately less than 1.

Edit: Ron, in the jt512 calculator, you have to put in the fall factor (like 0.7-0.8) not the fall distance (40).

rgold

Trad climber

Poughkeepsie, NY

Jun 16, 2013 - 02:09pm PT

5 kN peak load on the belayer is about 1124 lbf. Ouch!

gunsmoke

Mountain climber

Clackamas, Oregon

Jun 17, 2013 - 12:28am PT

I've taken two 20 footers onto the anchor with nothing in between. I wasn't hurt by either fall because I WASN'T belayed off the anchor. Wasn't nice on the belayer, but it prevented bailing.