Why is descending w/ Blake's + DRT the exception?

In TCC, Jepson says, "Never use the Prusik knot, or any other friction hitch, as
the sole means for descending when using a static climbing line system. The
only exception is when using the dynamic climbing system." Why is the
dynamic climbing system the exception?

Bradley,

Take a 25 pound weight, and tie it to a piece of dental floss. Lift the wieght from the ground with one hand and below that hand use a pair of tweezers to hold the dental floss. This is an example of SRT system: the weight is you, your hand is the prussik and the tweezers is your brake hand in such a descent as you describe. If you let go of the dental floss with your hand you will see that the tweezers have no hope of keeping the weight from hitting the ground HARD.

The human hand can produce at most 150# of grip. One friction is broken within a friction hitch it has no holding power, and moves pretty much freely. Therefore if one was to try to descend only on a prussik then he would be faced with the dilema of overcoming his 9.8m/s^2 acceleration towards the ground only with the help of friction formed between his palm and the rope. I don't know the math involved but the downward force would equal the climbers weight, and the resisting force (in the unfortunate climbers hand) would be much less, which would result in an unpleasant 'descent' and most assuredly a very burnt palm.

A Ddrt system has the added benifit of extra friction being produced at the support anchor. This is what keeps you from slamming into earth everytime you break friction on your Blake's Htch.

I understand you have no descent equipment, so look into other descent options such as a 'biner wrap, or stick to Ddrt.

Icabod

Icabod says:

"A Ddrt system has the added benifit of extra friction being produced at the support anchor."

Why does the friction at the support anchor with a dynamic climbing system allow safe descent using only a Blake's hitch?

The ability of any descent system to control a fall depends on it providing just slightly less friction than is required to overcome the force of gravity; thus allowing a controled descent.

A DdRT system loops a rope over an anchor point, over which the rope must travel in order to move the climber up or down, the amount of contact between the rope and the anchorage, and the inherent properties of the rope and the cambium of the tree (coefficent of friction), determines how much friction will be produced. Some types of trees have very tough bark, and I've had times where my brake hand did very little work during descent. By the same token I've climbed trees that I felt more comfortable using a descent control device because the friction in the system was too little to keep me from making a controled descent.

There is also the fact that the force applied to the running part of the line is 1/2 the total weight of the climber. In otherwords if you ate enough doughnuts to double your weight (I reccomend Dunkin Donuts) then you would find that the amount of friction you would need to control your descent would be greatly increased.

So the simple answer is:
half the weight+more friction=controled descent
& too many donughts=a trip to the cardiolgist.

Icabod

The first tree climbing system is a dynamic (traditional) rope system installed with a false crotch that uses a pulley that provides minimal friction at the tie in point (TIP). One 200 lb. climber ascends and descends with this system on a friction hitch. Descending on the friction hitch with this system is generally considered safe, right?

The second tree climbing system is a static rope system. One 100 lb. climber ascends with this system on a friction hitch, but descending on the friction hitch with this system is generally considered unsafe, right?

If we assume no friction at the TIP in the first system, then the weight supported by the hitches in both systems is 100 lbs., right?

So why is descending on the hitch with the first system considered safe, while descending on the hitch in the second system is considered unsafe?

I have one other thought about descending with a friction knot (prussik, Blake's etc) on an SRT vs a DRT. The speed of the line going through the knot.

With the DRT I have to pay out 2 feet of line for every one foot I drop, while with the SRT I run through one foot of line for every one foot of drop. The friction is greater with the DRT due to the line over the limb and the amount of rope going through the knot. Greater friction equals more control. That's why the rock climbers use descending devices when they rappel, to gain friction.

If you're planning to use SRT (my favorite also), do yourself a favor and get a "figure eight" descender - about $12.00 via EMS or most other sporting stores. Much safer, but still fast when you want it.

consider going for an ATC-style device or belay tube: they are also very cheap and can sometimes behave better than an 8.

What is an ATC-style device or belay tube? Never heard of them. Where can I get a look at them?

I'm all for anything that that behaves well - except myself of course.

hmmm....thats wierd, many rock climbers in the gym and outdoors have never used figure 8s.



i must confess I am (respectfully) puzzled by the prevalance of figure 8s in the SE US rec treeclimbing community, because belay tubes do everything the figure 8 can as well as doing it far better (except disperse heat).
No risk of girth hitch locking you off, no rope twist, better control, lighter, can be used for dynamic belays... I reckon I'd rather leave the 8 at home and use a Munter hitch if I felt like twisting the rope.
The major failing of the belay tubes is that they don't make such a cool clinking noise.

they are reviewed by storrick at
http://storrick.cnchost.com/VerticalDevicesPage/BelayDevices.shtml

you can see them for sale at

http://www.rei.com/online/store/Search?vcat=REI_SEARCH&stat=7889&langId=-1&storeId=8000&textQuery=black+diamond+atc


There is an excellent link at REI's site called "Expert Advice: How to Choose a Belay Device" which unfortunately cant be linked externally...worth checking out.
Storricks site is always worth a good long look...

http://storrick.cnchost.com/index.html


And dont forget the prussik backup BELOW the device....works like a charm.


am looking forward to the heated response from the figure 8 afficionados.

Following some of the statements here.....I have a question:

Icabod said that if you doubled your weight, then you'd be hard pressed to stop yourself on a descent while climbing DdRT.

I know people who weight double what I weigh and they, too climb- albeit with a different climbing hitch. But he CAN climb with the traditional doubled rope technique.

So, could I, weighing one half of him, Use his DdRT friction hitch and climb on SRT with that, and have it function as it should?

I know the answer is no. Something else is going on here and I don't know how to explain it.

Any ideas?

love
nick

Redpanda

(great name, by the way)

You noted that many folks here use figure eights rather than belays. I know that for myself it has to do with the rope size. Most of the belays I've seen go up to 11mm line. But tree climbers most often use 1/2 in (13mm) arborist rope. From what I can gather the belays don't hold that size.

Thanks for the links, however. I might pick one up for when I'm on my 11mm.

Try anchoring the belay device with two or more carabiners, more surface area for rope contact.

there are also braking biners, put them on a leg loop and pull away to get more friction. All cavers using petzl stops include a braking biner, give you more options.


of course, its just friction, below any device you can wrap around your legs, braid rope through your fingers, use rubber gloves, wrap rope under your rear, or even place that autoblock beneath your device and let IT do the friction work....

This comes closer to answering the original question in this thread than anything else I've seen:

Friction Hitch for SRT?