SRT advice

[pdqdl]

I pretty much prefer using a 1/2" screw link to my double-ring friction saver.

1. it's smaller and lighter, and I can attach it to a spliced eye.
2. Less chance of getting stuck in a tree: pull it down with the Drt rig in the same direction that it went up.
3. Cheap and simple. I have lots of screw links.
4. Easy to inspect, I have never even heard of a screw link failure that was properly screwed on.

 

[constantine]

SRT and mechanical descender

Although I am by no means an expert at SRT, a figure 8 is considered acceptable for descending. I have never worked on an SRT system, always switched to a doubled line but you could back the fig 8 up with a prussic cord or do a hard lock if you wanted to work off the system.

Get the book "On Rope" if you plan on getting serious about SRT. Indispensable. Best 10 bucks I have spent!

+1 for the book. Excellent instructions and very clear illustrations.
One mechanical descending device you may want to check out the "Scarab" from Conterra Technical Systems (www.conterra-inc.com). This is only a little larger than a figure-eight, but can be used in the same way as a rappel rack to vary the amount of resistance. Also, it gives a very positive "hard lock." The Scarab is used for high-angle rescue. It contains no levers or cams, so it won't break. Also, no teeth so it won't chew up the rope like some other devices. Like any other device, you can also back it up with a Blake's hitch, Prusik, or other friction hitch.

 

[matdand]

Hey NorwayClimber,
what does friction have to do with the doubling effect? I dont understand the relationship. I always tought the only thing that could change the the force at the crotch was the angle of the rope.

Matt

 

[pdqdl]

Perhaps you remember your 7 simple machines they taught you about in grade school? One of them was the pulley. When a rope goes over a pulley and back down to a 100 lb load on the ground, when you pull down on the rope with 100lbs of pull, the other end of the rope goes up with the same 100lbs of force, lifting the weight.

This means that BOTH ropes are pulling down on the pulley with a combined downward force of 200 lbs! In this fashion, mankind has been moving big loads with less force for thousands of years.

Unfortunately, there is a catch. If the pulley is only a wooden limb, a certain amount of force is lost overcoming the friction of the rope going over the limb. So the limb, acting as a pulley, gets considerably more downward pull than just the weight of the climber hanging from it. It also carries whatever force wraps over the top and is held by the other end of the rope. If that same weight is applied to a friction saver or a rigging pulley, then the force applied to the tie in point will be almost exactly doubled, less the small amount of friction lost on the pulley.

I'm sure you know how a groundman can hold a very heavy limb if the rope is wrapped around a branch? This same principle applies to every aspect of rigging that we do in the trees, not just lowering heavy branches.

 

[newsawtooth]

I would like to use a RAD system more but I am frustrated by the descender options. The only descender that would work in the system for 1/2" rope is the ID and I am not even sure it would work well. It is a little bulky and not built as well as the Gri-Gri, not to mention the cost for what is to me an inferior product. The Gri-Gri is made for 10 to 11 mm rope, which limits the rope options. I have used a pulley and prussic in place of the Gri-Gri on 1/2" ropes, with limited success. Previous posters were right that it locks hard on a single line. Anybody have ideas for equipment to replace the Gri-Gri in the RAD system so it can be used with 1/2" line?

 

[matdand]

Ya I undesrtand the principles of friction. But I tought we were talking about SRT and ascending on a single rope. Lets say your line goes over a branch then you tie it at the trunk (preferably with a ELS system) and you ascend SRT, there is friction on the crotch, but it isnt playing a role in your ascent because your line is static. There is two time the weight on the crotch than you are aplying on the ascending line (if your lines are parallel).

Maybe theres something I havent undestood about the SRT system or maybe Im just totaly out of whack, but this is what Ive always tought (and explained to people). Please corect me if Im wrong.

 

[pdqdl]

him him him him him himOk. IF you are climbing an SRT rope, the friction on the branch has no bearing whatsoever to your ascent. However... your ascent on the SRT line will put somewhere between one and two times your weight on the Tie In Point, according to how much friction is in the crotch.

If your rope gets stuck in the crotch, 100% friction: the TIP only thinks it is carrying your weight once.

If you are rigged over a perfectly smooth silver maple, right after a spring rain, your friction will be close to zero: the branch is acting like a pulley, and your weight is almost doubled. Add a little bit of shock loading from your climbing effort, and that branch is carrying more than twice your weight, due to the mechanical advantage the rope is applying.

That weak silver maple branch might just break off under your doubled weight and leave you wondering when the trip back down will stop.

 

[SINGLE-JACK]

him him him him him himok. If you are climbing an srt rope, the friction on the branch has no bearing whatsoever to your ascent. However... Your ascent on the srt line will put somewhere between one and two times your weight on the tie in point, according to how much friction is in the crotch.

If your rope gets stuck in the crotch, 100% friction: The tip only thinks it is carrying your weight once.

If you are rigged over a perfectly smooth silver maple, right after a spring rain, your friction will be close to zero: The branch is acting like a pulley, and your weight is almost doubled. Add a little bit of shock loading from your climbing effort, and that branch is carrying more than twice your weight, due to the mechanical advantage the rope is applying.

That weak silver maple branch might just break off under your doubled weight and leave you wondering when the trip back down will stop.

:agree2: ... and wondering when that branch will land on you ... pointy end first

 

[matdand]

I dont follow.

''However... Your ascent on the srt line will put somewhere between one and two times your weight on the tie in point, according to how much friction is in the crotch.''

I dont think it has to do with friction at all. The reason its doubled is because your tied in at the base, therefore the crotch needs to support the weight and it has the weight apllied to it. (A 200 lbs person apllies 200lbs and the crotch needs to withstand 200 lbs from the base = 400lbs). The same crotch that you choke the rope on (lets say with a running bowline) will only have to withstand 200 lbs, no matter how much friction is involved. The only thing that can reduce the weight at the crotch is opening up the angles, the more you open, the less weight on the crotch.

 

[SINGLE-JACK]

I dont follow.

''However... Your ascent on the srt line will put somewhere between one and two times your weight on the tie in point, according to how much friction is in the crotch.''

I dont think it has to do with friction at all. The reason its doubled is because your tied in at the base, therefore the crotch needs to support the weight and it has the weight apllied to it. (A 200 lbs person apllies 200lbs and the crotch needs to withstand 200 lbs from the base = 400lbs). The same crotch that you choke the rope on (lets say with a running bowline) will only have to withstand 200 lbs, no matter how much friction is involved. The only thing that can reduce the weight at the crotch is opening up the angles, the more you open, the less weight on the crotch.

him him him him him himOk. IF you are climbing an SRT rope, the friction on the branch has no bearing whatsoever to your ascent. However... your ascent on the SRT line will put somewhere between one and two times your weight on the Tie In Point, according to how much friction is in the crotch.

If your rope gets stuck in the crotch, 100% friction: the TIP only thinks it is carrying your weight once. {CONDITION 3}

If you are rigged over a perfectly smooth silver maple, right after a spring rain, your friction will be close to zero: the branch is acting like a pulley, and your weight is almost doubled. {CONDITION 1} Add a little bit of shock loading from your climbing effort, and that branch is carrying more than twice your weight, due to the mechanical advantage the rope is applying.

That weak silver maple branch might just break off under your doubled weight and leave you wondering when the trip back down will stop.

 

[matdand]

Interesting... I always assumed it was negligeable. Guess not. Thanks for the explanation, I bow to you in defeat.

 

[SINGLE-JACK]

Man ... NO DEFEAT! Leaning something is never a defeat - I learned stuff just making the diagram. I'm glad you asked and glad if it helped!!!

Have some rep for being cool about it.

Regards,
Jack

 

[matdand]

I was just playing. Kind of like master-student kinda samurai thing. Ive only been in the business for a year now, and I'm pretty sure I understand alot more than some hacks that have been doing this for years. I realize I dont know everything, but I wanna try to. More knowledge more power.

So I guess, if you wanna reduce the amount of weight at the crotch, you wanna have as much friction and as big of an angle as possible. Any other factors play a role in weight on anchors?

 

[pdqdl]

Nice graphics! Thanks for the help. I tried to rep you, but for some reason, I can't hit anybody anymore.