OK.....Let's Get Into The Meat Of SRT !!!!

[Lumberjack]

Originally posted by Rich Hoffman
Carl, are you thinking 2:1.......... like a gear ratio of sorts?

Well kinda... Your setup has a MA (mechanical advantage) of 2:1. For every 2 feet you pull the tail, you go up 1 foot. Twice the movement half the effort.

My setup is 1:1. Every 1 foot I pull the tail, I go up one foot. Half the movement, double the effort (kinda like walking up a ladder).


Carl

 

[Lumberjack]

Originally posted by RockyJSquirrel
I think Tom is the one who created the DdRT nomenclature do differentiate our doubled rope setup from rock climber's DRT which is two ropes hanging together- literally Double Rope.

So,

DbRT = Carl's spelling of DdRT- same as above.

I knew it was something like that!

Carl

 

[Tree Machine]

So, how do we designate clearly which DRT is which, (even though the thread is SRT)? We've only got three here, so it shouldn't be too difficult.

Carls post describing the 2:1 mechanical advantage vs the 1:1 is very clear. -TM-

 

[rahtreelimbs]

Originally posted by Lumberjack
Well kinda... Your setup has a MA (mechanical advantage) of 2:1. For every 2 feet you pull the tail, you go up 1 foot.Carl

I believe you are wrong on this one. When I footlock the tail....say..... 2 ft. I go up 2 ft. The crotch offers no mechanical advantage. To get that another pulley would have to be added.

 

[Tree Machine]

Nomenclature

SRT= Single rope t
DbRT= Doubled 2 ends on ground
DdRT= Doubled 1 end on ground

And DRT would be two ropes. I'm OK with that, but I'm not on the committee to decide. -TM-

 

[rahtreelimbs]

Originally posted by RockyJSquirrel
Rich, you're mistaken on that one.

Someone explain this one?

To get a 2:1 mechanical advantage I believe 2 pulleys have to be employed. The crotch acting as a pulley only offers one.

 

[Lumberjack]

Originally posted by Rich Hoffman
I believe you are wrong on this one. When I footlock the tail....say..... 2 ft. I go up 2 ft. The crotch offers no mechanical advantage. To get that another pulley would have to be added.

Think of the limb as the pulley. Or when you get stuck mud riding. The cable goes from the winch around the pulley and back to the truck. 2:1 advantage. I think that I am right no offense tho.

That is why SRT is great for acsending.

Carl

 

[rahtreelimbs]

No offence taken. That is why we are here to educate one and other.

 

[Lumberjack]

Originally posted by Rich Hoffman
Someone explain this one?

To get a 2:1 mechanical advantage I believe 2 pulleys have to be employed. The crotch acting as a pulley only offers one.

With 2 pulleys you get 3:1. Follow this as I try to explain what I think.

1 pulley:

Rope goes from you around the pulley and back down to you, 2:1. Think about this, I am fairly strong (IMO), I cannot body thrust up SRT (easily) because I have to lift my entire weight. DdRT I can because of the 2:1 advantage.

2 pullies:

From the limb, to the pulley on you back to the pulley on the limb, and back 2 you. That makes 3:1

An easy way to count MA is to count the legs of the system. DdRT 2 legs, 2:1, but there is an exception.

Carl

 

[rahtreelimbs]

I just played around with a small piece of cordage in my basement. I place the cord up over a beam, pulled on one side 1 ft. the other side went up 1 ft. Where is the 2:1 advantage?

 

[Lumberjack]

Originally posted by Rich Hoffman
I just played around with a small piece of cordage in my basement. I place the cord up over a beam, pulled on one side 1 ft. the other side went up 1 ft. Where is the 2:1 advantage?

Here is a quick way to show you. Do a chin up, and then body thrust DdRT. Which is easier?

The advantage is only there if the load is on all the legs, that is the exception. Like Rocky said, if your groundie pulls it is 1:1 because he is only touching one leg of the load. When you DdRT you are touching both legs, making it a 2:1 advantage.

Carl

 

[rahtreelimbs]

Originally posted by RockyJSquirrel
Rich-
Your rope is over a limb 20' up and you are tied in normally, standing on the ground. In order to ascend the 20', you must pass 40' of rope through your hands. That is 2-1 mechanical advantage.

If Bubba the Groundman tries to pull you up, it is not 2-1. for every foot of rope he pulls, you go up 1 foot.

Why isn't groundman pulling 40 Ft. of rope? He is doing the same thing that I am, only he is on the ground. The Sherrill catalog had something about this at one time. They stated that 1/2 of your weight is displaced on the limb. You are only pulling half of your weight because 1/2 of your weight is on the limb and the other half you are pulling. When the ground man does it he is pulling all of your weight.

 

[rahtreelimbs]

Try this one. If I pull myself up the 20 ft. I still have 20 ft. left from me to ground that the groundie can't get to. If he pulls me there is still that 20 ft. from me to the ground. Same thing.


Time to move on.

 

[Tree Machine]

so glad to get this hashed out

Well-said, Rock. This is precisely why I don't DdRT. You might footlock 2 feet, but one end goes up one foot, the other side comes down one foot. Plus any slop in your tress cord / attachment and you lose a fraction more. If you have any friction pushing a hitch up, that furthers your efforts.

That's why I'm fully into DbRT with ascenders. Or SRT with ascenders, for that matter. They don't slip back even a fraction. If the cord or web attaching you to the ascenders doesn't stretch, there's no loss there. I also like a non-stretch rope, for there is no loss in the elasticity. Ascenders offer next to zero friction, or resistance, on the up-push. You get ~100% of what you put in. This is just a viewpoint IMHO, not a slash on how others do it. I respect technical diversity. -TM-

 

[TheTreeSpyder]

Brian is right, an old puzzle....

You have to measure from you, to the controlling hold, if that is on the ground, you must count that length; if it is the knot next to your head then zero rope is left. Slack after that doesn't count, only the loaded part of the line that you would fall if it was cut is counted as in use at end of lift.

If you start off line over branch 20' high, you need 40' of line. If you pull your self up the 20',there is no line left. 40/20 = 2/1.

If Groundie pulls you up, there is still 20' left from you to where groundie holds line on ground (40 - 20 left over)/20 = 1/1

If ground control lifts you, 1 line is pulled, 1 support moves (1 line to climber)

If you lift your self, you pull 1 line, but 2 lines move that support climber, you must shorten both of them 1' to go 1'. The line dead ending to your saddle and the line your friction hitch lides on both support you.

The trick is, that the we are used to seeing each leg of line serve one purpose, in this unique event, the system is 'closed'; and 1 leg of line seves 2 purposes to lift and support. Notice that witht he 2 legs, either the suppoirt load is 2x or the power is 2x depending on if the system is open or closed.

Any mechanical conversion will always be distance x power still = distance x power but will have a friction cost, there can be no perpetual motion, constantly transfered back and forth, force must trickle out to nothing. So SRT would not pay that cost on ascending, making no power conversion as DdRT. That is good, as these guys polish this to within the human machine powerband. Polishing more and more play out of it it seems....

 

[rahtreelimbs]

Thanks Treespyder.

 

[Tree Machine]

Clarifying, for the record

I, too, admit this is a tricky concept. Mebbe some guys are still just a little fuzzy, so for fun, let's turn this problem upside-down, and solve it from the top down, rather than the ground up.

Say you are at the top of the tree. You're standing on a limb that allows your saddle to be at the same exact level as your tie-in point.

Rope attached to the left side if your saddle, goes through the fork and to your right hand. Pull tightly so your hips are pressed up against the TIP.

OK, start backing down the tree, letting the rope belay through your gloved hand. When your hips are exactly 10 feet away from the TIP, STOP.

How many feet of rope have gone through your hand? Ans: 20 feet; the 10 feet from saddle to the TIP, and the 10 coming back down to your hand. 20 feet of rope to go down 10 feet. Of course, to get back UP that 10 feet, you'll need to pull the same 20 feet of rope .

If the TIP were a pully (ridding the system of frictional losses), and you were to use only your arms to pull youself back up that 10 feet, letting your legs and feet dangle, you would be pulling with a force equivalent to half your weight, since your weight is split 50:50 on each line. That is a 2:1 mechanical advantage. Or you could grab both ropes and pull yourself hand-over-hand (you stud), pulling your full weight with half the number of pulls as previously described. This is 1:1.

SRT is a 1:1 ascent system, which I consider a main advantage over traditional DbRT since it gets you up there with half the amount of motion. -TM-

 

[TheTreeSpyder]

When you are up 50' and want to make sure enough line is left to descend; you pick the end of your climbing line up and touch it to your chest, the loop of the line should touch the ground.

Because ya need 2x as much line to come down as you have distance to descend.

If you only have 50' of line left to descend 50' with DdRT; it is preferable that ya have a stopper knot as a possitive, mechanical stop at the end of the line, or ya get to ride the 'short bus' home fer kids that don't learn as fast.


i think they even send special ice cream trucks down your street then, ya can tell the ones for the such handicapped learners; it says "Slow Children" on the back!

 

[Tree Machine]

HSRT?

Spidey, ya just gotta love your sense of humor. Here's a few SRT pics I dug out from six years ago, when I was really starting to dig on SRT, and was using it more frequently. These were from when I was pre-digital. I had to take digital photos of the prints, so I hope they look OK. -TM-

 

[Kneejerk Bombas]

Worked a tree today SRT.
The first problem I had was the two-to-one load it puts on the crotch. As I loaded and unloaded the rope it had too much slack. The rope would saw back and forth at the crotch from rope stretch and TIP movement. The same problem you have setting up a retrievable lowering point with a second rope.
Once I tied a running Butterfly the slop was gone, and it was still retrievable from the ground. This of course make some of Tom's built in safety features obsolete.
I climbed of a modified VT which worked really well except on decents more than a few feet. Yo-yoing with a Pantin was simple and working a single line is really nice.
The benefits are apparent, if only the bug of how to attach yourself to the climbing line could be slolved.