SRT/DRT Setup

[OTG BOSTON]

I was thinking it would be tough to anchor to the stem and still be able to release it in an emergency situation (while it is under load)

 

[alanarbor]

If you want the system to be availible for a rescue, I'd say tie off at the base with a porta wrap.

I've seen that system simplified down to a kleimheist with friction saver rings. You SRT to your working point, then tie a stopper knot in your ascent line, to prevent the possibility of the prusik slipping down, then descend Ddrt on your working system.

IMO fewer links=fewer points to fail.

 

[woodchux]

One thing to be aware of if anyone uses this system is the force generated at the crotch is 4x the climbers weight if the srt rope is anchored off at the ground. That means that a 200lb man when using the DRT will be putting 800lbs of force on the crotch. BE CAREFUL !

 

[OTG BOSTON]

One thing to be aware of if anyone uses this system is the force generated at the crotch is 4x the climbers weight if the srt rope is anchored off at the ground. That means that a 200lb man when using the DRT will be putting 800lbs of force on the crotch. BE CAREFUL !

How'd you come to that figure?

 

[woodchux]

How'd you come to that figure?

Basically while using the DRT you will have 4 parts of rope acting on the crotch. Hence the x4 factor.

 

[OTG BOSTON]

good stuff

So if you anchored to the limb itself would it change to 400 or remain 800?

The reason I ask is I've been studying up on SRT and the famous MC is referenced in a few articles as saying changing the angles and running the line through multiple crotches improves the overall strength.

Thanks for explaining this to me the pics make it much easier to grasp!

 

[woodchux]

Anchoring off at the limb via running bowline would place only 400 lbs of force on the TIP. I use this instead of anchoring off at the base. Not as good for rescue, but i still feel safer.

 

[alanarbor]

Basically while using the DRT you will have 4 parts of rope acting on the crotch. Hence the x4 factor.

This is off a bit. Since both ends of the working line are suppourting the 200lb weight, it does not double at the DRT attachment point. This is still only 2x climber weight load at the crotch, i.e 400lbs

 

[TheTreeSpyder]

You get the 4x - Friction if pulley is floated, then only use 1 end and the other end anchored; for a 2x within a 2x. The friction at TIP holding the pulley is biggest reducer to this effect normally. Wrapping that line around tree; or other high friction strategies can maximize this friction, to minimize the 4x effect. This all can make the system pretty bouncy. The same rope in DdRT has less load per leg, then the less elasticity, so SRT has 2x load, so more elasticity in support leg, + more on the control leg (that DdRT doesn't have); then into the 2nd pulley for double loading that line, at higher elasticity. The low point of the pulley can give more line loaded at 2x than if pulley was at top, so more elasticity there. So, all considerations more elasticity, especially if then we load that all onto a more 'dynamic' support that has some give.

The elasticity will help if you fall, buffering that input. But, in walking up rope; the elasticity will buffer that effort input too; making job harder!

 

[OTG BOSTON]

?????????????????

This is off a bit. Since both ends of the working line are suppourting the 200lb weight, it does not double at the DRT attachment point. This is still only 2x climber weight load at the crotch, i.e 400lbs

Well you're half right... but it's 4x ,and close to 800lb at the crotch. BE CAREFUL!

I guess this is why I have always been confused about this

Can I get some conformation here??:help:

 

[moray]

If both lines are used to ascend to the floating pulley the weight at the crotch is 2X minus a small amount of friction at the crotch.

Correct. Something nobody seems to explain further is that "small amount of friction." It is the limb that feels that friction, in the form of a twisting force (torque) on the limb. So there is no free lunch--the limb feels the full 2X as a combination of torque and vertical load. I wonder if anyone has studied the strength of wood in resisting torque?

 

[woodchux]

Ok i'm probably wrong. I'll edit my previous posts. Sorry guys
Seems like if you had the groundman tail the rope for you while ascending the DRT section of the system, then maybe the 4x factor might be reached? Like a floating false crotch... ?

 

[Tree Machine]

Correct. Something nobody seems to explain further is that "small amount of friction." It is the limb that feels that friction, in the form of a twisting force (torque) on the limb. So there is no free lunch--the limb feels the full 2X as a combination of torque and vertical load. I wonder if anyone has studied the strength of wood in resisting torque?

We're getting closer. Torque is not a 'form' of friction. Torque is the force to create a rotation about a point. If we're looking at where the rope goes over the limb there might be a small amount of twisting force (torque), but the fact that the rope only goes over a portion of the limb, there is no 'point' about which the limb could rotate. Just a minimal amount of surface friction, and when that equilibrates (quits moving in either direction), the force in the place comes from two different directions of pull and equals the sum of those two forces with the net direction of force being downward between the two directions of pull, a downward force, not a rotational force.

As in SRT, with the rope anchored at the base of the tree and you on the opposite end, the force is what you and your weight create. The equal and opposite force is generated on the rope coming up the other side of tree. The cumulative force is placed where the rope goes over the crotch or limb and it comes from two opposing directions, and there is no 'rotational' force as there is no singular, unidirectional force to create rotation about a single point.

As an analogy, think of the tool that twists off an oil filter. There is no friction between the filter wrench and the filter, just an inward force to create the 'grip' on the oil filter, and then the pressure on the handle creates the rotational force (torque) with the exact center of the filter being the point about rotation. This type of force isn't happening with a rope lacing over a limb

I was aware of torque being applied to a maple tree I was taking down today. The tie-in point was on one spar, and I sunk a redirect on another spar. Torque (rotational force) was being applied to the tree, but not at the tie-in point, nor the redirect. I've touched up a photo of that tree to show where the torque was. This will make sense to you.

 

[moray]

But not there yet...

We're getting closer. Torque is not a 'form' of friction. Torque is the force to create a rotation about a point. If we're looking at where the rope goes over the limb there might be a small amount of twisting force (torque), but the fact that the rope only goes over a portion of the limb, there is no 'point' about which the limb could rotate. Just a minimal amount of surface friction, and when that equilibrates (quits moving in either direction), the force in the place comes from two different directions of pull and equals the sum of those two forces with the net direction of force being downward between the two directions of pull, a downward force, not a rotational force.

As in SRT, with the rope anchored at the base of the tree and you on the opposite end, the force is what you and your weight create. The equal and opposite force is generated on the rope coming up the other side of tree. The cumulative force is placed where the rope goes over the crotch or limb and it comes from two opposing directions, and there is no 'rotational' force as there is no singular, unidirectional force to create rotation about a single point.

As an analogy, think of the tool that twists off an oil filter. There is no friction between the filter wrench and the filter, just an inward force to create the 'grip' on the oil filter, and then the pressure on the handle creates the rotational force (torque) with the exact center of the filter being the point about rotation. This type of force isn't happening with a rope lacing over a limb

TM, I have really enjoyed your posts in other threads, as your comments are always thoughtful and educated. However... you haven't got this quite right, IMHO.

Take the oil filter wrench for starters. Of course it is friction that holds the wrench to the filter, but it is static friction, not sliding friction. If you push gently against a cinder block sitting in your driveway, it is friction that prevents it from moving. If you push hard enough to make it move, the resistance you feel comes from sliding friction, which is almost always significantly less than static friction.

Now take a rope over a limb. Just wrap it around the limb 10 times and secure it with a clove hitch. Hang 100 lbs on it. Since the rope hangs from one side of the limb, the limb feels a rotational force (torque) equal to the product of the force (100#) times the diameter of the limb. If you cut the limb off and trimmed it down to a short length, mounted a bearing in each end, and supported it with two posts, you would have a large horizontal rolling pin. The 100# weight, hanging as before, is going to cause it to start spinning like crazy. Clearly torque is applied to the limb.

The "point" around which the limb wants to rotate is the axis of the limb. In your photo, if one imagines the two spars as two blades of a pair of scissors, the axis on which the torque acts is the axis of the scissors.

A final point about the equilibration of frictional forces on a limb in DRT. When you are rappeling, the frictional force resisting your descent not only applies torque to the limb, but also causes an imbalance in the load on the two legs of rope. The upward moving leg has less load than the downward moving leg, and the greater the friction, the greater the imbalance. When you stop, some imbalance would tend to be preserved. The imbalance doesn't automatically go away. You can force it into balance, thus removing any torque on the limb, by shaking and jerking the ropes. The fact that the system can be stable but way out of balance is demonstrated any time you lower a load by using a wrap around a limb to supply friction. Your hand never has to supply a force anywhere near the weight of the load. Even if you stop the load part way down, the load side of the rope still has far more tension on it than your side.

 

[TheTreeSpyder]

i think most everything works in ranges, like Goldilocks saw. No totally straight lines on the graph; just curves. i don't think our rope rotations are in the dynamic range of the graph of having effects on the limbs. Increasing the Turn/ redirect around the supporting limb to a Round Turn; decreases vertical loading; would more than double rope contact; but without branch failure.

 

[moray]

factor?

The torque on the limb is not a factor.

I'm not sure what you mean. The torque I am referring to doesn't tend to make the limb bend at all. It tries to twist it off the trunk. If the limb were a huge lag screw, the torque would try to screw it in deeper, or unscrew it from the trunk, depending on which side the rope was on. The friction between the rope and the limb produces this kind of torque on the limb.

The vertical load on the limb, which tries to break the limb by bending it downward, produces torque as well. The further from the trunk your rope is placed, the greater the torque. The torque in TM's picture is of this type, which we could call bending torque. But this torque has nothing to do with friction and has nothing to do with the imbalance between the two legs of the rope.

In physics, torque is represented by an arrow. The two arrows that represent the two torques we are talking about are at 90 degrees to each other.

 

[Tree Machine]

... you haven't got this quite right, IMHO.

Take the oil filter wrench for starters. Of course it is friction that holds the wrench to the filter, but it is static friction, not sliding friction.

I hesitate to get into non-motion friction. It is true what you say, that in the world of equal and opposite forces always at play that static and kinetic friction are forces working as (analogy) a 'mirror-image' to one another. For practical purposes we tend to think of friction as the resistance to motion (kinetic friction), the force opposing the motion of an object. In the practical application of friction, we tend to talk only of the outside force being applied to the object. Static friction plays more into 'an object at rest tends to stay at rest'.

Now take a rope over a limb. Just wrap it around the limb 10 times and secure it with a clove hitch. Hang 100 lbs on it. Since the rope hangs from one side of the limb, the limb feels a rotational force (torque) equal to the product of the force (100#) times the diameter of the limb. If you cut the limb off and trimmed it down to a short length, mounted a bearing in each end, and supported it with two posts, you would have a large horizontal rolling pin. The 100# weight, hanging as before, is going to cause it to start spinning like crazy. Clearly torque is applied to the limb.

Again, what you say is true, however in the practical application of this, we don't wrap our rope around the limb ten times and terminate with a clove hitch. We send a rope up and over and back down. The rope is only in contact with a portion of a horizontal limb. Yes, there is some torque applied, but the total rotational force leaves off where the sliding (kinetic) friction begins. If the rope is through a V-crotch, torque would be even less of a factor, surface friction in action, but a single point about rotation would be a really difficult place to determine. If you're ascending a doubled rope with dual ascenders, or abseiling DbRT, equal force is being applied to each leg, there is zero rotational torque.

A final point about the equilibration of frictional forces on a limb in DRT. When you are rappeling, the frictional force resisting your descent not only applies torque to the limb, but also causes an imbalance in the load on the two legs of rope. The upward moving leg has less load than the downward moving leg, and the greater the friction, the greater the imbalance. When you stop, some imbalance would tend to be preserved. The imbalance doesn't automatically go away. You can force it into balance, thus removing any torque on the limb, by shaking and jerking the ropes.

That depends on which DRT system you're using. In traditional DdRT, yes. In DbRT, both legs are treated identically and are perpetually in balance, with the exception of the climber swinging out away from the place below center (pendulum).

The fact that the system can be stable but way out of balance is demonstrated any time you lower a load by using a wrap around a limb to supply friction. Your hand never has to supply a force anywhere near the weight of the load. Even if you stop the load part way down, the load side of the rope still has far more tension on it than your side.

Can't dispute this in any way. Wrapping a rope at least one full time around the limb, and applying the load will generate increasing torque until the rope starts to slide. Maximum torque would be when the the load is stopped and let hang. More torque could be achieved if you let the limb run and then suddenly stop it (shock load); the torque would peak, and then quickly come back to the level of the limb hanging free.

From a practical standpoint, rotational torque being a major force would depend on if the rope were to make a full and complete wrap around a limb. Otherwise, surface friction will be the real player and torque can only reach a maximum force to the point at which the rope starts sliding.