Why no hitch for SRT?

in my distort(our)ed mind.
The DdRT has 2 legs of support yes, and feeds from a single leg pool to a dual leg support yes.
Half load per leg of DdRT is lesser load per leg vs full load on single leg SRT , yes.
.
BUT,
A 300# man can descend on a DdRT @150# per leg.
>>but a 125# man still has the same hitch cinching tight on same lifeline host in SRT @125# per (only)leg
>>even with less weight per(only)leg SRT still seizes??!!
.
So i name the operating component for the slide as
In DdRT the hitch unloads to slide on the 'dynamic'/continuous to the pool of rope by swapping load over to the static/saddle termination leg; just as 1 leg more elastic, or starting to shear etc.
>>the firmer/saddle termination/static leg carries the load; and the hitch slides on the continuing/dynamic leg.
Friction Buffer of overhead support redirect (rather than pulley on same) can capitalize more on the effect as offered i think.
.
 
So I think you’re saying in DdRT, the static/harness side is helping when descending as it is trying to pull through the hitch as the climber is compressing the hitch? In SRT the rope is just there and not assisting, how rude!
 
In DRT with a theoretically frictionless pulley (I. E. worse case), half the climber's weight is on the hitch side and half on the other side, less on the hitch side when moving. In SRT all of the weight is on the hitch side, less when moving but much more than DRT, so tons more friction required. Hitch gets pulled much tighter at each stop.
 
Mike Islander said:
In DRT with a theoretically frictionless pulley (I. E. worse case), half the climber's weight is on the hitch side and half on the other side, less on the hitch side when moving. In SRT all of the weight is on the hitch side, less when moving but much more than DRT, so tons more friction required. Hitch gets pulled much tighter at each stop.
Click to expand...
Quick and dirty. Best/easiest explanation.
 
Then 150# climber in SRT can descend as easily as 300# climber in DdRT??
>>as both are 150# at the friction hitch to host lifeline
Tried to show NOT of 125# climber can't descend SRT as easy as 300# (or even 400#climber) DdRT.
Think we should watch the magician's other hand on this.
.
i think DdRT is capitalizing more fully on the leg of support terminating at the saddle.
In either case, the system tries to work, SRT has no other way to except cinching tighter as try to slide hitch vs. DdRT hitch starts to slide load can swap over to the other leg, just as if friction hitch leg was more elastic or coming apart; 'buddy' leg will try to be a hero. Friction buffer/not pulley between the 2 support legs accentuates/expresses this more.
.
In either case of SRT vs. DdRT, the hitch must unload to slide, the mechanics of that part are the same, and can dial to the same force at that point with half load on SRT to equalize that load to friction hitch variable across both to cancel out of equation/model.
.
The force all ways and always goes to the most rigid resistance against it's own self; as seeks Equal/Opposite partner in this dance. As DdRT leg goes dynamic and 'stretches', force leans to the more static rigidity of the terminating leg on saddle in model i try to show.

.
 
Last edited:
Tom Dunlap said:
In the decades since I started dabbling in SRT I would have expected the cord on rope friction issue to be solved. So far..no solution.
Click to expand...
In a mechanical engineering sense I am not sure if this is correct but after playing around with all manner of hitch/ rope combos and with mechanicals I look at the systems as fundamentally different in the following sense. Seems to me a hitch depends far more on the surface textile properties of the hitch cord and the rope and their interaction - cover to cover. This is quite variable, new v.s. old, dry v.s. wet and icy. A mechanical functions by or at least adds the physical bendiness and perhaps compressibility of the rope. Easier to dial in (recognizing discussions about rope flattening or losing shape etc.) and less variable. There are variations with things like brake bars, munters or F8's or ATC's - somewhere in between? Or it could be the double malt talking again . . .

(Years ago one winter, I wondered about the effect of "fuzzing" ropes and hitch cords and played around with sandpapered rope/ hitch combos v.s. "new". I found results quite different, hence the comment.)
 
Where as the complex interactions that are describe undoubtedly occur, I don't believe they have much relevance in overall function in the two climbing systems.

I base this on how closely an SRT hitch works when compared to a DdRT hitch when combined with a friction modifier like the HH or RW. When properly tuned, modulation of grip and speed are on a par. With the friction modifiers being so close to the hitch, it pretty much defines what is needed and, indeed, happening.
 
Time for the current state of knowledge on the topic to be put into practice. A rigging pulley has a tension ratio of 1.2 meaning if the log is going downwards, the log side rope tension is 1.2 x higher than the other side rope tension. Pete Donzelli found this out because his anchor tip force was too small i.e. not 2x log weight.

Rope on a branch has tension ratio of about 1.4 That means that if you're DRT descending, the hitch side of your rope is going up and has the lower tension. 1.0 tension + 1.4 tension = your total weight, so the hitch sees 1 / 2.4 = 0.42 of your body weight. So a 200 lb guy has 84 lbs on his hitch and 116 lbs on the other leg of the rope.

When you ascend the friction reverses and you pull 116 lbs+ to go up. So much for the 2:1 pulley achieved by grinding your rope across the bark.

So the punchline is that in DRT your hitch sees/holds less than half your weight. Big guys can still have misbehaving hitches in DRT. Hitches have a small useable weight range to still comfortably unlock. I think I measured 40 lbs hand force on a sticky, uncomfortable hitch once. It would wear your arm out and frustrate you to have to work with a hitch like that. Interesting topic.



edit - well, it's old man grey hair I effed up correction time. Tree bark Mu is higher per my basal tip tie forces post, and the tension ratio is 2.0 to 2.5 going over tree bark. So it's way worse.

1.0 tension + 2.5 tension = your total weight, so the hitch sees 1 / 3.5 = 0.29 of your body weight. So a 200 lb guy has 58 lbs on his hitch and 142 lbs on the other leg of the rope. When you ascend the friction reverses and you pull 142 lbs+ to go up. !!! rough bark. tension ratio 2.0 for smoother bark.
 
Last edited:
A huge component of the puzzle is the climbing line diameter, and what happens to it when loaded in single or doubled configuration.

A rope with a high percentage of elongation will become significantly thinner when loaded in single configuration, and a hitch’s ability to grab while aloft if far different from how behaves when tying, dressing and setting on the ground. Much less so a rope loaded in doubled configuration.

Further, how the different systems are used.is a component. Let’s assume we’re using a Hitch Climber type system, and hip thrusting in MRS. the rope below the climber’s hand is only stretching due to its own tail weight, and that is what the hitch is to grab onto. This will behave much like it would when setting up the system on the ground.

Now let’s assume we take the same rope/hitch/pulley and add in a Rope Wrench and set it up in a stationary configuration. Let’s also assume we’re using a foot ascender to advance. This puts 100% of the climber’s weight into the line below the hitch, and the rope will basically be pulled straight as a pin. Same hitch has to grab that, and it’s apples to oranges compared to the MRS.
 
Foot ascender thinning rope under hitch can also remove the mushroom swell shelf below hitch from going from loaded to unloaded profile, as sometimes a safety issue.
 
oceans said:
A huge component of the puzzle is the climbing line diameter, and what happens to it when loaded in single or doubled configuration.
Click to expand...
The rope and the hitch cord diameter and "stiffness" (or hand) I expect would both affect the sizes of the "contact patches" of the hitch turns - maybe sorta like the contact patch of a tire on the road surface v.s. tire inflation. I did find different results with new rope v.s. slightly used and new hitch cord v.s. used hitch cord with respect to "grabbiness" which I found interesting - same rope and cord but what was different seemed to be the textile surface condition. And sanding and fuzzing had an effect also, as above.
I am not sure if elongation plays a factor in hitch cord given length, but I would bet that different rope constructions would - kern rope v.s. a softer 24 strand cover, ropes like tachyon with a tendency to keep their "round" etc. New rope with surface coating or treatment (over the years we found some new doubles with dry treatment to be quite speedy like on the way down until they get worn in/ a bit dirty - and this was with mechanicals like the ATC and Shunt).
My further 2 cents again for this morning.
Cheers all.
 
Reading this thread I think something @TheTreeSpyder and @Bart_ mentioned but has not been fully drawn out is the role of force. In a simple climbing system, the force of gravity is an acceleration, and the force of friction acts as a deceleration. Since force is mass times acceleration (F=m * a), the role of friction is not additive but multiplicative to the system.

In the weight calculation people are offering for the systems, nothing is being calculated for the acceleration or deceleration. Since these are multiplicative forces, their role is significant even though friction is often an unknown variable in our climbing systems.

In playing with the stitch hitch in SRT and DDRT, I have noticed that binding of the hitch can occur if I decelerate quickly in SRT.

In a climbing system acceleration is provided by gravity and the deceleration is produced by friction forces. In DDRT the rate of deceleration is modulated by two factors that are not present in SRT: the two legs moving and the friction at the top of the system.

The two legs of the system provide an additional factor that primarily contributes to the sensitivity of the system. Like in a 2 to 1 mechanical advantage system where the length of line has to move double the distance but reduces the force need to move the load, DDRT provides a sensitivity to the system. This is not really a force per se, but you have twice the distance of rope moving over the same distance of descent/ascent. Even though this is not a an explicit force, I believe it encourages a slower rate of change as our hands touch the rope. As a hitch is broken and an descent is begun, if the amount of rope sliding through your hand is the same between SRT and DDRT, the decent rate is doubled with SRT. I would call this a sensitivity factor. You have twice the sense of movement at your hands, with half the rate of descent.

The other major factor is the friction itself at the TIP. Obviously this varies tremendously. But significantly, this friction acts multiplicatively through the system as it decelerates. Even a small change will be significant to the force needed by the hitch to decelerate a descent.

There will obviously be a lot of variability in how friction is applied by the hitch knot and by the user. But the user will have a multiplicatively reduced demand on precision of knot tying and decent control by the added friction force at the TIP when operating in DDRT.

In SRT, it would seem that reduced friction force at the hitch by putting a bend in the line at another point above or below the hitch is enough of a friction force multiplier to allow the hitch to function. Thus the stitch hitch hardware, HH, or the wrope wrench.
 
Last edited:
kludge said:
Reading this thread I think something mentioned but has not been fully drawn out is the role of force. In a simple climbing system, the force of gravity is an acceleration, and the force of friction acts as a deceleration. Since force is mass times acceleration (F=m * a), the role of friction is not additive but multiplicative to the system.

In the weight calculation people are offering for the systems, nothing is being calculated for the acceleration or deceleration. Since these are multiplicative forces, their role is significant even though friction is often an unknown variable in our climbing systems.
Click to expand...
Remember too, that in Sir Newton's Second Law of Motion, Force = m X a , for climbers and for rigging, the variable acceleration, due to gravity, is a square quantity - m/sec(squared). In controlling all things falling, quicker is better . . . . or forces can quickly get out of hand.
 
kludge said:
Reading this thread I think something @TheTreeSpyder and @Bart_ mentioned but has not been fully drawn out is the role of force. In a simple climbing system, the force of gravity is an acceleration, and the force of friction acts as a deceleration. Since force is mass times acceleration (F=m * a), the role of friction is not additive but multiplicative to the system.

In the weight calculation people are offering for the systems, nothing is being calculated for the acceleration or deceleration. Since these are multiplicative forces, their role is significant even though friction is often an unknown variable in our climbing systems.

In playing with the stitch hitch in SRT and DDRT, I have noticed that binding of the hitch can occur if I decelerate quickly in SRT.

In a climbing system acceleration is provided by gravity and the deceleration is produced by friction forces. In DDRT the rate of deceleration is modulated by two factors that are not present in SRT: the two legs moving and the friction at the top of the system.

The two legs of the system provide an additional factor that primarily contributes to the sensitivity of the system. Like in a 2 to 1 mechanical advantage system where the length of line has to move double the distance but reduces the force need to move the load, DDRT provides a sensitivity to the system. This is not really a force per se, but you have twice the distance of rope moving over the same distance of descent/ascent. Even though this is not a an explicit force, I believe it encourages a slower rate of change as our hands touch the rope. As a hitch is broken and an descent is begun, if the amount of rope sliding through your hand is the same between SRT and DDRT, the decent rate is doubled with SRT. I would call this a sensitivity factor. You have twice the sense of movement at your hands, with half the rate of descent.

The other major factor is the friction itself at the TIP. Obviously this varies tremendously. But significantly, this friction acts multiplicatively through the system as it decelerates. Even a small change will be significant to the force needed by the hitch to decelerate a descent.

There will obviously be a lot of variability in how friction is applied by the hitch knot and by the user. But the user will have a multiplicatively reduced demand on precision of knot tying and decent control by the added friction force at the TIP when operating in DDRT.

In SRT, it would seem that reduced friction force at the hitch by putting a bend in the line at another point above or below the hitch is enough of a friction force multiplier to allow the hitch to function. Thus the stitch hitch hardware, HH, or the wrope wrench.
Click to expand...

Everywhere you say "multiplicatively", you can use "proportionally". May help folks visualize.
 
i think simple multiplier would be proportional as say;
but, much of this has at least 1 factor that goes beyond with squaring or other exponential expansion.
.
As like i think that in E=MCsquared
Mass is a multiplier expansion of more predictable proportions
but the C is on beyond simple multiplier rate of expansion/effect on sum as is squared, running away much more quickly as stated(Ghostice), to be beyond a simple proportional to me.
.
But the math goes to multipliers used as then an exponent in radial friction in capstan formula as in The Mechanics of Friction in Rope Rescue(link) and expanded spreadcheat (link) i made from it.
Working from the accepted standards of mated flat materials frictions like shown on engineering toolbox (link)
>>where pi X friction of mated materials as linear friction X # of arc180s are multiplied, then used as an exponent of Euler's number.
>>i look at the PI X standard friction of mated materials (nylon on steel etc.) as conversion from linear/flat friction to radial, then the amount of multiplier is the amount of 180 arcs on capstan
.
So arc180 is a key component in all rope and knot work to me*.
Euler's number is ~2.718 as the approximate logarithm of 1
>>used similarly in calcs of compound interest, population growth, as well as decay growth etc.
>>we use as compounding frictions.
*(i think all working rope forces in rigging, Hitches, Bends etc. have only 3 raw elements: arc0/linear, arc90 and arc180)
ghostice said:
Remember too, that in Sir Newton's Second Law of Motion, Force = m X a , for climbers and for rigging, the variable acceleration, due to gravity, is a square quantity - m/sec(squared). In controlling all things falling, quicker is better . . . . or forces can quickly get out of hand.
Click to expand...
To give more weight to E=MCsquared model ; see same jewel from different facets to show the consistent deep rules across all:
Total power watts = resistance X amps squared.
resistance = as weight,mass against change to dynamic state
Amps is speed, once changed to dynamic from static as current moves.
>>same thing as E=MCsquared only expressed differently to me; force is force, the capacity to give dynamic change from static after nominal resistance component to then exponential force gain component of dynamics, then multiplied to total sum. Have seen such shown in thermal and even chemical inertias etc. where same pattern is consistent across.
ghostice said:
The rope and the hitch cord diameter and "stiffness" (or hand) I expect would both affect the sizes of the "contact patches" of the hitch turns - maybe sorta like the contact patch of a tire on the road surface v.s. tire inflation
Click to expand...
In model of imposing rigidity against force density,
i find that we use a Friction Hitch raw stiffness and raw diameter xTension to impose rigidity against the force density of host lifeline's like wise raw stiffness, diameter xTension (it's own).

Matching-or-denser-force-nipping-other-friction-hitches.png


To be in command Friction Hitch must be of matching or greater tension density imposing/displacing against host lifeline density. I think hitch diameter should be able to fold in half so width = 2xDiameter as to be at least equal lifeline diameter as lower limit. Then comes 'frictive' (@knudeNoggin strikes a gain) value in the vertical stack of an instance in horizontal timeline (kinda computer model).
.
Dispersed Radial is very key in it's maths here , and most organic clean forces are in round rope and hosts, as even ropes as rope hosts. Especially as take linear force thru Standing Part(s) to then dispersed radial to control in Hitch terminations and Bend dbl.sided opposing terminations to make coupling. Different then in radial binding where input is internal radial force instead of external linear.
 
Last edited:
The + sign on my ascending calculated pull force is there to represent the required acceleration of the climber's mass into upward motion beyond just fighting gravity. Similar effect for stopping. I measured guess-recollecting 10 or 20% for normal climbing motion ascent or stops.

It would be interesting to see a measurement of enthusiastic hip thrusting ascent. My bet is both DRT rope legs would spike up 30 or 40% during the thrust, then the climber's total "weight" on the two DRT rope legs would decrease momentarily by guessing 40% and at that point the 2.0 or 2.5 tension ratio calculation would apply (lower size arm yank required) and right after that the total climber's "weight" into the two DRT rope legs would spike somewhat as he landed back into the rope's support after his hip thrust bounce.

I'd do the measurement but it's a bit sketchy setting up and executing in a lower ceiling height basement. Plus the math on the load cells may not work out if there's some non-co-linear loading.

Rope friction on tree bark is pretty consistent in my measurement results.
 
@Bart_ what do you get as you coefficient of friction for rope on tree bark? Have you developed any coefficients for rope types or tree bark types?

Also, do you have any coefficients for rope on rope or rope diameters?

Not expecting you have worked on all the variables, but would love to know if you have.
 
Alternate hip thrust analysis. As you do the Tom Jones or glow worm wriggle, you apply arm force to the pull side, balanced by your leg spasm on the other end, and the "gravity" weight your pelvis presents to it's rope attachment temporarily drops. During this brief drop you seize the instant and pull with your arms because the required rope tension to do so has briefly reduced. During this transient the 2.0 or 2.5 tension ratio calculation applies but basically without a net acceleration of your whole body because you kept it in the same location and only twitched your pelvis up while your legs went down. You land from this twitch with perhaps a small transient. Then you un-glow-worm-bend yourself and this slower motion accelerates your CofG to it's new higher position.

This analysis probably better represents the secret to why hip thrusting helps at all. The earlier analysis represents a possible situation, kind of a bouncing DRT.

As to foot ascenders on the rope tail, it just splits the load off your arms but you still crank out the same big suck-wind force with your leg.

Minus too-cold-for-the-fingers degrees outside today, stuck inside. Even the garage is too cold to work in. Froze a bit of fingertip skin yesterday trying to hero it out. Doh!
 
Look up the SRT Basal anchor TIP forces thread. Odds are I described the test rig there. Cross grain 4x4 pine and 2 or 3" locust bark for sure were done. I calculated a matching Mu for the 180 degree rope wrap too. The raw data was the two rope tensions. DRT climb and SRT TIP force are almost the same analysis if you think about it. The tensions were selected out of a time recording, I think 4 or 5 points per recording - which is what lead to the principle of the tension ratio - it was a constant ratio whether low or high rope tensions.
 
You must log in or register to reply here.

New threads New posts

Back
Top Bottom