I completely agree with pdql. Im simple minded and this is the way I see it. If what you are saying were true, I would be better off jumping off a bridge attached to 1" manila rope instead of a bungee cord.
Forces in rigging
- Thread starter Dan@JBT
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I think your argument might be much better than mine. Thanks!
PassionForTrees
ArboristSite Operative
You have to simply work with in your equipments and the Trees and your workers abilities! Knowing what that is , is more than half the battle! know the tree your rigging from, and we all know there is only so much you are going to know, consider species and condition. work within your equipments ratings, if you know how to do that, and what your workers are capable of understanding and doing well. meaning let it run to a slow smooth stop doesnt mean let it drop 10 more feet and stop it dead hard. knowing and doing are two different things, Practice Practice and more Practice! Remember guys.. you can always take a smaller piece right! but everyone wants to get down quicker and this usually compromises something.
yoyoman
ArboristSite Operative
I don't think we disagree, I just don't think we are understanding each other yet or I'm not thinking of good hypotheticals.
No energy is being added it just is not being dissipated or absorbed.
Without using a hypithetical let me present it this way.
Car shock absorbers are like a good groundie.
You want a rope and a groundie that absorbs the shock like a high priced car not an old worn suspension that lets the wheels bounce.
Same bumps, different shocks and car, are like the same log with a different rope and groundie.
Example: 1: log falls and between a good rope that has some stretch and a good groundie all the energy is absorbed and the log stops and comes to rest in a distance of X'.
2: same log falls and it is tied off, does not slow over this same distance and the energy is not absorbed and so it bounces back upward.
Which is going to offer the smoothest ride?
Another example.
You have to get down from a high place over concrete (work with me here) and you have two choices, get into a man sized nurf ball or a man sized supper ball (remember those things that bounced like crazy). Again which ride?
I understand what you say here, it is a little differant than the (hypithetical) example I used. Let me express my point using your example.
The 60 mph car using the brakes stops in 100 feet, there is a G force there.
The 60 mph car using a tow strap and not allowed to "run", stops the car in 50' and "yanks" the care back 50', total 100' traveled. It is not letting it run, or using the brakes, as you would say that caused the bounce.
Again, I'll take the ride with the brakes that absorbs the energy rather than something that causes it to bounce.
No energy is being added it just is not being dissipated or absorbed.
Without using a hypithetical let me present it this way.
Car shock absorbers are like a good groundie.
You want a rope and a groundie that absorbs the shock like a high priced car not an old worn suspension that lets the wheels bounce.
Same bumps, different shocks and car, are like the same log with a different rope and groundie.
Example: 1: log falls and between a good rope that has some stretch and a good groundie all the energy is absorbed and the log stops and comes to rest in a distance of X'.
2: same log falls and it is tied off, does not slow over this same distance and the energy is not absorbed and so it bounces back upward.
Which is going to offer the smoothest ride?
Another example.
You have to get down from a high place over concrete (work with me here) and you have two choices, get into a man sized nurf ball or a man sized supper ball (remember those things that bounced like crazy). Again which ride?
I understand what you say here, it is a little differant than the (hypithetical) example I used. Let me express my point using your example.
The 60 mph car using the brakes stops in 100 feet, there is a G force there.
The 60 mph car using a tow strap and not allowed to "run", stops the car in 50' and "yanks" the care back 50', total 100' traveled. It is not letting it run, or using the brakes, as you would say that caused the bounce.
Again, I'll take the ride with the brakes that absorbs the energy rather than something that causes it to bounce.
imagineero
Addicted to ArboristSite
It's pretty hard to take you seriously with the name 'yoyoman'. All I can picture in my head is yoyos ;-)
You've all got it right, and the truth is somewhere in the middle. No rope has zero stretch, and no rope has bungee cord stretch either. Understanding rope construction is of some benefit.
Class 1 double braids are the most common lowering ropes in our industry. The load is shared by the core and the sheath, and they are commonly made out of nylon and/or polyester. They're quite stretchy.
Class 2 double braids are less common in our industry but still used for some technical rope work. The load is carried primarily by the core, and the sheath is there mainly to prevent abrasion and protect the core from grit. They're very common in sailing where very low stretch is highly desirable. Class 2 double braids are generally made of exotics like dyneema, vectran, technora etc...
Single braids are not used as lowering ropes very often. They are used as winch lines, and for whoopie slings and loopies etc. Many are of 12 strand construction and most are made out of common fibres like nylon or polyester. They splice easy but lose strength quickly because they have no cover.
To make matters more complicated, there are 12 strand, 16, 24 etc which all have different handling characteristics. And lets not even get started on kernmantle or 3 strand! Manufacturers use different materials and constructions to produce ropes which are ideal for specific purposes and can alter the important criteria of strength, abrasion resistance, stretch, heat resistance and handling. In a class 1 double braid, which is what we use generally for both climbing and lowering ropes in our industry, the construction of the sheath will most often affect the way the rope handles and wears, while the construction of the core will most often affect the stretch characteristic of the rope.
Higher strand count sheaths are associated with nicer handling and more wear. Lower strand count sheaths are a lottle rougher to handle but wear better. Of more interest though, is what's going on in the core.
[video=youtube;EWmzdsfeeZM]http://www.youtube.com/watch?v=EWmzdsfeeZM[/video]
There are endless variables with tension, material, and numbers of strands and also numbers of fibres per strand besides the previously mentioned sheath configuration. There are available very high strength very low stretch ropes which are completely unsuitable for the purpose of general tree rigging. In absolute terms, most class 1 double braid lowering ropes are relatively stretchy, and that's a good thing. It helps absorb some of the force of the fall. Manufacturers can construct ropes with extreme stretch by altering the tension and twist rate, making the rope much like a spring. What's worth knowing, is that it doesn't act like a bungee. You'll never see a bull rope launch a log back up into the air. Neither will you see it stop it dead (even if you think it did!). What it does is something in between.
It stretches, and slows down. Then it shrinks back a little. But it doesn't shrink back immediately to where it was before that catch. If you take a big hit on a rope, it will take that rope some time to slowly shrink back to where it was and regain its absorption qualities. If you take a series of big hits on a rope, the impact on the rope is greater with each hit, and the rope is destroyed quickly. You can take a huge hit on a fresh rope. But the same hit 3 or 4 times in succession will break that rope. This is very obvious from the CTF data published by manufacturers. The rope obviously didn't get weaker after only 3 or 4 hits, but it lost its stretch and didn't recover. Ropes used at the lower end of their WLL will last a long time, because they retain their stretch. That means the rope and the anchor see a lot less load. Over time, ropes lose much of their strength (up to 60%). Partly due to strength loss from wear and abrasion (this is especially true of webbing slings and single braid ropes!) and partly due to loss of elasticity. The more stretch a rope loses, the more of the actual shock it is subjected to. It's easy to see that ropes taking constant heavy hits are being burnt at both ends of the candle and it's only a matter of time.
Shaun
You've all got it right, and the truth is somewhere in the middle. No rope has zero stretch, and no rope has bungee cord stretch either. Understanding rope construction is of some benefit.
Class 1 double braids are the most common lowering ropes in our industry. The load is shared by the core and the sheath, and they are commonly made out of nylon and/or polyester. They're quite stretchy.
Class 2 double braids are less common in our industry but still used for some technical rope work. The load is carried primarily by the core, and the sheath is there mainly to prevent abrasion and protect the core from grit. They're very common in sailing where very low stretch is highly desirable. Class 2 double braids are generally made of exotics like dyneema, vectran, technora etc...
Single braids are not used as lowering ropes very often. They are used as winch lines, and for whoopie slings and loopies etc. Many are of 12 strand construction and most are made out of common fibres like nylon or polyester. They splice easy but lose strength quickly because they have no cover.
To make matters more complicated, there are 12 strand, 16, 24 etc which all have different handling characteristics. And lets not even get started on kernmantle or 3 strand! Manufacturers use different materials and constructions to produce ropes which are ideal for specific purposes and can alter the important criteria of strength, abrasion resistance, stretch, heat resistance and handling. In a class 1 double braid, which is what we use generally for both climbing and lowering ropes in our industry, the construction of the sheath will most often affect the way the rope handles and wears, while the construction of the core will most often affect the stretch characteristic of the rope.
Higher strand count sheaths are associated with nicer handling and more wear. Lower strand count sheaths are a lottle rougher to handle but wear better. Of more interest though, is what's going on in the core.
[video=youtube;EWmzdsfeeZM]http://www.youtube.com/watch?v=EWmzdsfeeZM[/video]
There are endless variables with tension, material, and numbers of strands and also numbers of fibres per strand besides the previously mentioned sheath configuration. There are available very high strength very low stretch ropes which are completely unsuitable for the purpose of general tree rigging. In absolute terms, most class 1 double braid lowering ropes are relatively stretchy, and that's a good thing. It helps absorb some of the force of the fall. Manufacturers can construct ropes with extreme stretch by altering the tension and twist rate, making the rope much like a spring. What's worth knowing, is that it doesn't act like a bungee. You'll never see a bull rope launch a log back up into the air. Neither will you see it stop it dead (even if you think it did!). What it does is something in between.
It stretches, and slows down. Then it shrinks back a little. But it doesn't shrink back immediately to where it was before that catch. If you take a big hit on a rope, it will take that rope some time to slowly shrink back to where it was and regain its absorption qualities. If you take a series of big hits on a rope, the impact on the rope is greater with each hit, and the rope is destroyed quickly. You can take a huge hit on a fresh rope. But the same hit 3 or 4 times in succession will break that rope. This is very obvious from the CTF data published by manufacturers. The rope obviously didn't get weaker after only 3 or 4 hits, but it lost its stretch and didn't recover. Ropes used at the lower end of their WLL will last a long time, because they retain their stretch. That means the rope and the anchor see a lot less load. Over time, ropes lose much of their strength (up to 60%). Partly due to strength loss from wear and abrasion (this is especially true of webbing slings and single braid ropes!) and partly due to loss of elasticity. The more stretch a rope loses, the more of the actual shock it is subjected to. It's easy to see that ropes taking constant heavy hits are being burnt at both ends of the candle and it's only a matter of time.
Shaun
yoyoman
ArboristSite Operative
Dan,
pdqdl, makes a good example ( I tried) of the car using brakes to stop vs. a tow strap.
Both of your computations use the car and brakes, energy absorbing method for stopping. I'm not explaining this well but I'm just trying to say if that energy absorption method of stopping is not used you will have bounce that you have not included in your formula. That "bounce" will reintroduce the energy back into your system (your perch more than likely) and cause unwanted forces.
The formula and note that I posted about "if it bounces back the impact forces will be even greater...." comes from Georgia State University where my son is a student.
Sorry if I added confusion to your question. I think you have some good examples though of the importance of letting it run, not just to add length to the deceleration of the falling piece but to add energy absorption and dissipation to the system. Teach them to be good little shock absorbers and the guy in the tree will thank them for it, math or no math formula
Cheers
Richard
P.S. I just read Shaun's post. So well said!
I love the math, the formulas and the physics but when it is all said and done it is about using the right equipment, ropes and wraps to dissipate energy in a controlled fashion, that is the hard part. Any guy swinging yoyo's can do the math. https://www.youtube.com/watch?v=9iHYndoVpmw
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