Bungee Cords

I learned on a mil. spec. system. In pirate jumping it has a number of advantages. You have a set of 5 individual cords that you group together in a set of 3,4,or5. So you can jump from 100-250lbs with one set of cords. They are covered in Nylon (or Cotton) sheaths that protect them from dirt when you lay them down. Also, the redundancy of multiple cords, as well as the sheaths which act as a static back up in the case of over elongation, provide (I believe) a greater degree of safety. Traditional mil. spec. cords have an elongation of about 110%. By the way, in the “bungy or bungee” page a common mistake is made about this word. Elongation refers to the change in length of the cord, not the ultimate loaded length. So 110% elongation in a 100 foot long cord is 110 feet for a total stretched length of 210 feet, or 2.1 times it’s original length. Since mil. spec. cord stretches less, a longer length is often used, resulting in more initial free fall.

After the 1995 X-games I realized that, because of the bigger rebounds, more acrobatic stunts were possible on all rubber cord. So I set about learning how to build that type of cord. New Zealand style cord is built with a ribbon, about 3 inches wide, composed of a number of stands of rubber. This is wrapped around teflon spools, tied together, stretched to it’s ultimate elongation and wrapped down it’s length, with the same type of rubber, to hold it all together.

The actual building of an all rubber cord, or tying loops on the ends of mil. spec. cord is not something that can be learned through words only. You need to learn first hand from someone who knows what they are doing. So you can watch them and they can check your work. The same goes for the bungee system, whether it’s a lowering system, a man basket on a winch, or a raising system, you either need to learn it first hand or start from scratch with the engineering done on all the materials, and many practice “jumps” done with sandbags.

New Zealand specification cord has an elongation of 200-300%, i.e. it will stretch to 3-4 times it’s original length. I believe the Kocklemans came up with the idea of building a length of webbing into all rubber cord. This back up is the length of the ultimate elongation of the cord. This gives it the redundancy that I like. The Kocklemans build their cords thicker than New Zealand specs. This gives the cord a long life (over 1000 jumps) but results in higher G-forces, and less ability to do rebound tricks. I believe their elongation is 150-200%.

It is difficult to determine actual elongation because many people will tell you the length of the all rubber cord before it is stretched out and tied down it’s length. This makes the cord a little longer (about 5-10%) than when the ribbon is first layed around the spools. For example, for a 100 foot jump you would first lay the rubber out at 22.5 feet, which would need to stretch to 4.5 times it’s original length to get to 100 feet. But once the cord is stretched and tied it would be about 25 feet long. So then it would need to stretch to 4 times it’s length to reach 100 feet.

Well, I’m starting to ramble so that’s all I have to say about that. Here’s some info for your bungee sites list. Total Rebound and Yesh in California are friends of mine that are no longer in the business. My company will be active again in Northern California in December 1996. I’ll update you later. The sites at Raging Waters and Big Bear were both temporary stops for Thrill Sport Productions, who produces our show in Taiwan. They will have arches at state fairs this summer in Wisconsin, Michigan, Ohio, and other places. Their Corporate office is 88 A Elm St. Hopkington MA 01748 Tel.(508)435-0420 Fax(508)435-6594. Total Hard Core Gear, in Redding California, makes what I think are the best harnesses. They make both ankle harnesses and chest and waist harnesses. They are fleece covered, well padded, and use self locking parachute buckles, which are easy to use and secure. Sorry, I don’t have their address or phone number. I used the N.Z. Bungy Knot ankle binding at the 95 X-games and didn’t like it. Like you said, you feel trapped. Also, the force is transferred to your legs with a 1 inch strap, which, even though padded, digs in after a while. The worst thing though is that when it tightens there is a 2 foot strap of webbing hanging off which can wrap around your calf on a rebound and then your are hanging from an unpadded 1 inch strap which really abrades the skin and can cause hematomas. This happened to three of my friends at the ’95 Extreme Games.

The Euro Cord

The European/New Zealand cords described above look like this

Steve who wrote that section above believes that the US mil spec system is safer because of the extra redundancy, but the Euro cords do have the advantage that they can be inspected after each jump for signs of rubber failure or possible problems. The sheath on Mil spec cords prevents this. Some Euro cords have an extra static line built into the core to prevent over stretch and provide extra backup. The term Euro cord is a little misleading – this cord is used and made in many countries.

Mil Spec Cord

Mil Spec cord is cord that is made to US military specifications, these specifications were designed to hold down tanks on boats and in planes rather than to suspend adrenaline junkies jumping off bridges. As it turns out they work rather well for both!.

Picture of Mil Spec Cord

Here is some mil. spec cord made by Glenn from Bungee Experience. The pink/white stuff is the cord, the blue webbing is attached in a very special way to the ends of the cords so that carrabiners can be attached to each end. The way the end of the cord is connected with a piece of webbing is called termination. The Red Tape is part of the special way the cords are terminated.

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Specs

Lots + lots of people have asked me for bungee cords specs and I just got emailed this from J Kockelman (cheers!)

Most Bungee Jumping Cord is made from “natural rubber” whose physical constants are in most college libraries. The most common bungee cords stretch 2 to 4 times the original length and the jumper feels 2.5 to 3.5 G’s.

I found a page on bungee.com (link expired) which had a technical paper on Bungee jumping.

Energy Conservation bungee jumping

Study for the MCAT. Anytime. Anywhere.

Kaplan topical Physics waves question 1: woman bungee jumping from a bridge. dmax = max distance from bridge.

1) Why does the woman NOT hit the bridge on her way
back up from dmax?

C . The force of gravity prevents her from reaching
the bridge.
D . Energy is not conserved due to the frictional force
of air resistance.

The back of the book says gravity is a conservative force so C is the incorrect answer. But when I try to logically think about this, even when I assume no friction force of air resistance, I still see C as being a correct answer as well. Gravity is always pulling “down” on the woman. It will aid in her descending downwards and then will fight her on going back upwards. Thus keeping her from ever returning to the original bridge height. Why is C wrong? Or is it actually correct but D was just a better answer? Thanks

justwantanmdphd

Full Member

If total energy is conserved, the PE at the highest will all be transformed into KE at the bottom. Regardless of gravity, this KEmax at the bottom will be used to drive the woman to the same position where PE was highest and the woman will hit the bridge.
Think of the bungee cord as a spring. PE max on top, KE max on bottom and PEmax=KEmax. the KE max on bottom causes the spring to go back to the very top where PE is max.
The only reason why the spring will not go to the very top is because some E is lost on the way due to friction. So with frictional force. PE max on top, KE max on bottom but PEmax > KEmax because some E is lost. This KEmax will be used to drive the woman up to a certain point under the bridge when that PE = KEmax. (check out dampening spring). hope that helps

Jack08

Full Member

If total energy is conserved, the PE at the highest will all be transformed into KE at the bottom. Regardless of gravity, this KEmax at the bottom will be used to drive the woman to the same position where PE was highest and the woman will hit the bridge.
Think of the bungee cord as a spring. PE max on top, KE max on bottom and PEmax=KEmax. the KE max on bottom causes the spring to go back to the very top where PE is max.
The only reason why the spring will not go to the very top is because some E is lost on the way due to friction. So with frictional force. PE max on top, KE max on bottom but PEmax > KEmax because some E is lost. This KEmax will be used to drive the woman up to a certain point under the bridge when that PE = KEmax. (check out dampening spring). hope that helps

Thanks. That is a good explanation for the correct answer D. I’m trying to understand why C would be wrong in real life. If someone were to bungee jump in a frictionless environment where gravity was still around however, they would return 100% to the top? I just can’t overcome that thought. Gravity is trying to pull them to the earth, even with no friction of air. So gravity will pull hard on the bungee jumper accelerating her towards earth. Once shes at the bottom and the bungee cord begins to pull upwards on her, gravity is trying to still pull her downwards. Makes no sense to me how she’d ever reach back to the top 100% . What am I missing?

el_duderino

Some men play tennis, I erode the human soul

C is wrong because if none of the energy were lost due to friction from the air and within the rope, she WOULD hit the bridge on the way back up.

Think about it in terms of energy. She has some potential energy x standing on top of the bridge. She jumps off, and as she falls she loses potential energy as it becomes kinetic energy. Then she bounces up, and she is motionless at the very top. Zero kinetic energy. If she hadn’t gained back all her original potential energy, where is the rest of it?

justwantanmdphd

Full Member

Thanks. That is a good explanation for the correct answer D. I’m trying to understand why C would be wrong in real life. If someone were to bungee jump in a frictionless environment where gravity was still around however, they would return 100% to the top? I just can’t overcome that thought. Gravity is trying to pull them to the earth, even with no friction of air. So gravity will pull hard on the bungee jumper accelerating her towards earth. Once shes at the bottom and the bungee cord begins to pull upwards on her, gravity is trying to still pull her downwards. Makes no sense to me how she’d ever reach back to the top 100% . What am I missing?

So the PE=mgh means that you are doing work against gravity. So you are already taking into account the problem of g pulling it down. The Gravitational potential energy is the work needed to apply against gravity. you have already taken into account the pulling down of gravity once you apply GPE (or in short PE) to an object. in another words, think of the all the energy you put into raising something to the top against gravity. once you drop it and once at the bottom all that PE you have put become available to the object. the object can then use it to reach to the top in a frictionless environment. Once at the bottom, some form of E must be present since all the PE is no longer there. This E at the bottom is in the form of KE.

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Jack08

Full Member

So the PE=mgh means that you are doing work against gravity. So you are already taking into account the problem of g pulling it down. The Gravitational potential energy is the work needed to apply against gravity. you have already taken into account the pulling down of gravity once you apply GPE (or in short PE) to an object. in another words, think of the all the energy you put into raising something to the top against gravity. once you drop it and once at the bottom all that PE you have put become available to the object. the object can then use it to reach to the top in a frictionless environment. Once at the bottom, some form of E must be present since all the PE is no longer there. This E at the bottom is in the form of KE.

Thanks so much. That helped me understand or at least get the idea. Crazy to think that if there is no friction/air resistance, only gravity, and I was lifted 100m in the air, hooked to a bungee cord, then released, that I’d be a self perpetual machine for eternity. Thanks for the help though, truly appreciate it!

justwantanmdphd

Full Member

Thanks so much. That helped me understand or at least get the idea. Crazy to think that if there is no friction/air resistance, only gravity, and I was lifted 100m in the air, hooked to a bungee cord, then released, that I’d be a self perpetual machine for eternity. Thanks for the help though, truly appreciate it!

DrknoSDN

Full Member

Thanks so much. That helped me understand or at least get the idea. Crazy to think that if there is no friction/air resistance, only gravity, and I was lifted 100m in the air, hooked to a bungee cord, then released, that I’d be a self perpetual machine for eternity. Thanks for the help though, truly appreciate it!

Ignoring the original question because that seems to be resolved I still have to correct this.

You would need a bungee cord that has zero length. By that I mean it would need to start building potential energy the moment you were lower than the height of the bridge.
The only way for that to happen would be if it was made of a magical material that compressed to zero length on it’s own but had stretching properties that followed a hook’s law spring constant relationship.

Otherwise the potential energy required to launch you back up to bridge level would not begin building until you passed the natural rest position and you wouldn’t attain enough stored potential energy at the bottom to launch you back up to bridge level. In that aspect, even if you were doing a bungee jump in a vacuum (no air resistance), you still would not bounce back up to bridge level without a bungee made of flubber.

Even without air resistance it can’t happen because like you said, it would allow perpetual motion machines to be built anywhere that doesn’t have an atmosphere. (the moon etc)

el_duderino

Some men play tennis, I erode the human soul

Ignoring the original question because that seems to be resolved I still have to correct this.

You would need a bungee cord that has zero length. By that I mean it would need to start building potential energy the moment you were lower than the height of the bridge.
The only way for that to happen would be if it was made of a magical material that compressed to zero length on it’s own but had stretching properties that followed a hook’s law spring constant relationship.

Otherwise the potential energy required to launch you back up to bridge level would not begin building until you passed the natural rest position and you wouldn’t attain enough stored potential energy at the bottom to launch you back up to bridge level. In that aspect, even if you were doing a bungee jump in a vacuum (no air resistance), you still would not bounce back up to bridge level without a bungee made of flubber.

Even without air resistance it can’t happen because like you said, it would allow perpetual motion machines to be built anywhere that doesn’t have an atmosphere. (the moon etc)

I thought about that as I was writing my initial post, but I don’t think it’s correct.

After all, when you boil it down, you’re either losing energy into the system or not. With a frictionless environment and ignoring internal friction in the cord and whatnot, you need to conserve energy. You start with some x of PE and when you bounce back up and are motionless at the top of the bounce, there’s zero kinetic energy. There’s nowhere else for that PE to have gone, so you must have gotten all the energy back as PE.

And when you think about it logically, you will have some x m/s velocity at the moment the bungee starts to elongate. When you bounce back up and pass the point when the bungee returns to its original length, whatever that was, you will be at -x m/s.

So no matter the length of the bungee, you will end up hitting the bridge (or returning to your original height) if you’re not losing any energy due to wind resistance/friction.

Bungee Cords

I learned on a mil. spec. system. In pirate jumping it has a number of advantages. You have a set of 5 individual cords that you group together in a set of 3,4,or5. So you can jump from 100-250lbs with one set of cords. They are covered in Nylon (or Cotton) sheaths that protect them from dirt when you lay them down. Also, the redundancy of multiple cords, as well as the sheaths which act as a static back up in the case of over elongation, provide (I believe) a greater degree of safety. Traditional mil. spec. cords have an elongation of about 110%. By the way, in the “bungy or bungee” page a common mistake is made about this word. Elongation refers to the change in length of the cord, not the ultimate loaded length. So 110% elongation in a 100 foot long cord is 110 feet for a total stretched length of 210 feet, or 2.1 times it’s original length. Since mil. spec. cord stretches less, a longer length is often used, resulting in more initial free fall.

After the 1995 X-games I realized that, because of the bigger rebounds, more acrobatic stunts were possible on all rubber cord. So I set about learning how to build that type of cord. New Zealand style cord is built with a ribbon, about 3 inches wide, composed of a number of stands of rubber. This is wrapped around teflon spools, tied together, stretched to it’s ultimate elongation and wrapped down it’s length, with the same type of rubber, to hold it all together.

The actual building of an all rubber cord, or tying loops on the ends of mil. spec. cord is not something that can be learned through words only. You need to learn first hand from someone who knows what they are doing. So you can watch them and they can check your work. The same goes for the bungee system, whether it’s a lowering system, a man basket on a winch, or a raising system, you either need to learn it first hand or start from scratch with the engineering done on all the materials, and many practice “jumps” done with sandbags.

New Zealand specification cord has an elongation of 200-300%, i.e. it will stretch to 3-4 times it’s original length. I believe the Kocklemans came up with the idea of building a length of webbing into all rubber cord. This back up is the length of the ultimate elongation of the cord. This gives it the redundancy that I like. The Kocklemans build their cords thicker than New Zealand specs. This gives the cord a long life (over 1000 jumps) but results in higher G-forces, and less ability to do rebound tricks. I believe their elongation is 150-200%.

It is difficult to determine actual elongation because many people will tell you the length of the all rubber cord before it is stretched out and tied down it’s length. This makes the cord a little longer (about 5-10%) than when the ribbon is first layed around the spools. For example, for a 100 foot jump you would first lay the rubber out at 22.5 feet, which would need to stretch to 4.5 times it’s original length to get to 100 feet. But once the cord is stretched and tied it would be about 25 feet long. So then it would need to stretch to 4 times it’s length to reach 100 feet.

Well, I’m starting to ramble so that’s all I have to say about that. Here’s some info for your bungee sites list. Total Rebound and Yesh in California are friends of mine that are no longer in the business. My company will be active again in Northern California in December 1996. I’ll update you later. The sites at Raging Waters and Big Bear were both temporary stops for Thrill Sport Productions, who produces our show in Taiwan. They will have arches at state fairs this summer in Wisconsin, Michigan, Ohio, and other places. Their Corporate office is 88 A Elm St. Hopkington MA 01748 Tel.(508)435-0420 Fax(508)435-6594. Total Hard Core Gear, in Redding California, makes what I think are the best harnesses. They make both ankle harnesses and chest and waist harnesses. They are fleece covered, well padded, and use self locking parachute buckles, which are easy to use and secure. Sorry, I don’t have their address or phone number. I used the N.Z. Bungy Knot ankle binding at the 95 X-games and didn’t like it. Like you said, you feel trapped. Also, the force is transferred to your legs with a 1 inch strap, which, even though padded, digs in after a while. The worst thing though is that when it tightens there is a 2 foot strap of webbing hanging off which can wrap around your calf on a rebound and then your are hanging from an unpadded 1 inch strap which really abrades the skin and can cause hematomas. This happened to three of my friends at the ’95 Extreme Games.

The Euro Cord

The European/New Zealand cords described above look like this

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Steve who wrote that section above believes that the US mil spec system is safer because of the extra redundancy, but the Euro cords do have the advantage that they can be inspected after each jump for signs of rubber failure or possible problems. The sheath on Mil spec cords prevents this. Some Euro cords have an extra static line built into the core to prevent over stretch and provide extra backup. The term Euro cord is a little misleading – this cord is used and made in many countries.

Mil Spec Cord

Mil Spec cord is cord that is made to US military specifications, these specifications were designed to hold down tanks on boats and in planes rather than to suspend adrenaline junkies jumping off bridges. As it turns out they work rather well for both!.

Picture of Mil Spec Cord

Here is some mil. spec cord made by Glenn from Bungee Experience. The pink/white stuff is the cord, the blue webbing is attached in a very special way to the ends of the cords so that carrabiners can be attached to each end. The way the end of the cord is connected with a piece of webbing is called termination. The Red Tape is part of the special way the cords are terminated.

Specs

Lots + lots of people have asked me for bungee cords specs and I just got emailed this from J Kockelman (cheers!)

Most Bungee Jumping Cord is made from “natural rubber” whose physical constants are in most college libraries. The most common bungee cords stretch 2 to 4 times the original length and the jumper feels 2.5 to 3.5 G’s.

I found a page on bungee.com (link expired) which had a technical paper on Bungee jumping.

Source https://www.bungeezone.com/equip/cord

Source https://forums.studentdoctor.net/threads/energy-conservation-bungee-jumping.1076852/

Source https://www.bungeezone.com/equip/cord

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