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I need a math Nerd

DirtyMike

Turbo Fluffer
Aug 8, 2005
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My own world inside my head
Ok, so I am planning on going for a drive witha buddy of mine, and we will be partaking in an event that cares the fcuk out of most people leaving them with a feeling of your Effin nutts and need to be locked up, and those that dont get scared usually take bets if the rop will snap.



So what I need to know<Imnot good at math, I know alot of people here are really good with this stuff> is how much force will there be on a rope of a given length, swinging a given weight.

265 pound weight, 60 yard rope, starting with rope totally horizontal, with a vertical drop.... What will the max force be when the rope reaches vertical<weight at bottom most>

Think I am asking that right.....

Here video to explain

 

HAB

Chelsea from Seattle
Apr 28, 2007
10,952
1,241
Seattle
Neglecting the mass of the rope, and assuming your fat ass doesn't have any drag, i.e. all the potential energy you have up top gets converted to kinetic, about 530 lbf.
 

DirtyMike

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Aug 8, 2005
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Neglecting the mass of the rope, and assuming your fat ass doesn't have any drag, i.e. all the potential energy you have up top gets converted to kinetic, about 530 lbf.
Not as bad as I thought...... But yes, assuming no drag, rather go off the side of caution and have more rope strength than actually needed.
 

HAB

Chelsea from Seattle
Apr 28, 2007
10,952
1,241
Seattle
Not as bad as I thought...... But yes, assuming no drag, rather go off the side of caution and have more rope strength than actually needed.
The mass of the rope will bring that up a little, though the drag will bring it down a little. Also, if you push off at all, which you inevitably will, that'll increase the force a little too.
 

DirtyMike

Turbo Fluffer
Aug 8, 2005
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Yeah I figure I when at the rock climbing place I will ask for a rope that is double or near double the the lbf needed..... so I figure that something that will handle 1k load will work great.
 

nmjb

Monkey
Apr 26, 2005
217
0
Idyllwild, CA
Yeah I figure I when at the rock climbing place I will ask for a rope that is double or near double the the lbf needed..... so I figure that something that will handle 1k load will work great.
Math nerd here. Neglecting the weight of the rope for everything below:

The maximum force felt by the rope in this situation will always be three times your mass . So the rope will feel a max of 795 lbf from your weight when you are at the bottom of your swing (closest to the water). If it were me, I'd buy at least 2000 lb rope.

Decent explanation here: http://www.vernier.com/discussion/index.php?topic=32.0
 

HAB

Chelsea from Seattle
Apr 28, 2007
10,952
1,241
Seattle
Oops, I spaced and forgot to include his weight over again in the calculation. Durr. I agree with the 795. When I did it I found the net force required to give adaquate acceleration, but forgot to include the 265lbs of weight on the end in the opposite direction. 530+265=795.
 

DirtyMike

Turbo Fluffer
Aug 8, 2005
14,416
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Math nerd here. Neglecting the weight of the rope for everything below:

The maximum force felt by the rope in this situation will always be three times your mass . So the rope will feel a max of 795 lbf from your weight when you are at the bottom of your swing (closest to the water). If it were me, I'd buy at least 2000 lb rope.

Decent explanation here: http://www.vernier.com/discussion/index.php?topic=32.0
Okies..... That makes sense.
2k rope it is.


Oops, I spaced and forgot to include his weight over again in the calculation. Durr. I agree with the 795. When I did it I found the net force required to give adaquate acceleration, but forgot to include the 265lbs of weight on the end in the opposite direction. 530+265=795.
Hmmm.... were you really trying to kill me David??? My Bike is already willed to my brother.:thumb:
 

Ithnu

Monkey
Jul 16, 2007
965
0
Denver
Math nerd here. Neglecting the weight of the rope for everything below:

The maximum force felt by the rope in this situation will always be three times your mass . So the rope will feel a max of 795 lbf from your weight when you are at the bottom of your swing (closest to the water). If it were me, I'd buy at least 2000 lb rope.

Decent explanation here: http://www.vernier.com/discussion/index.php?topic=32.0
I'm more of a structures/mechanisms guy but from my dynamic experience this seems reasonable to me. We usually use a 2.0 factor on dynamic events. This will typically be lower but it depends on the sin/cos (I forget which) shape of the impact curve, anyway 2.0 is the safe way to go. And that is with a direct impact, not the additional force caused by your weight on the rope attached to the bridge.

*So yeah, I agree 3.0x factor makes sense and is a safe way to go.
 
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DirtyMike

Turbo Fluffer
Aug 8, 2005
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Bacon?

Math nerd and a climbing nerd here. Make sure you buy a static rope. Most climbing rope is designed to stretch to absorb impact. I don't think that is what you want in your situation.
Yeah static rope is what I was told before I decided I wanted to even give this a go.... I think I am getting a harness as well, I dont like the idea of wrapping it around the waste and through the crotch.... Buddy swears by it, but I dont buy it.
 

ultraNoob

Yoshinoya Destroyer
Jan 20, 2007
4,515
1
Hills of Paradise
Yeah static rope is what I was told before I decided I wanted to even give this a go.... I think I am getting a harness as well, I dont like the idea of wrapping it around the waste and through the crotch.... Buddy swears by it, but I dont buy it.
You mean tying up a swiss seat right? Looks painful when you tie it up, but so long as you move your junk outta the way, it's pretty comfy.

 

DirtyMike

Turbo Fluffer
Aug 8, 2005
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My own world inside my head
let me know before you go!!!! i want in!:thumb:
Noooooo, something will happen and youll endup breaking your other femur!!!


You mean tying up a swiss seat right? Looks painful when you tie it up, but so long as you move your junk outta the way, it's pretty comfy.

Yeah I think thats it, but still....... I would rather have the harness so that if something goes wrong I can sue the **** out of the harness manf!!!!



EDIT.... actually after watching the video, that would be fine.... but tahts not what Homie does...... he ties the end of the rope......
 
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DirtyMike

Turbo Fluffer
Aug 8, 2005
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Well, I think taht will depend alot on what we have when we get to were we are going, I was thiking just a basic overhand squareknot...........
















Seriously though, will depends on what we have to tie up to, if we can attach some type of metal hook, or D ring.... or it we are litterally wrapping around part of teh bridge, will also depend on if the rope will slide or hold tight. Albright is out, seeing its one for ataching two differnet size ropes/lines together, mainly used in attaching flyline backing to flyline, posibilities that may prove usefull... Uniknot, simple, strong effective, Arbor knot is a possibility, gets tighter as its pulled, Maybe a Bowline hitch or a buntline hitch............ I am guessing though that we will be using a hi strength coated cable system to actually attach to the bridge itself, then tie some type of loop onto that, something like a rapala or a figure eight follow through with a stopper knot in the end. If conditions allows, might just find a tensionless hitch setup as well.
 
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stoney

Part of the unwashed, middle-American horde
Jul 26, 2006
15,887
2,101
Colorado
You need to find the weak link in the equation. As of now, it's the attachment points and the weight bearing load of the harness itself. When you look at mounting, you need to figure friction on any rubbing points, how much weight each component will be bearing, and what the redundancy plan is. If you have a dual rope setup (suggested when I was doing this back in college), you also need to make sure that you have redundant or at least extremely overlapped mounting hardware.

Also, factor in "whip" if you don't keep the rope taught as you fall.
 

rockofullr

confused
Jun 11, 2009
7,356
918
East Bay, Cali
Also, factor in "whip" if you don't keep the rope taught as you fall.
Hmm... didn't think of this when recommending static rope.

The force of impact from falling just a few feet on static line can be enough to seriously injure you. Climbers have broken their backs from short falls onto runners (webbing).

Also, when tying your rope into anything please use a figure 8 follow through with a stopper. A figure 8 knot reduces the rope strength to 80% of it's original while a bowline will reduce the rope strength to 60%, other knots reduce rope strength more (disclaimer: these numbers are approximate).

Good luck! Be sure to have a friend with a camera rolling.
 

mandown

Poopdeck Repost
Jun 1, 2004
15,080
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Transylvania 90210
for the rigging needs my friends and i have, we use 3X the weight as the "rough" calc, so it is good to see the math nerds here have the same idea.

agreed with the questions regarding the method of securing the rope. all the rope in the world won't do you any good if your security point sucks. investigate.