Author Topic: Coiled Leash Freak Accident or Not?  (Read 15060 times)

Nate Burgoyne

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Coiled Leash Freak Accident or Not?
« on: March 09, 2010, 12:59:38 PM »
I think you could count on 2 fingers how many times I've posted a link to our mag in a post, but this was pretty nutz and I see it as a safety issue. Ever wondered what would happen if your coiled leash broke mid-session. Check out these pics and be glad it wasn't your forehead. http://www.supsurfmag.com/equipment-stand-up-paddle/gear-talk/stand-up-paddlers-caution:-coiled-leash-freak-accident?-20100309910/
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Keau

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Re: Coiled Leash Freak Accident or Not?
« Reply #1 on: March 09, 2010, 01:07:38 PM »
Wow!  That is unreal!  I have a Dakine coiled leash and it says "Flat water leash, not intended for waves".  Good advice from Dakine.

kwhilden

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Re: Coiled Leash Freak Accident or Not?
« Reply #2 on: March 09, 2010, 01:20:13 PM »
Is this necessarily an issue with a coiled leash? It looks like the part that failed had nothing to do with coil itself. Perhaps the coil caused a higher recoil when the leash rebounded immediately after failing, but wouldn't a non-coiled leash do the same thing when suddenly released from high tension? I don't have enough experience with the elastic properties of leashes to say one way or the other... anyone?
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H2Oman

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Re: Coiled Leash Freak Accident or Not?
« Reply #3 on: March 09, 2010, 01:44:24 PM »
My coiled leash says the same thing, don't use for surf. I use it for my flatwater board and a regular heavy duty for my surfing sup.  I'm not sure this is something you can just blame on a coiled leash.  Any thing that stretches is going to have recoil.  That said, the fact that it penetrated the board might very well be due to the stiffness in the coiled sections.   

DavidJohn

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Re: Coiled Leash Freak Accident or Not?
« Reply #4 on: March 09, 2010, 02:33:03 PM »
IMO.. that has nothing to do with the coils.. that could have happened with a straight leash as well.

If you stretch a leash to the point of something breaking the recoil is amazing..I'm guessing that if coming through the water is nowhere near as bad and through the air.. and add to that a brass fitting on the end of it it would be like a bullet.. straight leash or coiled.

I use coiled leashes all the time in the surf.. so do many of my friends without any problems.

That could have been nasty for sure... I'm thinking freak accident.

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Easy Rider

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Re: Coiled Leash Freak Accident or Not?
« Reply #5 on: March 09, 2010, 02:40:34 PM »
Same thoughts here - doesn't look like the coil had anything to do with it.
That said I only use coiled leashes for flat water.
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ehrawn

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Re: Coiled Leash Freak Accident or Not?
« Reply #6 on: March 09, 2010, 03:15:23 PM »
You can’t see it from the picture, but I would guess that the leash failed in shear. As the leash is pulled at each end, the coiled structure will twist about the central axis of the material, loading the leash in shear. I’m guessing it was made of urethane, or some sort of polymer. In general, shear strength is much less than tensile strength for these materials. I don’t work with polymers, so I can’t give you actually numbers, but I would guess the difference is probably in the range of orders of magnitude.

Strand Leper

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Re: Coiled Leash Freak Accident or Not?
« Reply #7 on: March 09, 2010, 06:18:06 PM »
Ehrawn... What?

Tim
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ehrawn

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Re: Coiled Leash Freak Accident or Not?
« Reply #8 on: March 09, 2010, 06:56:55 PM »
It not only matters how much you pull on the leash, but how you load the material. A given material can exhibit different material properties for different loading materials. For simplicity’s sake, we’ll stick to isotropic materials in 2-D. If you look at a small element of material (square), and define the coordinate axis (x horizontal and y vertical with the origin in the middle of the square), tensile forces would be those applied parallel to each coordinate axis, and shear stresses would be those applied parallel to the sides of the square. For example, if you have positive tensile force along the x axis, and negative tensile force (compression) along the y axis, your square will deform into a rectangle. If you only have shear forces, your square will deform into a rhombus.

http://en.wikipedia.org/wiki/Tensile_strength
http://en.wikipedia.org/wiki/Shear_strength

The strength (the amount of force the material can withstand before failure) is different in tensile than it is in shear. I don’t know for sure, but the difference for polymers is likely around 100 times difference. So, if you’re loading the leash in pure shear, it will only take 100 time less force to break it than it would if you loaded it in tensile.

It’s obviously a little more complicated in 3-D. You can start here:
http://en.wikipedia.org/wiki/Mohr%27s_circle#Mohr.27s_circle_for_stress

Nate Burgoyne

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Re: Coiled Leash Freak Accident or Not?
« Reply #9 on: March 09, 2010, 07:07:55 PM »
A traditional leash isn't going to recoil like that to penetrate so clean and forcefully through a deck pad, glass and foam. If that action happened to traditional leashes when they broke, there'd be plenty stories about people getting impaled by the swivel on their leash. I've never heard one.
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DavidJohn

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Re: Coiled Leash Freak Accident or Not?
« Reply #10 on: March 09, 2010, 08:03:22 PM »
A traditional leash isn't going to recoil like that to penetrate so clean and forcefully through a deck pad, glass and foam. If that action happened to traditional leashes when they broke, there'd be plenty stories about people getting impaled by the swivel on their leash. I've never heard one.

I don't see how the coil could change anything.

DJ

ehrawn

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Re: Coiled Leash Freak Accident or Not?
« Reply #11 on: March 09, 2010, 09:30:44 PM »
For a ductile material, like urethane, when you bring it past it’s yield, the material will begin to neck down. Thing about pulling a piece of gum. At first, the cross sectional area  begins to shrink (like the square mentioned in my earlier post), evenly, but, as you pull it apart farther, the cross sectional area near the middle becomes increasingly smaller. Also, the deflection increases dramatically. This serves to release a lot of the tension in the material, so that when it does finally fail, the “stored” energy in the material is decreased.

If the leash fails in a brittle mode, this necking and energy dissipation will not occur. Essentially, failure occurs at maximum stress and, by extension, maximum force. My guess is that this leash wasn’t new- maybe a little sun baked- and, with its geometry, loaded primarily in shear. Failure with this powerful snap back seems to make sense.

I hope someone who has some experience working with polymers could weigh in on this.

PonoBill

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Re: Coiled Leash Freak Accident or Not?
« Reply #12 on: March 09, 2010, 09:53:14 PM »
It can indeed change a lot of things. When you coil something you transform it into a torsion rod. Most of the forces applied to it are in shear. Even a weak coil does that. Further, leash coils are not very springy at first, but as they reach full extension the torsion effects are much greater. You can experiment with that for yourself, the residual bit of coil enables the leash to become a fairly strong torsion bar, it stretches a lot further than a conventional leash will and stores a lot more energy, just as a spring can store a lot more energy in elastic deformation than the steel wire it's coiled from.

The swivel failed, or rather the collar the swivel attaches to,  not the leash material, but the extra energy did the dirty work. The brass bit shown in the picture is the end of the swivel, and it still has the allen screw through it. That means the plastic collar attached to the cuff broke off and released the leash.

Theoretical issues aside, I determined last year that coiled leashes and big surf do not go together. After having my board flung at my head four or five times, I cut my first session with a coiled leash short, and went to the jeep for a standard leash. I use them for downwinders, but Da Kine is quite right, they have no place in the surf.
« Last Edit: March 09, 2010, 09:56:17 PM by PonoBill »
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ehrawn

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Re: Coiled Leash Freak Accident or Not?
« Reply #13 on: March 09, 2010, 11:12:31 PM »
somehow i totally missed that the collar broke- that makes a lot of sense. what i said still applies, just for the brittle plastic collar.
« Last Edit: March 10, 2010, 12:02:55 AM by ehrawn »

DavidJohn

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Re: Coiled Leash Freak Accident or Not?
« Reply #14 on: March 10, 2010, 05:30:46 AM »
I don't think there's much extra spring effect in a coiled leash.

It doesn't take much force to pull it straight..and once it's stretched so there is no coil it becomes just like a straight leash.. and then the straight leash just stretches like a straight leash.

I'll ask the Balin guys here and they have been making leashes for longer than anyone here and do a lot of testing and it will be interesting to hear what they think about all this.

The coiled leashes that we use only have a small section that's coiled so that may make a big difference.

DJ

 


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