the bridle ripped clean off

[Tornolf 0]

Quote

The main advantage to different lengths is that if something such as a sharp railing cut the break cord prematurely, there's a chance it won't happen to the second one.

and no, it isn't supported by the above calculations. It is, however, common sense.

A waddling elephant seal is the cutest thing in the entire world.
-TJ

[jools 0]

May be I am wrong but from what I can see there is 2-3 inch difference between the first loop on the bridle and the second loop, which must mean the 1st loop tied the 1 nearest the pins is loaded first then the 2nd tied or PC end is loaded 2nd?
or am I way off?

surely 2-3 inch is enough difference between loadings on the SL?

"If you don't like it then fuck off" BASE 621

[stitch 0]

Shouldn't you be repling to TJ?? He's the math geek.

"No cookies for you"- GFD
"I don't think I like the sound of that" ~ MB65
Don't be a "Racer Hater"

[Tornolf 0]

You're right on both counts. As long as the first cord has enough room to stretch and break, the cords will be loaded separately.

Also, A PM I got made me realize I might not have been as clear as I could have. My main point was that there is absolutely no fundamental difference (neglecting an extra inch of freefall) between 2 break cords of equal length and 2 break cords of unequal length. The reason to use different length pieces is that it gives all the advantages of 2 equal length pieces along with a couple others. Any advantage to using 2 pieces of equal break cords is also an advantage to 2 different lengths, but not the other way around.

A waddling elephant seal is the cutest thing in the entire world.
-TJ

[Calvin19 0]

speaking of common sense, doesnt it seem right that if there is a mishap, and the 1st one breakes pre-maturly, that the same problem will probably cause the 2nd to break? even if its 8' up the bridal?

i know that 2 loaded seperatly would solve a SL rigging problem, but if you accidentaly have a wrap to break the first, can you clear it in 8' (.4sec)?

maybe the wrap-SL break would start the limb or body rotation to clear it, i dono. just thinking.

[Tornolf 0]

Keep in mind that the break cord has to overcome relatively little momentum to get the canopy out and to linestretch. It's very possible that whatever caused the first cord to break early will do the exact same thing to the second cord, but not break early because the first cord significantly reduced the momentum of the canopy.

dp/dt yo

A waddling elephant seal is the cutest thing in the entire world.
-TJ

[Calvin19 0]

Aye, agreed there, but in the case of something like a arm bridal wrap, i dono. and it could happen on a crowded A. or B. Or E. Or S. or O.

[Tornolf 0]

Pretty sure if you wrap on the static line you're fucked unless you get lucky.

I'm looking at you, twoply!

A waddling elephant seal is the cutest thing in the entire world.
-TJ

[Calvin19 0]

Quote

Pretty sure if you wrap on the static line you're fucked unless you get lucky.

!

of course, if your doing a SL with no PC, just flail a lot and you will be fine. well. live through it kinda fine. maybe. flailing is good. i love sport where flailing is a technique.

[980 0]

that's a lot of math to prove something that's not correct

do some simple tests in the real world with measurements and you will find that:

- a single loop of 80lb break cord will break at almost 160lbs given a gradual and static loading, this is almost not affected at all by whatever kind of knot you use (it is really much harder to measure a dynamic load than a static load, unless you happen to have a load cell and data logger)

- two sufficiently different length loops of break cord will break at a load of around 160lbs each, whereas two loops with not enough length difference will generate a significantly higher load before breaking

arguing this point with fancy math only proves you do not understand the system sufficiently well to model it correctly with math, as a few simple real life tests will prove

'In theory, there is no difference between reality and theory. In reality, there is.'

cya

[base689 0]

...
- a single loop of 80 lb break cord will break at about 0.6 · 160lbs = 96 lbs due to the 40% degradation factor caused by the knot for closing the loop.

Stay safe out there
Blue Skies and Soft Walls
BASE #689 - base_689AT_NO_123_SPAMyahoo.com

[MyTwoCents 0]

Quote

- a single loop of 80 lb break cord will break at about 0.6 · 160lbs = 96 lbs due to the 40% degradation factor caused by the knot for closing the loop.

Is that based on an actual experiment you've done with a tensile tester?

My own experiments have been in line with 980's. The cord broke between 150 and 160 lbs.

[base689 0]

> Is that based on an actual experiment you've done with a tensile tester?
——————————————————————————————
No. Rather, it's based on Dan Poynter manual, which states that a knot onto a oline decreases its breaking load by 40%.
Indeed, it's very reasonable that a knot decreases the breaking load of a line: in fact, we use the surgeon knot to minimize the curvature.
Indeed, (nearly) every time tha a break cord breaks, it breaks AT the knot or very close to the knot.
The loop of a 80 lb break cord should break AT 160 lb if we could close the loop in a veru clean way (i.e., with several stitches in a row of seam along few inches).
I do believe that the tests you performed caused your loop to break at 150÷160 lb: this means that YOUR piece of break cord (single strand measured in a laboratory with special clamps) would have broken at 120 lb or so.
If it were a PURE 80 lb break cord, it's sort of strange that the know did NOT introduce any degradation in the breaking load.

Stay safe out there
Blue Skies and Soft Walls
BASE #689 - base_689AT_NO_123_SPAMyahoo.com

[Tornolf 0]

Quote

- a single loop of 80lb break cord will break at almost 160lbs given a gradual and static loading

F = dp/dt

all my tests show breakcord breaking at the knot when a 100lb weight is dropped from 3-4 inches. every time.

your real world tests do not accurately model a static line base jump.

breakcord's breaking strength is shock load, measured by dp/dt, not m*a.

suggesting that knots do not noticeably degrade tensile strength goes against decades of scientific and practical (rock climbing) knowledge. I'm pretty sure this knowledge will go back centuries/millenia as well, but I don't know for sure. to plausibly disagree with this precedent, you're going to need a lot of math and evidence

exactly how did you do your tests?

Quote

(it is really much harder to measure a dynamic load than a static load, unless you happen to have a load cell and data logger)

there is a very good reason labs and private companies spend hundreds of thousands of dollars on measuring equipment.

edit: I phrased "breakcord's breaking strength is shock load, measured by dp/dt, not m*A." wrong. technically it is measured by m*A. however, in this case, A is more easily described as dv/dt since A will not be constant, and is definitely not equal to gravity. A will be a vector going in the opposite direction of the load, which is usually in line with gravity's vector.

A waddling elephant seal is the cutest thing in the entire world.
-TJ

[Ghetto 0]

TJ, your entire arguent above can be summed up quite easily with the following equation:

sh*U/T( t*H^e) |=uC*k/(uP) + j*u/mp

Web Design
Cleveland Skydiving
"Hey, these cookies don't taste anything like girl scouts..."

[Tornolf 0]

Quote

If it were a PURE 80 lb break cord, it's sort of strange that the know did NOT introduce any degradation in the breaking load.

I'm fairly certain the properties of the break cord change when gradually loaded and stretched. This most likely explains why all force is not equal when applying it to break cords. It could be anything from the style of the cotton weave to the electromagnetic interaction of polymer chains at an atomic level. The latter seems like it could plausibly explain the reason why it breaks at 96lbs of shock force but can easily suspend more than that.

I think I'll ask one of my professors next week about it. He taught a class I took last semester which dealt with things like this.

Do you have any ideas?

A waddling elephant seal is the cutest thing in the entire world.
-TJ

[Calvin19 0]

poynter manual? we are base jumping home boy.

dude, break cord is cotton.

40%?
what knot?
every knot, in every kind of material, in every weave, in every angle, in different temperatures and saturation of knot. it all matters.

from 0% up tp 80%.

and yes, i have a dynometer/tensile tester, and yes. i have tested it.

cotton is in the lower margins of knot-weakening percentages.


while i have not tested 80lb break cord, i would say that it breaks closer to 140lbs than 80 when gradualy pulling/gradual load increased.

[Tornolf 0]

Quote

while i have not tested 80lb break cord, i would say that it breaks closer to 140lbs than 80 when gradualy pulling/gradual load increased.

yeah, but what does that have to do with break cord relating to the base jumping environment?

edit: nm. forgot 689/m2c/sam were talking about loading it that way.

A waddling elephant seal is the cutest thing in the entire world.
-TJ

[base689 0]

> poynter manual? we are base jumping home boy.
——————————————————————————————
Really? I am afraid we are mushroom packing our PC's according to what is stated (among another 1000 issues about parachutes, PC's e relevant equipment) on Dan Poynter's manual

> dude, break cord is cotton
——————————————————————————————
I perfectly know break cord is cotton, so what?!?!?!?

> 40%? what knot?
——————————————————————————————
Dude, don't try to break the hair into 10 pieces. We are BASE jumping, do you remember? Dan Poynter Manual says that a generic knot can decrease up to about 40% the break load of a line, that's why we do the surgeon knot (to minimize this decrease).
Yes, if you do a crap knot you can decrease break load up to 80%, as well as can be 0% if you sew two overlapping ends with a sewing machine; our responsability to do "proper knots" when closing break cord loop, as well as jolly impossible to have a sewing machine on exit point.

> and yes, I have a dynometer/tensile tester, and yes. i have tested it.
——————————————————————————————
It's a pity you didn't test it under REAL working conditions, i.e., a BASE jumper exerting his full weight about 10 m - 30 ft lower.
I do understands there has been lots of real tests but only with static weights applied.
In then end, in our great experience of SL jumps, it has never happend to any of us any premature failure of break cord, as well as we have never experienced an abnormal break load of such break cord (the 80 lb nominal break cord bought by ParaGear), doing the surgeon knot as a standard, also when using the break cord in the CWY device.

> while I have not tested 80 lb break cord, I would say that it breaks closer to 140 lbs than 80 when gradualy pulling/gradual load increased
——————————————————————————————
I could believe that, but I am afraid that is NOT the real condition of deployment of our break cord in a real BASE jump. We use it (=break it) applying istantaneously (...after the forces needed to open container and extract canopy...) our weight 10 m - 30 ft below; jolly difficult to test THAT real situation.

Stay safe out there
Blue Skies and Soft Walls
BASE #689 - base_689AT_NO_123_SPAMyahoo.com

[base689 0]

> My own experiments have been in line with 980's. The cord broke between 150 and 160 lbs
——————————————————————————————
Did you do your test under static load gradually applied to break cord loop? While THIS setup states indeed the break load under static load, it is NOT the break load under the dynamic conditions we apply onto break cord loop when we SL BASE jump.

Stay safe out there
Blue Skies and Soft Walls
BASE #689 - base_689AT_NO_123_SPAMyahoo.com

[base311 0]

Quote

TJ, your entire arguent above can be summed up quite easily with the following equation:

sh*U/T( t*H^e) |=uC*k/(uP) + j*u/mp

That's awesome!

-Gardner

[Calvin19 0]

hmmm, my dynometer is pretty small. ill bring it on a jump sometime. but i never do SLs. your right though...

ill bring it to potato and jack some cord off a new kid and his super-BASE prep kit. that should solve the problem. ill even make it jump(the newbie)

on a different note, Cotton is about as in-consistant as it gets. even break cord varies a bit.


PSS-what is a surgeons knot? I do a figure-Eight. then a million little retard knots.

[GooManChew 0]

http://en.wikipedia.org/wiki/Surgeon's_knot

[Calvin19 0]

Aye, got it. thank you.

[base689 0]

> hmmm, my dynometer is pretty small
——————————————————————————————
I do believe that to perform a test of break load under real dynamic conditions is jolly impossible.

> Cotton is about as in-consistant as it gets. even break cord varies a bit.
——————————————————————————————
Agreed. I knew that. So what? I hope that the nominal 80 lb break load value is an average (not to go below, possibly to go over). Being something used by military on military soecifications, I hope such a 80 lb value is "guaranteed".
We see the 80 lb break cord by ParaGear as the most reliable break cord that we can afford to buy. It has worked very well indeed up now.


> 1) but I never do SLs
> 2) what is a surgeons knot?
—————————————————————————————
From statement 1 I can understand question 2
Here is surgeon knot (well studied by us WELL BEFORE attempting our first SL jump (in our group)) Surgeon knot is a knot which minimize at best the curvature of line forming knot itself, so decreasing the minum possible the "Break Load of line loop WITH knot" with respect to "Break Load of simple line loop".
Surgeon knot it's on our "handbook"

Stay safe out there
Blue Skies and Soft Walls
BASE #689 - base_689AT_NO_123_SPAMyahoo.com