# Thread: Bridge Hammock #2 Build

1. ## Bridge Hammock #2 Build

I just finished my first bridge hammock and now I'm on to number 2, learning from all the mistakes I made on number one I learned that planing head makes for a much easier job. To that end my end caps on my first try didn't go so well. I tried having some on lay in it and then take measurements. problem is that sewing a conner is not as easy as it sounds and makes for some odd problems, I know I know Grizzly said this in his DIY write up... but what better way to learn then to bash one's head into desk while learning.

Being that Im a math major I set out to solve the problem of solving for the hight of a parabola while only knowing the spreader bar length and the lenght of fabric... it ended up being a really long proof... took 3 pages to solve! I ended up having to re work this formula...

Arc Length = 0.5√(16h²+w²) + [w²/(8h)][Ln(4h + √(16h²+w²)) - Ln(w)]

into

ln(√(h²+0.625w²)+h)(w²+16)(h√(h²+0.625w²)-0.5(h+FL+.125w²(ln(w)-1.3862936112)))=0

Yeah.... lots of fun!

Anyways, I was wondering if anyone knew how well the parabola settled when loaded? I was thinking I would make the cap out of 1.1oz fabric or a stretchable fabric. What Im trying to stay away from is having a bunch of floppy fabric when its loaded?

Heres the arc when ploted. Total arch lenght comes out to 54.0028"... not bad

The formula for this 54" fabric with 36" spreader arch incase some one wants to replicate it is
y=-0.057099x^2+2.055555x

y ends up being your hight (drop if you flip it)

2. [BRAIN HURT]

So... that's math, right?

[/BRAIN HURT]

I think GrizzlyAdams on here might be the man you're looking for. He's super-scary smart.

3. ho-whee! A contender for the king of the bridge-hammock-math-geek throne! I declare thee a bridge design mensch.

Sometimes I want to know the height of the parabola given the arc length over a given interval length (a.k.a. "spreader bar length"), sometimes I want to determine the arc length given the interval length and parabola height.

Not trusting to the correctness of my own derivations if they spill over more than one page, I trust instead my ability to write a script that estimates by a piece-wise linear approximation the arc-length over the interval, for a given parabola (described simply by "a" in f(x) = a*x^2), and to embed that calculation inside of a binary search over values of "a" to hone in on a parabola with either desired height, or desired arc-length.

All that said, a parabola or any reasonable approximation thereof seems to "load" just fine. Truth---the last endcap I made I knew what arc-length I wanted and what height I wanted, so I marked off half the arc-length on a string, marked some fabric where the corner and the deepest part of the arc would be, and laid out the string with its endpoints aligned at those marks. Pushed the string around to make it kinda parabolic-like, traced the line, and had me half a "parabola". Folded it over for the cut to get symmetry.

I are an engineer.

4. I'm with you, Syb. I made the mistake of looking at this post before i had had a full dose of morning coffee. My synapses are shorting out left and right--along a nice arc--I think. Good to have folks who can do these things and then translate. I think I get Grizz's string method.

5. A string is also my "math" of choice in these situations for pattern making...IMG_3226 (Medium).JPG...also ~18" of drop, zero pages and about 2 minutes of my life.

6. Originally Posted by Syb
[BRAIN HURT]

So... that's math, right?

[/BRAIN HURT]

I think GrizzlyAdams on here might be the man you're looking for. He's super-scary smart.
Originally Posted by Les Rust
I'm with you, Syb. I made the mistake of looking at this post before i had had a full dose of morning coffee. My synapses are shorting out left and right--along a nice arc--I think. Good to have folks who can do these things and then translate. I think I get Grizz's string method.
Originally Posted by gmcttr
A string is also my "math" of choice in these situations for pattern making...IMG_3226 (Medium).JPG...also ~18" of drop, zero pages and about 2 minutes of my life.
LOL thats actually a really good idea. but im a nerd... there for i must work harder, not smarter! lol no really I might actually use this method. Right now Im trying to work out blue prints for a hammock that I can change on the computer with out having to cut though a bunch of fabric.

ho-whee! A contender for the king of the bridge-hammock-math-geek throne! I declare thee a bridge design mensch.

Sometimes I want to know the height of the parabola given the arc length over a given interval length (a.k.a. "spreader bar length"), sometimes I want to determine the arc length given the interval length and parabola height.

Not trusting to the correctness of my own derivations if they spill over more than one page, I trust instead my ability to write a script that estimates by a piece-wise linear approximation the arc-length over the interval, for a given parabola (described simply by "a" in f(x) = a*x^2), and to embed that calculation inside of a binary search over values of "a" to hone in on a parabola with either desired height, or desired arc-length.

All that said, a parabola or any reasonable approximation thereof seems to "load" just fine. Truth---the last endcap I made I knew what arc-length I wanted and what height I wanted, so I marked off half the arc-length on a string, marked some fabric where the corner and the deepest part of the arc would be, and laid out the string with its endpoints aligned at those marks. Pushed the string around to make it kinda parabolic-like, traced the line, and had me half a "parabola". Folded it over for the cut to get symmetry.

I are an engineer.
Thanks Grizzly! I've leanred alot in the past few weeks from reading your posts and your videos... Just this morning I was watching the video on your light weight bridge where you used the small peices of alumminum to put the spreaders between! This could solved my massive over use of webbing problem I had with my 1st go. My first try also put the spreader below the suspension causing some odd pulling, My next try will keep the bars in line with the suspension webbing. I also need to work on my sewing skill

7. Originally Posted by MrDieselTwitch
Thanks Grizzly! I've leanred alot in the past few weeks from reading your posts and your videos... Just this morning I was watching the video on your light weight bridge where you used the small peices of alumminum to put the spreaders between! This could solved my massive over use of webbing problem I had with my 1st go. My first try also put the spreader below the suspension causing some odd pulling, My next try will keep the bars in line with the suspension webbing. I also need to work on my sewing skill
There's a gotcha with the aluminum pieces---there's a whole lotta torque on the tip if it fits the hole tightly. The aluminum piece does NOT want to be at 90 degrees to the tip. That's OK with the hardy tips on the bars you illustrate, not so good with hiking pole tips. The solution is to be sure the hole that accepts the tip is drilled out enough to allow the aluminum to align with the forces on it but not twist off the tip while doing it.

I've played a lot with webbing and grommets at the corners too. Not confident enough yet in the safety of it to post and lead someone else to try The motivation was exactly this torque on the tip problem---webbing twists easily.

Keep up the good work. Even if you are a "math major" (and I double majored in math and more math in college), bridge building is an experimental activity.

There's a gotcha with the aluminum pieces---there's a whole lotta torque on the tip if it fits the hole tightly. The aluminum piece does NOT want to be at 90 degrees to the tip. That's OK with the hardy tips on the bars you illustrate, not so good with hiking pole tips. The solution is to be sure the hole that accepts the tip is drilled out enough to allow the aluminum to align with the forces on it but not twist off the tip while doing it.

I've played a lot with webbing and grommets at the corners too. Not confident enough yet in the safety of it to post and lead someone else to try The motivation was exactly this torque on the tip problem---webbing twists easily.

Keep up the good work. Even if you are a "math major" (and I double majored in math and more math in college), bridge building is an experimental activity.
Yeah Im a little worried about the tq on the tip also. I don't hike with poles so all I will have is those spreader bars, I would like however to use a smaller sized pole. I'm going to play around with some designs today and make take them over to the machine shot to have them cut out of billet. I have an idea in mind that might just work.

Got another quick question for ya, In another guys video he said that you keep the foot end width less than the head to help with stability? Any explanation about that?

9. Originally Posted by MrDieselTwitch
Yeah Im a little worried about the tq on the tip also. I don't hike with poles so all I will have is those spreader bars, I would like however to use a smaller sized pole. I'm going to play around with some designs today and make take them over to the machine shot to have them cut out of billet. I have an idea in mind that might just work.

Got another quick question for ya, In another guys video he said that you keep the foot end width less than the head to help with stability? Any explanation about that?
My mechanics is 35 years rusty so any quanty answer I'd be making up. What seems to happen is that the two ends tend to want to rotate a little differently on a small perturbation, so the overall effect is to rotate less.

I are a computer engineer, not a mechanical one!

10. Originally Posted by MrDieselTwitch
IWhat Im trying to stay away from is having a bunch of floppy fabric when its loaded?
While I applaud all the math here, I wanted to get you to think about this statement just a bit more.

On the Warbonnet Ridgerunner, there is an intentional bit of loose fabric at the head end. It creates a small pocket which is perfect to hold keys or a wallet and cell phone. In my opinion, a bit of floppy fabric is an EXCELLENT thing to have.

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