Bridge Side Cuts and Flatness

[JerryW]

After studying some of the bridge designs I have come to a couple conclusions. The flatness of the lay comes from a combination of two things.

1. How deep the sides are curved. Grizz and others use about 6" deep. TeeDee and others use about 12" deep, similar to the Aussie design.

2. How tight the hammock is strung, thereby stretching those curves toward being straight and pulling the center up.

Has anyone tried a curve in between 6" and 12" deep? Maybe split the difference at 9", anyone?
Does it really make that much difference?
Am I over-thinking this(as usual)?
Am I missing something else that affects the flatness?

I made a quick test hammock with 6" curves and it is pretty flat, but it seems to vary a little depending on how tight I hang it.

All comments and opinions are welcomed.



Jerry

[GrizzlyAdams]

After studying some of the bridge designs I have come to a couple conclusions. The flatness of the lay comes from a combination of two things.

1. How deep the sides are curved. Grizz and others use about 6" deep. TeeDee and others use about 12" deep, similar to the Aussie design.

2. How tight the hammock is strung, thereby stretching those curves toward being straight and pulling the center up.

Has anyone tried a curve in between 6" and 12" deep? Maybe split the difference at 9", anyone?
Does it really make that much difference?
Am I over-thinking this(as usual)?
Am I missing something else that affects the flatness?

I made a quick test hammock with 6" curves and it is pretty flat, but it seems to vary a little depending on how tight I hang it.

All comments and opinions are welcomed.



Jerry

welcome to the bridge club Jerry.
I think you understand what is going on. The suspension cut allows one to pull up the middle of the hammock more tautly when the ends of the hammock are pulled. The deeper the cut, the tighter that pull, the more the middle comes up...to a point. The point is when the suspension webbing has been pulled into a straight line.

With a deep cut like 12" per side it is possible to pull up the middle so much that when you get in you experience an inverted bananna effect.

Intuitively a cut could become so shallow that it doesn't pull up the middle enough to make the hammock flat. In the limit of course, there is no cut at all, and we know what we have then is a blanket toss, not a bridge hammock.

So how shallow is too shallow? I think I cut one once to 3", just to see, and as I recall I could pull it tight enough to get flatness. Adds a lot of stress on the suspension though.

My standard template cut for my lighter narrower hammocks has a 5.5" cut. Perfectly adequate.

I choose a cut depth now more to help guide the hammock to other objectives. One of those is minimum width at the center. The other is a range of hammock depths (distance from the center of the body up to the spreader bar.

My personal preference is for a width to be around 36". That's wide enough so that I can bring my legs up and not have them spill out of the hammock.
cut-3.jpg

At the same time I don't want the hammock depth to be more than 17 or 18 inches, because more than that creates a wall of useless fabric around my head and feet. More than that also lowers the center of gravity when loaded though, making the hammock harder to turn about its horizontal axis that runs between the apexes of the two suspension triangles).

This shot is of a hammock that is spread 34" at the foot over 51" of fabric, is 16" deep, has a 36" wide middle, and 7.5" suspension cuts.
cut-2.jpg

Grizz

[Walking Bear]

The one that I have been using has about a 6" cut to the sides. It's about 36" wide in the middle and about 48" at the spreaders. I had moved the spreaders in about 9" in it to make the triangle longer.

[JerryW]

Oooo, thanks guys! This is just the kind of info I was hoping to get.


Out of curiosity, does anyone know how deep the arcs are on the Jack's BMBH?




Jerry

[Smee]

We took a slightly different approach. The key parameter was the angle of the suspension coming off the hammock. The target was 30 degrees from the horizontal to establish the stress level on the suspension system. The other key was the height of the side wall in the center so that it was comfortable when sitting down. Those two points will define the curve of your side.


Regards,

[GrizzlyAdams]

hey thanks for the information Smee. Nice to have your perspective.

What you forgot to tell Jerry though was that your suspension cut is 8", the width of the hammock at mid-point is 39", and the depth of the hammock is 19".

I found a tape measure

Grizz

[TeeDee]

Jerry - please check this thread

[TeeDee]

hey thanks for the information Smee. Nice to have your perspective.

What you forgot to tell Jerry though was that your suspension cut is 8", the width of the hammock at mid-point is 39", and the depth of the hammock is 19".

I found a tape measure

Grizz

Grizz - when you had your tape measure, did you happen to measure the fabric end width and the spreader bar length(s) - I believe they use equal length spreaders?

Ignore my request for end width - I put the mind into gear and re-read your post.

End fabric width: 39" + (2 x 8") = 55"

If I remember correctly the spreader bars are 31". Correct?

[JerryW]

Thanks for the info Smee and Grizz.

Thanks for the link TeeDee. I don't know how I missed that in my searches. I'm going to have spend some time absorbing all that.




Jerry

[GrizzlyAdams]

Thanks for the info Smee and Grizz.

Thanks for the link TeeDee. I don't know how I missed that in my searches. I'm going to have spend some time absorbing all that.

Jerry

I had forgotten that post of TeeDee's, it deserves a bump.

Also I mis-remembered the width of the BMBH, it is 37" at the middle, not 39".
The fabric length under the spreader bar is 53", so the cut is 8".

The BMBH spreader bar is 31", but slot-edge to slot-edge it is 30.25". Then consider that these slots are in 2" diameter rings that are forced to be largely horizontal under load, so the actual spread in the hammock body is closer to 28".


Grizz

EDIT : had a look at the corner rings, they look to be 1" in diameter.