TeeDee,
I have looked at hammock geometry from time to time looking at various things. Usually I am looking at the forces involved, the sag angle, how a structural ridgeline plays into it, etc. Anyway, I often generate sketches and as you know, sometimes they quickly show what is going on. Here are a couple that might add some insight in what you are looking at. Note that I chose to normalize the height the bottom of the hammock rests above the ground.
Youngblood AT2000
Dave, thanks for the graphics. I think they may make the tables I generated more understandable by more people.
The right hand graphic is especially pertinent for the hang rope since it depicts the situation with a ridge line.
It graphically shows that small changes in the suspension length can make what are large changes in the sag angle. For example you graphic shows that at 15' span, changing the suspension length on one end by 5 7/8" changes the sag angle by 23 degrees.
That's what I was trying to explain with my last post - changing the suspension length by even small amounts results in large changes in the sag angle and that's why getting a really accurate measurement for the span distance is especially important to eliminating the adjustments of the suspension.
The diagrams being normalized to the bottom of the hammock staying at the same point above the ground shows some aspects of what is happening, but it masks another point that can be very aggravating when using a structural ridgeline that is initially tied off with the suspension line taut. And that is something is going to stretch when you get into the hammock and it doesn't have to stretch very much to cause the hammock to drop a great deal when the suspension line is initially taut.
That is not the case when the suspension line is not initially tied taut. Tying off higher and introducing some sag into the unoccupied hammock will result in less drop when the hammock is occupied and put less stress on the hammock suspension and the trees it is tied off to. It figures to require less readjusts as well.
Youngblood AT2000
Yes - if you read my original article on the hang rope method, I mentioned that as one of the reasons for setting-up so that the hang rope itself has a sag angle instead of trying to pull it taut.Originally Posted by Youngblood
That is why I output the sag angle table with a minimum of a 20 degree sag angle for the hang rope. And I only use 20 degrees at very long spans so that it is possible to place the tree huggers at a reachable height.
I usually try for a hang rope sag angle between 24 degrees and 30 degrees and I get only a little drop when I get in the hammock. I measured the drop a couple of times. I don't remember the numbers any more, but it wasn't much.
I forget the sag angle of my Bridge Hammock itself, but it's slightly greater than 30 degrees. When I set-up with a hang rope line sag angle of 30 degrees, the ridge line portion of my hang rope is under almost no tension. Very little. The few times I have experimented with 32 degrees or greater for the hang rope sag angle, the ridge line portion is hanging limp.
When I used my Carabiner Hitch method for my suspension, I followed the Hennessy advice and would pull the suspension taut with the hammock empty. I liked that method since I could easily exceed a 20' span and still place the tree huggers within reach. The biggest thing I didn't like about doing that was that the forces on the trees was so great that I couldn't use cord tree huggers at any time. The forces pulled a cord tree hugger right into the tree no matter what. That forced me to use webbing tree huggers no matter what. In my endeavors to use rope tree huggers, I needed to allow greater than a 20 degree sag angle in the suspension at the tree with a ridge line.
With the hang rope method, at small hang rope sag angles (say less than 20 degrees), it is very difficult to tie the bowlines at the ends of the hang rope so that the hang rope is taut since you have to pull the rope tighter than that to slip the bowline loop over the Marling Spike or Pile Hitch attachment point on the tree hugger. I can manage that with a 20 degree hang rope sag angle, but the forces involved become abundantly evident even then.
does hh really recommend that the suspension be pulled fully tight?
teedee, when you speak of 20 deg. or less of a sag angle, are you setting it up fully tight and then observing 20 deg when fully loaded? or are you hanging it with 20 deg angle when empty?
what is the lowest sag angle (weighted susp.) that you guys have achieved? i guess that may depend on if you use a trucker's or not, and the closeness of the trees may matter as well.
I remember the directions on the stuff sack saying to pull taut - it leaves the definition of taut up to the user.
fully loaded.
The sag angle I compute and set-up for and measure with my hang rope method is fully loaded. I measure the sag angle after set-up when empty using the hang rope method and then again when loaded. The sag angle when empty is usually about 2 degrees less than loaded.
Using my Carabiner Hitch method (which is a variation of the Trucker's Hitch), I would usually have the suspension sag angle between 15 degrees and 20 degrees. Say 18 degrees average. Loaded. Getting down to 15 degrees loaded is a real bear. I have to pull that Carabiner Hitch really tight. Also, that is for 16' span or less, mostly less. For a span greater than 16', I doubt that I could achieve less than 20 degrees, loaded. With the Carabiner Hitch Method, I only ever used a 3:1 advantage. Griz says that he used more times around the circuit. That would get you a higher mechanical advantage, but the frictional forces build really fast and I doubt that using more circuits really buys that much because of the frictional forces. And the harder you pull the greater the frictional forces and the more you lose to them. I think that adding more circuits really only lets you use less force in pinching the rope to hold the place while you cinch it off. I never had any problem with the 3:1 advantage pinching to hold while cinching, when without that advantage, pinching to hold while I cinch it would be extremely difficult.
Empty when I used the Carabiner Hitch method - never measured, but guessing it was probably on the order of 10 degrees, maybe, maybe between 5 degrees and 10 degrees, but I doubt that it was less than 10 degrees.
Last edited by TeeDee; 08-09-2008 at 17:25. Reason: Clarify
thanks teedee, that's exactly what i was wondering. so ~15 deg would probably be the minimum. i think there was an equation in the "hitchcraft for hammock" thread that gave % of bodyweight on each support rope based on the degrees, where 30 degrees put approx. 100% on each end. i've been curious for awhile about how much force is on a tight suspension so this at least tells me a minimum sag angle.
didn't realize the hh stuff sac said that. must be why there have been so many breaking, they put cord in there that is too weak and then tell people to set it up with as much force on it as possible
Yes - it's been a long time since I last used my HH ULBA, but that ridge line cord Hennessy used isn't exactly what I would have chosen for a ridge line. It reminds me more of a shoelace. Long ago I took the whipping socks apart on both ends, and removed their ridge line cord and replaced it with the BPL spectra guy line cord and replaced the whipping socks. I would have preferred a color other than yellow of the BPL cord and maybe someday I'll get ambitious and replace the BPL cord with the Lash-It cord. Gray is much better for the ridge line... they put cord in there that is too weak and then tell people to set it up with as much force on it as possible
Safari Deluxe Set Up Tips---
Directly from the stuff sack the Safari Deluxe cam in."Hammock Set Up
Center hammock between supports and tie ropes tightly. Adjust hammock level until you are comfortably centered. Entrance should be a chair height under tension of your weight. Depending on the weather conditions, adjust width of hammock by changing tension or angle of side elastic cords to tree branches, bushes, rocks, improvised stake or to canopy side adjustment cords."
I may be slow... But I sure am gimpy.
"Bless you child, when you set out to thread a needle don't hold the thread still and fetch the needle up to it; hold the needle still and poke the thread at it; that's the way a woman most always does, but a man always does t'other way."
Mrs. Loftus to Huck Finn
We Don't Sew... We Make Gear! video series
Important thread injector guidelines especially for Newbies
Bobbin Tension - A Personal Viewpoint
I've heard of a few HH ridgeline failures over the years but not that many considering how many are in use. I don't think failure is something to be expected. I also don't think it is necessary to stress the suspension system like some do (including me when I initially used one years ago) which is a contributing factor to that.
As far as the ridgeline cord, at one time I was puzzled about it's strength in comparison to the rest of the suspension system. It is fine for initial setups with some sag but seems like a weak link if you use the taut-as-a-drum setup. But after thinking about that, something is going to be the weak link in the taut-as-a-drum setup and if I had to chose between the ridgeline or the suspension lines, I would pick the ridgeline.
The way I look at it, the ridgeline cord is fine, it is the instructions that are the weak link.
Youngblood AT2000
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