The physics of pyramid tent design

[Ell]

I have been doing some thinking and have decided, against my better judgement,to design and build a pyramid style single walled tent (hexamid-ish).

However, my background isn’t engineering (CS sorry !) so I am a bit unsure where to look/ what to consider in the design.

I am sure I could eyeball things so I don’t accidentally make a very expensive and dangerous kite, but that doesn’t really sit well with me. So ideally I would like to guide design choices by the physics of it.

Some inshial questions:

 

Guylines:

– Mid panel and/or on the seams?

 

Panels:

– Is more always better?

– What is the point of diminishing returns?

– Is actually the additional panels or the additional stakes needed?

 

Wind Loading/ distribution/ Shedding?

– Can any of these be reasonably estimated?

– How could you optimise these ?

 

I know that any one of these could be a topic in its own right and that there may not be huge benefits to this approach as there are significant factors that are not able to be controlled for – my sewing being the main one!

But any advice/ aerodynamics book recommendations would be much appreciated!

[Jerry Adams]

a lot of topics on backpackinglight about this

they tend to be more experience than theoretical/physics

[Eric Blanche]

My take on the guylines is you need and should want both! They do not all have the be setup all the time. Only when necessary/concerned.

 

With regards to panels, IMO one of the beautiful aspects of the pyramid is the super simple four corner/stake design. Setup is very easy. Adding panels and required stakes may add robustness but also adds complexity in shelter setup. Perhaps, depends on what one is looking for.

[Jerry Adams]

I have been pondering the guylines question.  I’ve tried mid panel, ridge.  Center, 1/3 from bottom.

Currently, my opinion is it’s better to have more stakes at the bottom.  1 on each corner = 4 total in calm winds.  8 total for windier.  16 total for the worst winds.  I don’t carry 16 stakes but I can use rocks.

With guylines on the sides, either ridge or mid panel, it distorts the shape of the panels.  Better to just have more stakes at the bottom.

But, I like to have one guyline on the side about even with where my head would be if I was sitting up.  Then there’s more room for my head.  So I don’t brush against condensation on the inside of the tent.

Although after reading the article about the Dan Durston X mid, that is another solution to more headroom in a mid.  Maybe some day I’ll play with something like that.

[Jeff McWilliams]

I REALLY like how much additional headroom one gets by using a diagonal 2-pole design, like the Xmid or the Tarptent Stratospire.  Single pole pyramid tents like MLD’s pyramid tents seem like a big waste of surface area in order to get a usable amount of volume under the fly/inner.   2-pole tents like Gossamer Gear’s “The Two” or ZPacks Duplex are somewhere in the middle.

[Jerry Adams]

I just looked at the stratospire.  That looks similar to xmid.  I wonder who copied the other or if they both came up with the idea.

[Jeff McWilliams]

The Stratospire predates the XMid (and the Sierra Designs High Route FL 2).

[Ryan Jordan]

Jerry wrote:

With guylines on the sides, either ridge or mid panel, it distorts the shape of the panels.  Better to just have more stakes at the bottom.

I want to make a case for:

1. Yes, add guyline mid-panel tieout points. They don’t need to be super tight – they just need to prevent some deflection of the panel in wind that hits the panel. Doing so preserves as much of the original structure as possible in a storm, which relieves stress on the rest of your stakes, minimizes flapping, preserves some interior volume.
2. I’m assuming you are using DCF, since you mentioned the Hexamid. DCF does not stretch. So a mid-panel guyline tieout is not appropriate – that relies on stretch to work. This is one of the fundamental rules broken by most tent makers, because it saves them money. But you’ll see the problem with this when you experience it – guying out the middle of a DCF panel introduces wrinkles and concentrates, instead of distributes stress throughout the panel. You can see this on DCF shelters from ZPacks, Mountain Laurel Designs, Gossamer Gear, and others. Mid-panel tie-outs require stretch fabric to get the most performance out of them.

The solution to #2 is to split your big side panel into two parallel panels joined with a vertical seam. Put the “mid-panel” tie-out along that seam. Now, you can tension that mid-panel tie-out even more on a DCF shelter, because you are distributing the tension load along the seam instead of creating some wacky stress line between the panel tie-out along some arbitrary (and weak) load line towards one of the corners.

This is one reason why Locus Gear pyramids outperform other DCF mids in very stormy weather. The arrows in this photo point to the mid-panel guyline tie-outs, which are actually on the vertical seam joining two parallel panels:

[Geoff Caplan]

– Ryan

This makes a lot of sense!

More work to build, but worth it for those apocalyptic nights when you want to grab a little sleep!

Would you say that this principle applies more generally to silnylon and silpoly as well as DCF panels? I can’t see why not. A mid-panel tie-out will work better on those fabrics, but a tie-out on a seam is surely the optimal design?

[Jerry Adams]

That makes sense

Similar idea with stakes on the bottom.  You want to tighten the stakes at the 4 corners quite hard.  The rest of the stakes you mostly just want to tighten any slack, but don’t tighten so much that it distorts the tent.

I wonder if anyone has done the experiment with side guy lines at various points, in strong winds.  Optimally, at some point the wind so strong that the tent collapses.  MSR did experiments with mounting a tent on the top of a car, then driving at various speeds.

In the 1967 “Mountaineering: Freedom of the Hills” they describe a “McKinley Tent” as being appropriate for mountaineering:

Things have moved on since 1967, but there must have been people using that in strong winds.  Their guylines are on the seams, about 1/4 of the way up from the ground.

[Diane “Piper” Soini]

I think a simple pyramid tent is a flat tarp set up with one pole in a half pyramid pitch. Once you set up a shelter like that for yourself, you start to think this would be a lot better if there was some kind of beak on the front. This then becomes something like the Hexamid without doors, or maybe something like the Trailstar. If you make something like that then you might think doors for privacy or even more weather protection would be nice. Now you have a Hexamid with doors. Then you decide other things, like maybe more vertical sides or more mid-panel tieouts would give you better inside space, or maybe something more equilateral will be hardier in bad weather. You sort of evolve like that. Through iteration like when you write code.

[Jerry Adams]

Same here Diane, I have evolved sort of like that

[jscott]

“Same here Diane, I have evolved sort of like that”

any tent that I made would have gills…

[James Marco]

Shelters are usually a series of compromises leading to a specific solution for a specific problem. I do not use pyramids, but, an elongated pyramid makes an excellent wet weather shelter, for example. Why? it is far lighter for the coverage.

[HkNewman]

Panels: – Is more always better? … diminishing returns

Iirc an interview with a cottage gear maker (here?), more panels were better in terms of minimizing area exposed to a strong wind, but then IMHO there’s more sewing and bonding = more opportunities for a defect, more time, and (for their companies) way more expensive.   Pretty sure they stopped due to economics before the additive weight of extra sewing and bonding became an issue.

Also look at what the major gear makers have already done.  Believe the hexamid has the most panels available commercially  (maybe cutting a little weight too), but many report needing more guy lines.  I know Yama shelters add the weight of guy lines and other “rigging”, so look at those designs and maybe extrapolate?  If I were to do MYOG I’d probably start by emulating or modifying existing designs … a lot of the work has already been done in several decades of backpacking and mountaineering (cottage, major, and/or historical gear makers).

[Craig B]

The solution to #2 is to split your big side panel into two parallel panels joined with a vertical seam. Put the “mid-panel” tie-out along that seam. Now, you can tension that mid-panel tie-out even more on a DCF shelter, because you are distributing the tension load along the seam instead of creating some wacky stress line between the panel tie-out along some arbitrary (and weak) load line towards one of the corners.

You could also accomplish the same thing by making a faux seam in the center of a panel by bonding a 1″ or so strip of the DCF tape along it.  Or two strips for that matter.  You could also make reinforcing seams in this way in other directions if you figure out where the stresses are coming from.  This is one of the great benefits to construction using bonding rather than sewing….

[Stumphges]

I really like 8-sided symmetrical pyramids. I own several, all regular octagons, and think they are an overall better shape than squares and rectangles. I also think octagon beats hexagon. Eight sides means eight tensioned ridgelines and eight relatively small panels. Made well, an octagonal pyramid tensions almost into a cone shape, shedding wind exceptionally well.

The downside is that they need to be pretty big to provide enough length for a human to sleep. Perhaps a better design is an octagaon that’s been stretched into a more rectangular or eliptical shape. I have such a one, made by the Chinese company Knot – about the size of a Dyomid or Khufu, but with two vestibules and two zippers. Pitched with poles in an A-frame, it’s a pretty sweet 2p tent. This one is silnylon and not as high quality as something from MLD or Locus Gear, but I like it a lot.

If I were to make a mid in DCF I would make such an elliptical pyramid with two doors and use the rectangular inner for the above inside it. The elliptical shape is not as aerodynamic as the regular octagon, but offers a large enough rectangular space inside for humans to sleep without the overall footprint being too massive.

Here is a great post from a couple years ago about a very similar design: https://backpackinglight.com/forums/topic/mid-set-up-issues-got-advice/page/2/#post-3443441

I guess the Seek Outside Cimarron is kind of an ellitpical octagon too, although larger than 2p.

I remember Kevin Timm from Seek Outside remarking on here some years ago that the more sides a mid has the less cat cut it needs. The octagonal mids I have all have significant cat cuts, but the blue one in the post above does not, and seems to pitch just fine. However, for reasons i can’t fathom, octagonal mids with cat cuts pitch into a conical, almost tipi-like, shape, whereas octagonal mids without cat cuts don’t seem to (you can still see defined ridgelines in the latter).

Another nice thing about tipi-style mids is that mid-ridge or mid-panel tieouts seem unecessary unless the wind gets very strong, like 50+ mph, according to accounts on trek-lite.com, where such mids are favored for very windy UK conditions. I’ve not had any of mine in 50 mph yet.

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