The Volumetric Efficiency of Trekking Pole Shelters

Discussion on the “volumetric efficiency” of a tent might immediately cause some heavy eyelids, but it simply refers to how weight efficient a tent’s geometry is [I can already tell this post isn’t going to rank in my top 10]. Thus any outdoors person interested in lightweight gear should possess at least a cursory familiarity.

If the goal of a lightweight shelter is to provide living space at the lowest weight, then starting with a basic shape (or “geometry”) that uses the least material to provide that space should clearly be desirable (amongst numerous other criteria). So we are looking to optimize the volume:surface area ratio.

To introduce the topic and demonstrate how it can be non-intuitive, I will compare this volume:surface area ratio for two popular styles of trekking pole shelters against a tent of my own design – the X-Mid. The first popular tent geometry is a single pole pyramid with a rectangular base (the MLD DuoMid being a classic example). Single pole mids are widely referred to as being the most simple tent, so intuitively many folks think this is the most efficient or the “lightest” design (if other attributes like fabric choice are equalized).

duomidclosedzipper.jpg

The second comparison is a standard “pup tent”. This tent is also based around a rectangle but uses two poles set nearly as far apart as possible. Obviously this maximizes the volume you can get from two poles but also increases the surface area, so it’s unclear if it is more efficient in terms of the material required for the amount of living space (again the volume:surface area ratio).

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The last tent is my X-Mid design, which again uses a rectangle shape at the base but places the two poles at an internal position rather than at the perimeter. This is roughly halfway in between the previous two shelters.

omsivg0isomd7w7cwqlv_x-mid - 4 copy

These poles are inset from the perimeter of the fly so the tent can pitch without the guylines required for the pup tent, and the poles are on a diagonal which avoids conflict in the doorways present on these other two shelters. The layout is like this:

screen shot 2019-01-06 at 9.51.03 pm

Readers with a fair recollection of high school geometry might already be able to guess the most efficient shape. As a hint, recall that the best possible volume:surface area ratio is possessed by a sphere, as the graph below shows (area on the y-axis, volume on the x, lower lines are more efficient). The core message of this graph is that for any given volume, a sphere always has the least surface area.

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Below I have sketched out these shelters to scale to show their geometry. I have used the same base dimensions (100″ x 67″) for all of them and assigned the pole height at 54″ for the single pole shelter and 45″ for the dual pole shelters since these are typical values.

screen shot 2019-01-08 at 2.40.58 pm

From these dimensions I calculated the volume and surface area of each shelter to get the volume:surface area ratio as you can see above. Note that the surface area excludes the floor since I am only calculating the material you would need to build a fly for this tent. Any floor wouldn’t necessarily have to use the full area of the base.

Before we get into this though, I’ll give away the theoretical answer right now and then we’ll see how close these tents come. The most efficient possible shape in terms of the volume:surface area ratio is a dome tent since it has half the volume of a sphere and also half the surface area (if we are not including the base). Thus the ratio is just as good as a sphere and unlike a sphere, you can actually build a dome shaped tent.

But of course creating a tent that is both a perfect dome and trekking pole supported is impossible because you’d need to have a perfectly round base (requiring an infinite number of stakes) and an infinite number of support poles (for a perfectly domed fly). Thus the closer a trekking pole design gets to a dome, the more stakes and seams it requires. Somewhere there is an optimum where you minimize fabric with a semi-dome shape while also avoiding too much seam and stake weight. So basically we’re looking to optimize the volume:total weight ratio, not just the volume:surface area ratio.

[Side Note: Larger tents are more efficient in the volume:total weight ratio. Notice that the 2P version of any tent provides about 70% more volume while only weighing 30% more. Also notice that the lines in the graph above have a slope of less than 1, showing an improving ratio. So it’s important that we are comparing geometries for similarly sized tents.]

Okay onward to comparing these tents and then at the end we’ll compare them to both domes and shapes that are closer to a dome, such a hexagon based shelters.

First, the single pole mid has a volume of 70 cubic feet and requires 78 square feet of fabric to build that, for a ratio of 0.89:1 (or 0.89 cubic feet of space for each square foot of fabric). Unsurprisingly the pup tent possesses an impressive 24% more volume (87 cubic feet) but also requires 26% more fabric  (99 vs 78 square feet) so it actually is less efficient with that fabric (a slightly lower ratio of 0.88:1).

The X-Mid on the other hand possesses 16% volume than the single pole mid despite only using 7% more material, so its volumetric efficiency is about 10% better at 0.97:1. In other words, if you were to scale these tents so they all have the same volume, the X-Mid would require about 10% less material. Thus of these three, the X-Mid is easily the most weight efficient geometry.

However, the dimensions I’ve used thus far for the single pole mid aren’t that realistic because single pole mids have such low wall angles on the ends that they need a longer floor to allow for a decent length of sleeping area. Shown below are the actual dimensions of perhaps the most popular single pole pyramid (MLD DuoMid), which is 10″ longer than the X-Mid:

screen shot 2019-01-08 at 2.41.11 pm

Here, we see that the DuoMid has a bit less volume (79 vs 81 cubic feet) despite using more fabric (85 vs 84 square feet). This continues to show that single pole mids in general are less efficient designs. The X-Mid is both more spacious and lighter.

As a side point, the longer length of the DuoMid looks better suited for tall hikers but this is not the case because the heavily sloping end walls result in very low and unusable area at the ends. Despite the DuoMid being 10″ longer (100″ vs 110″), both of these accommodate the same length of floor (87″) and the X-Mid is actually about 6″ longer if you look at the height about 15″ off the ground (where the top of a sleeping bag would be). Thus the DuoMid can not accommodate as tall of hikers despite having a larger footprint.

Now you might agree that the X-Mid is more volumetrically efficiency on paper but wish to point out that gleaning an extra 2.3 cubic feet from 1.5 square feet less of fabric isn’t that much of a difference. Indeed it would hardly show on the scale. However if we go beyond the total volume and look at where that volume is located, we can see that the X-Mid is actually far more spacious.

As mentioned, a single pole pyramid “wastes” about 5% of its volume at the ends were the canopy is very low. This is why the DuoMid is 10″ longer than the X-Mid, yet actually has a shorter floor length in the inner tent. Basically a portion of a single pole mids volume isn’t “useable” (admittedly a subjective term, but let’s define it as the volume that you or your gear might reasonably occupy). That why it needs to be 10″ longer for a similar length of sleeping area. Conversely the X-Mid avoids the very low slopes of the end walls such that virtually all its volume is useable. Thus the X-Mid really has closer to 10% more “useable volume”.

But even this doesn’t really reflect how the space feels in these shelters because if you actually sit in both of these shelters, you’ll find the X-Mid feels much larger. This occurs because the volume in a single pole mid is heavily biased towards the lower half. It does have a lot of volume but it is mostly close to the ground so headroom is sparse (you can only sit up adjacent to the pole).

Looking only at the volume in the upper half (let’s say above 24″), a single pole mid has about 15% of its volume above this point (about 12 cubic feet out of its 78 cubic feet total), whereas the X-Mid has about 30% of its volume above 24″ (24 of 81 cubic feet). Thus the X-Mid volume in the upper half (aka “headroom”) is roughly double that of a single pole mid and which is why it feels and is much more spacious. You can sit up anywhere inside. So to sum it up, the X-Mid has slightly more total volume, substantially more “useable volume” and way more “headroom” – all from less fabric.

Durston X-Mid showing ample volume in the upper half.

Next lets discuss how this compares to a dome tent and a wider range of popular designs for trekking pole shelters. The elephant in the room here are the tents with a hexagon base since these are quite popular and seemingly closer to a dome than the X-Mid. We can assess this by circling back to my earlier statement that a dome is the most efficient shape. If you were to build a dome tent with the same 81 cubic foot volume as the X-Mid, you could do it with 72 square feet of fabric (rather than 83.75). So that’s a theoretical savings of 11.75 square feet or 1.3 square yards of fabric. Lightweight materials today are 0.5 – 1.4oz per square yard, so the possible savings are 0.65 – 1.8oz for a perfect dome. Thus any design that aims to be more efficient than the X-Mid needs to realize some of those savings, without adding more weight than it saves in additional seams, stakes and struts. This task is not easy.

The closest you might reasonably come to a trekking pole supported dome is an 8 sided shelter supported by 4 trekking poles. The math here is hard but this would be roughly halfway between the X-Mid and a sphere in terms of fabric requirements. This would save 0.3 – 0.8oz of fabric but also necessitate 4 more stakes – more than offsetting the fabric weight saved (plus there is further weight in the additional seams). Thus while the ratio of volume:surface area would improve, the volume:total shelter weight ratio would be worse. The same thing is true for a hexagonal shelters. They only marginally improve the volume:surface area ratio with a theoretical fabric weight savings of 0.2 – 0.5oz which will never translate into a net weight savings because any stakes is at least 0.3oz so two of them will more than offset the fabric saved (and again, there are also additional seams). Thus any two pole hexagonal tent is going to be more complex and heavier for the same volume than a comparably sized rectangular based shelter.

On the flip side, a possible 3 sided shelter would require a lot more fabric than a four sided one (scroll up to that graph earlier and notice how much worse a three sided shelter is than anything else). Despite a lot more fabric, a three sided shape would only save the weight of one stake, so four sides is indisputably more efficient. Thus overall four or five sided designs are the most efficient. Whether 4 or 5 is better is hard to say as they end up almost identical.

Lastly, you may wonder about the efficiency of the myriad of other trekking pole shelter designs out there. In short, anything with a single pole that is off center (aka asymmetrical such as the SMD Lunar Solo) is always going to be less volumetrically efficient than the same thing with the pole positioned centrally (just from the basics of geometry). This is only done to get the pole out of the sleeping area. That’s why you’ll often see this type of shelter often adding various struts to increase volume or improve the distribution of that volume (e.g. supplement headroom). But these struts will almost always cause a further decline in the overall weight efficiency of the shelter because the additional volume you get relative to the weight of a strut is almost unavoidably poor. You can add enough struts to achieve a reasonably sized living space but it would be much more weight efficient to start with a more efficient geometry rather than tacking weight onto a less efficient one.

General Principles
The most weight efficient trekking pole tent will:
– 
Use a four or five sided shape
– 2+ poles
– Position the poles a moderate distance apart to approximate a dome shape
– Avoid struts
– Avoid asymmetry

Take home message
The worst case scenario for weight efficiency is to start with a shape with many sides (e.g. hexagon or octagon), add a single pole in an offset position and then add struts to that.

Conversely, the best case scenario is a two pole shelter based around a 4-5 sided shape and with the poles located at a moderate distance apart. I’m not aware of anything more volumetrically efficient than the X-Mid but some other two pole designs do come close like the Black Diamond Beta Light (below). Thus if you find a tent that is lighter than the X-Mid, it’s almost certainly not because of the geometry but rather because it is smaller, using lighter materials or less fully featured.

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Black Diamond Beta Light
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6 thoughts on “The Volumetric Efficiency of Trekking Pole Shelters

  1. Very helpful, Dan. And my eyelids didn’t get heavy at all. How would the Tarptent Stratospire Li compare? And do their end struts make a difference?
    Thanks.

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    1. Hi Dennis,

      It’s difficult to explain the end struts effect on volumetric efficiency without some diagrams but I’ll take a shot. Also note that those “PitchLoc” corners affect more than just the volume. They also have both good and bad effects on wind and snow loading.

      As for volume, if you were to take a normal corner of a trekking pole tent (e.g. any corner on a DuoMid) and then add a PitchLoc corner to that then certainly you’d get more volume at that corner but you would also add more weight. So more volume and more weight. Alternatively, you could instead achieve the same increase in volume by increasing the size of the tent (e.g. making the base larger). Actually I do need a diagram, so here is one that shows a regular corner, same corner plus a PitchLoc, or extending the fly further to achieve the same volume as the PitchLoc:

      Notice how the length of the green “PitchLoc” line is only a little bit shorter than the red “extended fly” line because the PitchLoc fabric still wraps around the corner and extends to the ground. So despite bringing in the edge of the fly perhaps a foot, the PitchLoc corner only saves a small amount of fabric versus the “extended fly” option to provide the same volume. Note that I am talking about “useable volume” here because the extended fly version adds more “total volume” than the PitchLoc corner but the additional volume on top of the PitchLoc is “wasted” since the roof is so low.

      Some companies make strut corners with just one strut (e.g. Hilleberg, Nemo) but TarpTent uses two struts because there are some stability advantages. But of course two struts will be heavier than one. I haven’t crunched the math on this, but even with a single light carbon fiber strut (perhaps 0.3oz) that strut will almost always weigh more than the amount of material that it saves. The amount of material saved does vary a lot though depending on the panel slopes, if you have a relatively flat panel like the Nemo Rocket then an extended fly version would need to be massive so the strut does save a lot of material there.
      https://www.rei.com/product/144622/nemo-rocket-2-tent

      But in any of the TarpTent designs the material saved is relatively small. So with a single strut corner I suspect the strut is heavier than the volume it saves in most cases, but there are circumstances where it likely does translate into a lighter tent. With a dual strut corner like TarpTent uses and given the relatively modest amount of fabric it saves, I’m quite certain that these two strut PitchLoc corners are heavier than the fabric they save in all of TT’s designs. This would certainly be true for the Stratospire Li because the DCF fabric is so light that the PitchLoc is barely saving any fabric weight. I estimate those PitchLoc corners save less than half a yard of DCF each (e.g. 0.25oz of material) and probably more like 1/4 yard (0.1oz) yet the dual struts likely weigh about an ounce for a set. So I haven’t run the math but I’m quite sure there is measurable net loss in volumetric efficiency. That’s a big part of why the StratoSpire Li is 26oz, whereas as a hypothetical X-Mid 2P would be several ounces lighter in DCF.

      Overall, I don’t think TarpTent uses these corners to aid volumetric efficiency because there is little to gain and more likely a loss. Rather the main reason that TT does it (I suspect) is to decrease the footprint of the tent. A PitchLoc corner does trim area off the footprint of the tent, so it’ll pitch in smaller sites than an “extended fly” version. So you can have a high volume two trekking pole tent without a huge footprint. That is a real advantage I’ll readily admit, although whether it’s worth the added weight/complexity/cost depends on whether fitting into smaller sites is frequently an issue where someone hikes. An alternative approach is to figure out a design that simply has steeper walls (e.g. raise the poles taller) so you aren’t faced with the dilemma of either an awkwardly large footprint or needing struts to have enough volume around the edges. But there are a variety of reasonable ways to end up with a good shelter and the SS Li certainly is a good shelter. I’m just splitting ounces here.

      As for why TarpTent uses their dual strut (“PitchLoc”) design rather than one strut like everyone else, that appears to be for corner “stability”. With two struts, the corner is supported in two directions by solid rods (rather than stretchy material) so if you have severe weather conditions and snow loading then it should be a bit more robust (e.g. handling snow loads better). But again, a steeper wall may avoid this dilemma too since it would shed snow better so it would be less likely to have the sort of snow loads that are a problem. So again there are a variety of ways to do and each have their own advantages. Which is best really depends on what characteristics you’re after in the end product.

      One last thing I’ll mention that is commonly a major problem with TT’s PitchLoc corners but is not fundamental to the design (e.g. it can easily be fixed) is that they are typically set up in such a way that it multiplies the stresses on the stakes so they are much more likely to rip out. I’ve written a post on that and how it can be avoided here:
      https://www.massdrop.com/buy/massdrop-x-dan-durston-x-mid-1p-tent/talk/2290630

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  2. Thanks, Dan. Is there going to be a mid-term? Looking forward to delivery of my X-Mid. As we’ve discussed, I’m going to have to improvise something as to poles, as I don’t otherwise use trekking poles and the lightest ones I’ve seen so far pretty much negate the weight savings of the X-Mid. Ideas, anyone?

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  3. Anyone who puts that much thought into volumetric efficiency is probably going to design a pretty impressive tent! Thanks for the read.

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  4. I only use one trekking pole, have no intentions of ever use two, so I’m stuck with single pole shelters. I agree that their footprint to usable space ratio is kind of low. A major issue is rubbing up against the wet tent walls. I find it very difficult to avoid it in my lunar solo. Even in my duomid, I find it difficult to totally avoid. I need to be sitting as close as possible to the pole. If I lean over to grab something at the end of my tent I can easily rub up against the wall. Using 2 poles in an inverted V shape would be better because I could sit up directly under the peak, maybe I should trying finding a stick to achieve this.
    I find using a really big mid to be a decent option for extended periods of wet weather, especially with two people. I have shared a 10×10 oware mid with a friend, the tent is only 2 lbs without center pole and you can literally stand up in it. 4 people can sleep in it comfortably without any wall rubbing. But you need a large clear area to set it up.

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    1. What about carrying a second folding pole just for shelter use, so you can use that along with your trekking pole in a two pole design? You can get a folding pole to use in a shelter for ~2oz from places like Ruta Locura. If that provides a lot more living space and avoids low angled walls and large footprint tents, then it could be weight well spent.

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