Volume: Adding a New Dimension
Areas marked with a red “X” indicate where the occupant’s body extends past the spatial boundary of the tent body.
Can you really call it a two-person tent if it’s not usable by 2 people? The rush to make lighter and lighter tents has led manufacturers to make tents that are so small they are often not livable or functional for their stated capacity.
Weight is important but so are livability and function.
The users should ask themselves what functional features they are willing to give up in order to reach their target weight. What is the balance between having a light tent versus a livable or functional tent? Is the vestibule large enough for your pack, boots and gear? Is the vestibule large enough to cook in? Does the tent have a dry entry? Is the stated square footage usable or are the sidewalls and tent corners so steep that the measurements are, for all practical purposes, useless?
In the era prior to the Internet, a tent buyer would shop for a tent by first looking at catalogs then visiting a store and getting inside the tent before picking the “right” one. These days the average tent buyer (of technical, ultralight or lightweight tents) spends 4 to 6 hours researching their options online prior to making a purchase. The trouble with relying on current online tent metrics is that the livability and functionality of a tent are not known until you get inside the tent and see for yourself.
To help the tent buyer, SlingFin has added several new specification metrics to its current list of tent specifications: Tent Body Volume, Vestibule Volume and Weight. We encourage the outdoor specialty tent manufacturers to include tent volume specifications to their list of tent specs. Some companies like Nemo and Sierra Designs have already begun similar efforts and we really hope the momentum continues leading to an industry-wide consensus. We also want to encourage gear testers to include tent and vestibule volumes with their reviews.
Calculate the tent body and vestibule volumes.
With the improvements in technology and 3D imagery, it is now much easier to create 3D models, calculate volumes of complex shapes and convey spatial images to customers virtually.
Tent and flysheet measurements can be manually entered into a 3D solid modeling CAD program or, if you have access to a 3D scanner, then by all means scan away. When the tent and flysheet are modeled, the 3D CAD program can calculate the volume of the tent body and/or vestibule(s). You can also calculate the area or volume at any height or cross section of the tent/vestibule.
We have 3D modeled our two TreeLine tent style (2Lite and CrossBow 2) as well as a few popular models from other manufacturers.
Calculating the weight-volume index
Once we have the weight and volume measurements of the tent body and vestibule(s) we can calculate the weight to volume ratio, giving us what we call the weight-volume index, which is the total minimum tent weight divided by the total volume (tent body and vestibules). This measurement, in ounces per cubic foot, will allow users to compare tent models based on how much living and storage space the tent offers for the weight. This number is a valuable piece of information because it describes numerically how efficient the weight is that you are carrying.
For example, a tent with a weight-volume index of 0.61 oz/cu-ft is a better use of weight than a tent with an index of 0.82 oz/cu-ft, even though the latter tent may be lighter overall. In the example pictured and above, both tents have a square footage of approx. 28 sq. ft, but have much different weight and volume measurements. Overall weight is important, but for those who count ounces, understanding that an extra 13 oz may also translate into an additional 34 cubic feet of usable volume, it could easily justify the weight (stated trail weight of 44 oz vs. 31 oz). This means there is an additional 34 cubic feet of volume for yourself, your gear, your partner, dog, etc. Note, that there is some ambiguity in the term “weight“ and that will need to be precisely clarified in any index using “weight“ (see *1 below).
3D imaging software has the ability to “map” the inside of the tent or vestibule with shapes and objects. Human figures (lying down or sitting up), pads, sleeping bags, etc. can be easily added to scale. Using 3D models to visualize, or map, interior spaces is the next step in evaluating tent specs, particularly when the tent is not physically in front of the shopper. Mapping will be especially helpful for tall individuals who are looking for lightweight tents. It will also help communicate how a tent solves, or fails to solve, other factors impacting tent livability.
Every individual situation has its own optimum shelter ranging from a simple tarp (or no shelter at all!) to mountaineering tents that must block spindrift, blunt extreme temperatures and withstand winds that would take many houses apart . We believe it is our job to provide the information a consumer needs to make the purchase that is best for them and their intended use.
When looking for lightweight shelters, buyers need to find that balance between the weight of a tent, its livability and how functional it is. By adding a new dimension into the provided specifications (the tent and vestibule volumes as opposed to just square footage), we hope to help the buyer make the best informed purchasing decision. Better yet, we hope to help pair them with the right gear so that they can best enjoy the great outdoors.
We will leave you with 2 thoughts our friends have shared with us.
“Pick the right tent for extreme conditions because you are only a millimeter away from spending the night outside” (Robert Link , www.Mountain-Link.com) and “The heaviest tent in the world is a lightweight tent that failed” (Phil Scott, designer with 50+ years of outdoor design and production experience).
Enjoy the journey as you discover what works best for you!
Martin Zemitis- Designer
Richard Ying- COO and Mechanical Engineer
Scott Chenoweth- Mechanical Engineer & CAD Specialist
Copyright SlingFin 2018 under Creative Commons License: okay to use with attribution. Please link to this article if quoting or reusing.