The Arc Bag Concept: Saving Weight with Variable Girth Sleeping Bags Having an "Arc" Shaped Cross Section

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by Ryan Jordan | 2001-04-29 03:00:00-06

Editor's Introduction

As we prepare to release BackpackingLight.com, it is probably appropriate to introduce a teaser about what we intend to spend a great deal of time with this year: clothing and sleep systems for ultralight hiking.

This editorial provides some insight into the concept of the ARC BAG: a variable girth sleeping bag having an arc, rather than an elliptical (or oval), cross section, and are going to ride what we think will be a leading edge in sleeping bag design in coming years. Enjoy, and get ready to 'lose some weight'!

Overview

Conventional sleeping bags are manufactured with an elliptical (or oval) cross section (at least, that is the approximate cross section of a bag occupied by a user).

It has long been known that the insulation beneath you is compressed while sleeping, and thus, may not be as effective in maintaining warmth in the bag while sleeping.

Both Rab Carrington (UK) and Macpac (New Zealand) have recognized this and are manufacturing so-called "top" bags, i.e., sleeping bags that only have insulation along the top and sides, and simpy a fabric sleeping pad sleeve (in the case of MacPac) or a single layer of fabric (in the case of Rab) for the bottom.

However, such designs, and in fact, most sleeping bag designs, suffer from a fundamental flaw that has been recognized and promoted by Ray Jardine beginning in the late 1990s: girth cannot be controlled, and insulating efficiency is lost.

Enter the Quilt

And so, we have the 'sleeping quilt'. The quilt is just that - a sheet of insulated material, not unlike your bed comforter at home, that you simply lay over you. On cold nights, you grab the edges, and tuck it under you. Guess what you are doing when you do this? That's right - changing the interior girth of the bag to increase its insulating efficiency.

Do-it-yourselfers are realizing the benefits of quilt designs, creating "variable girth sleeping bags" with sewn footboxes and open bottoms, where the sides can then be tucked beneath you on colder nights.

Advantages of Variable Girth Sleeping Bags

The primary advantage of variable girth bags is that you can adjust your insulating clothing underneath and extend the bag to a wide range of conditions and seasons. This is economical (you may only need one bag), you can layer two ultralight variable girth bags together to extend temperature ranges (something that is far more difficult to do with conventional sleeping bags), and the resulting bags are lighter (lack of bottom insulation and hoods save weight and give you more flexibility for integrating your sleep system with other components, such as hooded jackets, hats, sleeping pads, etc.).

The Arc Cross Section

The ability to escape the confines of the "ellipse" and enjoy the freedom of the "arc" cross section gives ultralight hikers and climbers tremendous versatility, as outlined above. Arc-cross-section bags will evolve as a category, although manufacturers will likely drag customers kicking and screaming for a few years, because they will face the emotional barriers that consumers hide behind, the most common of which is the psychological security of being wrapped up, head to toe, in a draft-free down cocoon.

Arc designs are available on the Internet for the do-it-yourself crowd. One of the best has been published by Hungry Howie: http://www.newsushi.net/quilt.html. Howie is an experienced thru-hiker of the Appalachian Trail, and to our knowledge, is the first to put the variable girth, sewn-footbox arc design into action (November 2000). He will be using the quilt this year in adventures that will include a thru-hike of the Long Trail. We're eagerly awaiting his report!

Don Johnston, of the eGroup BackpackingLight (http://www.egroups.com/group/backpackinglight), has additional thoughts on the variable girth arc design. An experienced user of the Rab Top Bag (which we are reviewing - stay tuned this summer as a review is released), Don has issues with it, most of which are largely related to its lack of variable girth functionality. In an email to the BackpackingLight eGroup on 4/25/03, Don describes the elements of what may be an ideal sleeping bag design:

"These are the specs I developed for an alternative to the RAB top bag that should work better for me: Picture in your mind a sleeping bag with a full length zipper that is open and the open side is facing down so the bag is a quilt over me. Start with a sleeping bag like the PHD Minimus or Nunatak Alpinist. If using the Alpinist request it be constructed with a 2" baffle height and 4" baffle width. Open up this zipper less bag soon to be quilt except for a 10 to 12 inch foot pocket. The quilt is open for the remainder of it's length. No Zipper, Mesh, or other closure. The width of the quilt tapers from the foot box to 45" wide at the hip which is 39" from the foot end of the Quilt. The width remains 45" wide for the remainder of the quilt. No hood. The head end of the quilt has a velcro tab to close the top of the bag and the top of the bag is encircled with a draw cord and toggle that is adjustable on the top side. This allows closing the bag off at the neck. Microlite shell fabric. 8 oz of 800 fill down or more up to a total bag weight of 17 oz. The bag can be held down to a pad by adding a string to each side of the bag at hip height. The string is simply tied under the ground pad. An alternative is grosgrain ribbon and velcro."

In essence, Don has taken the best features of top bags (insulation on sides and bottoms), the variable-girth flexibility of quilts, and engaged an arc design with bottom cords/straps. We think this may approach an ideal design for lightweight hikers, and are looking forward to the fruits of his efforts. Last we heard, Don was soliciting a few manufacturers to make this custom design for him. We hope the design becomes commercially available. I know my pocketbook is ready.

Perhaps Western Mountaineering, manufacturer of cadillac down bags for backcountry enthusiasts, will adopt the concept. Could we see an "Arc UltraLite", "Arc Dakota", or "Arc Puma" in the near future? (Answer: not soon enough).

Using the Arc Design with a Sleeping Pad

The Jardine concept of an arc design employs the edges of the bag mating with the edges of a closed cell foam pad to regulate the ground seal. The problem with this design is that forcing the edges of the bag to mate with the edges of a sleeping pad eliminate the variable girth flexibility of the system, and you are left with a traditional elliptical cross section that is unable to accomodate various layers, and thus, the system loses insulating efficiency for the underdressed sleeper on a cooler night (more volume of dead air in the sleeping bag to heat).

Others (including Johnston) have proposed using the sleeping pad inside the bag, with the edges of the quilt tucked underneath. Johnston proposes the use of straps or strings to secure the edges of the bag underneath the pad. The main advantage of this technique is the ability to fully seal the sides of the bag where it reaches the ground, minimizing drafts. The main limitation of this technique is that the pad forms a semi-rigid bottom that, unless as narrow as the body, can cause compression of the down near the sides, and cold spots. For pads that are greater than body width, the ability to create an efficient arc shaped cross section around the user is impaired by the semi-rigid pad, and more volume than necessary is created for the user to heat up.

A final alternative is to secure the bag edges (using Johnston's proposal of straps or strings) above the pad, so that the bag's girth can be more precisely controlled around the user and the excess interior volume is minimized, making for a very efficient insulating system. The main disadvantage of this technique is that when the sleeper rolls, if s/he rolls with the sleeping bag, the bag's open bottom gaps will be exposed to drafts.

Summary

The following table summarizes the features of arc vs. elliptical cross sections for sleeping bags.

Table 1. Comparison of Arc (Variable Girth) vs. Elliptical (Fixed Girth) Sleeping Bags

Cross Section Arc Ellipse
Girth Variable Fixed
Hood No Sometimes
Bottom Insulation No Sometimes
Weight Lighter Heavier
Commercial Availability Limited Widespread
Do-it-Yourself Ability Easier More difficult
Overbag Layering Layers well as an overbag Must be oversized to layer as an overbag
Clothing Layering Layers well with winter insulating clothing Must be oversized to layer with winter clothing
Warmth Draft protection is compromised by lack of hood and bottom insulation Drafts can be sealed better with a hood and fixed fabric elliptical (closed) shell

Credits

Many thanks to Hungry Howie, Don Johnston, and Ray Jardine for their efforts in advancing the concept of arc-cross-section sleeping bags and variable-girth quilts.

Updates

August 2001 - Don Johnston has successfully convinced a manufacturer, Nunatak Gear, to bring his design to market! Don's bag weighs around 18 ounces, and is reported to have 2.5 or so inches of single layer loft (800 fill down) and uses the 0.85 oz ultralight nylon similar to that used in the Western Mountaineering ExtremeLite series bags. The design, presumably, was a derivative of Nunatak's "Alpinist" bag, and they're calling the new design the "Arc Alpinist". Hey, we like the name, what can we say?!

November 2001 - Well, I bit the bullet too and had Nunatak customize an arc-section variable girth down bag for me. Microlight shell, 0.85 oz lining, with a few unique specifications that I asked for: differential cut (for which I specified the exact differential), variable down fill across its length (more in the foot area, less in the chest area, so the bag is ideally combined with an insulating jacket), and a slightly wider "expedition" cut so it can be used as an overbag over a smaller down bag. Overall, I'm very happy with it and expect that it will take me down into the single digits easily. I've written a complete review for BackpackGearTest. Read it here.

July 2002 - Don provides a comprehensive description and review of his arc-cross section variable girth sleeping bag. Read it here..


Citation

"The Arc Bag Concept: Saving Weight with Variable Girth Sleeping Bags Having an "Arc" Shaped Cross Section," by Ryan Jordan. BackpackingLight.com (ISSN 1537-0364).
http://backpackinglight.com/cgi-bin/backpackinglight/00017.html, 2001-04-29 03:00:00-06.

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