Featured photo: one of the first TOM BIHN backpacks, circa 1972. 1000d Cordura back panel.

Many of the first of "modern" outdoor packs (circa 1950s) utilized a metal external frame to help control the shape of the load and support the weight of the pack. The rigidity of the frame also allowed for the stretching a fabric or mesh back panel, and hence the creation of air circulation between the pack and the user's back. On larger packs designed for multi-day trips, the addition of a padded hip belt allowed the user to carry most of the weight of a load on their hips. Carefully packing the bag with the heavy stuff up high, so the center of gravity could more easily shift to be over one's hips, further enhanced distribution of the weight to the user's hips rather than their shoulders. These packs were, and still are, very good for carrying heavy loads on relatively even trails. Off trail or cross-country travel was less fun, as that same high center of gravity that allowed transferring the weight to the hips becomes unwieldily with any sort of athletic jumping or clambering around.

In the early 1970's, manufacturers started introducing alternatives to external frame packs more suited to off-trail travel. Rivendell Mountain Works made the Jenson pack, one of a few frameless packs that enjoyed brief popularity. Chouinard had their Ultima Thule; Yak Works in Seattle had their own version, and Wilderness Experience offered their R.O.R wrap-around pack ("R.O.R" stood for "Rip Off Rivendell"). Rivendell used Corduroy as their back panel material specifically to add comfort to those who wore their packs shirtless or in tank tops in the heat of the summer; Chouinard used cotton canvas and later Cordura®, while Wilderness Experience used 420 Parapack. Tom's early frameless and internal frame packs -- circa 1972 - 1985 -- featured back panels of 1000d Cordura, 420d Parapack, nylon mesh, cotton corduroy and cotton canvas.

A frameless pack inspired by Rivendell, made by the 15-year-old Tom Bihn in the mid-1970s: cotton canvas back panel.

Rivendell Jensen pack from the early 1980s: waleless corduroy.

A TOM BIHN daypack from the mid-1990s: nylon mesh over closed-cell foam.

Other manufacturers introduced internal frame packs, which have since largely replaced external frame packs for most backcountry travel. Some of these had mesh-covered foam back panels that promised a compromise between an external frame pack and the body-hugging back panel of frameless designs. However, with the exception the breezy airflow of an external frame pack, we've mostly come to live with more-or-less sweaty backs.

Our own compromise is Dri-Lex® Aero-Spacer®. Since the late 1990's we've been using this mesh over closed cell foam for most of our backpack back panels. It's used on the back panels of the Synapse 19, Synapse 25, Brain Bag, Smart Alec, and Guide's Pack. Dri-Lex® Aero-Spacer® Mesh is a patented moisture management fabric; it's a 3-D knit construction which creates small chambers to promote air circulation to ventilate and cool one's back. At the same time, it's a polyester and nylon blend: the two fibers absorb moisture in different ways, and together they create a wicking action that helps transport moisture away from the contact area with your back to the outer edges of the back panel, where it can then evaporate. It remains comfortable next to the skin, plus it's gentle on one's clothes, including the fine wool that many of us wear as an alternative to synthetic base layers. You'll find this modern fabric often utilized as a lining fabric in sports and performance footwear, but we like it as a back panel fabric.

That's a bunch of materials-geek speak for something that we've long experienced as avid hikers: Dri-Lex® Aero-Spacer® mesh, in use, provides noticeable cooling and relief from moisture build-up on our arduous or simply hot summer hikes.

Still, that's our experience; and everyone sweats and experiences sensations — like heat and moisture — differently.

In an effort to better understand what we were experiencing, Tom's longtime friend Robert Swarner (sculptor, helicopter pilot, product designer, big wall climber and machinist) came up with a device that'd allow us to scientifically evaluate the effectiveness of Dri-Lex® Aero-Spacer® mesh. Tom and Robert modified a Synapse and equipped it with Robert's mobile heat sensors and data recording device, which we named The Swarner 5000 Datalogger. The Synapse had a back panel split down the center vertically: one half was spacer mesh and the other half was 1050 Ballistic (both sides were padded with closed-cell foam). Over the course of the last year, we've used this special Synapse on day hikes long and short, on hot summer days and cooler winter ones, and usually on hikes with big elevation gains. (Watch The Swarner 5000 Datalogger in action at 00:52 in our Materials Testing video.)

The Swarner 5000 Datalogger

The Dri-Lex® Aero-Spacer® back panel of the Synapse.

The Synapse backpack.

The results? The Dri-Lex® Aero-Spacer® mesh would start off early in the hike about one degree Celsius warmer than the ballistic. Once we started sweating though (which we seemed to do with or without the mesh), the wicking action of the mesh kicked in and it started cooling us. So, for the remainder of our hikes, our backs were ~2 degrees Celsius cooler on the spacer mesh side. And we found, as we always had, that our shirts were less damp on the Dri-Lex® Aero-Spacer® side because it wicked the moisture away. Not a huge measurable difference, but a difference that seems to us to make a difference in field use.