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Mountainsmith TrekLight AT 55 Backpack REVIEW

Cutting edge design and materials, loaded with features, user-friendly, load capable, and fits like wearing a vest - but a tad heavy by lightweight standards, and needs a few tweaks here and there.


by Will Rietveld | 2006-04-04 03:00:00-06

Mountainsmith TrekLight AT 55 Backpack REVIEW


Mountainsmith's popular MountainLight series is gone and is being replaced with a new TrekLight series that so far consists of the AT 55 and PCT 45/CDT 45. The new packs depart from the familiar beige Dimension Polyant fabric, and are frankly “softer” and more user-friendly. The AT 55 essentially replaces the popular Auspex, but the new pack is so different that they can hardly be compared. A closer comparison is with the new 2006 Osprey Aether 60 which we recently reviewed. How does the new TrekLight AT 55 stack up as Mountainsmith's new frontrunner lightweight pack, and how does it compare with Osprey's Aether 60? Read on.

What's Good

  • X frame is flexible and effectively transfers weight
  • Suspension system easily adjusts for a perfect fit
  • Firm, comfortable shoulder harness and hipbelt
  • Top lid converts to a day pack
  • Eight outside pockets provide plenty of convenient storage
  • Water-resistant zippers operate smoothly
  • Zippered map pocket under top lid
  • Zippered access to main compartment
  • Lightweight yet durable fabrics

What's Not So Good

  • Heavy by lightweight standards.
  • Zipper on the main compartment opens from the top (it would be more convenient if it opened from the bottom)
  • Inside PU coating on side stretch-woven pockets resists when sliding in gear or water bottles
  • Straps and buckles on the top pocket make conversion to a day pack cumbersome
  • Hipbelt pockets are too small
  • At 4 lbs 3.3 ounces, the AT 55 is heavier than we would like





2006 TrekLight AT55


Internal frame, top loading, drawstring closure, floating top pocket


Size LT tested: standard volume 3356 ci (55 L), extended volume 5309 ci (87 L)


4 lb 3.3 oz (1.9 kg) measured weight; manufacturer's specification Weight: 4 lbs 2 oz (1.91 kg)


Main body is 210d PU coated Cordura nylon, bottom and reinforcements are 420d Cordura nylon, pockets are 500d nylon Kodra


Two 0.75 in (1.9 cm) T6 6061 corrugated aluminum stays in an X configuration, secured in Hypalon pockets at the ends


Floating top lid with zippered mesh map pocket on the underside, 5-inch extension collar with drawstring closure, internal white PU coating, two side and one top compression straps, zippered access to the main compartment, water-resistant outside zippers, two stretch-woven side pockets, large gusseted stretch-woven front pocket, stretch-woven lower front pocket, two hipbelt pockets (one zippered), two ice axe loops, two side accessory attachments, hydration sleeve with one port, haul loop, load lifters, hipbelt stabilizers, sternum strap

  Volume To Weight Ratio

49.9 ci/oz size L (based on 3356 ci and a measured weight of 67.3 oz)

  Comfortable Load Carrying Capacity

35 lb (16 kg) estimated comfortable load for an average person carrying the pack all day

  Carry Load to Pack Weight Ratio

8.3 (based on 35 lb and a measured weight of 4.21 lb)


$199 US


The rather low, 3356 cubic inch (55 L), volume specification for the new Mountainsmith TrekLight AT 55 is deceiving. In reality it has about the same capacity as the old MountainLight Auspex/Chimera packs and the new 2006 Osprey Aether 60. The weights of these packs are equivalent within an ounce or two, so the real considerations are features, load carrying ability, and comfort. I will start with a full description of the AT 55.

While the previous Mountainsmith Auspex was rather utilitarian with its dominant (stiff and very durable) Dimension Polyant fabric and minimal features, the new TrekLight AT 55 is constructed of a variety of fabrics and is loaded with features. The pack body is 210 denier Cordura nylon with a white PU coating on the inside for better visibility, while the bottom is heavier 420 denier Cordura. Stress points are reinforced with Hypalon. Overall, the AT 55 pack fabrics are an excellent balance of lightweight and durability.

Mountainsmith TrekLight AT 55 Backpack REVIEW - 1
Mountainsmith is replacing their popular MountainLight pack series with a new TrekLight series beginning in 2006. The flagship of the new TrekLight series is the AT 55 (left), which is completely different from its predecessor, the MountainLight Auspex (right).

The outside bungee attachment system on the Auspex is replaced by real pockets, lots of them. The pack has a total of eight outside pockets: two stretch-woven side pockets, two stretch-woven front pockets, two hipbelt pockets, and a top lid with a zippered map pocket on the underside. The stretch-woven pockets have ample bellowing and gusseting so they have good capacity even when the pack is stuffed full. I am thankful for the hipbelt pockets, but they are small and will hold only the smallest items.

Mountainsmith TrekLight AT 55 Backpack REVIEW - 2
Want outside pockets? You got ‘em; the AT 55 is loaded, eight in all. There are two large stretch-woven pockets on the frontpanel (left top and bottom), a top lid with a map pocket on the underside (top right), two stretch-woven side pockets (bottom right), and two hipbelt pockets (next photo).

Mountainsmith TrekLight AT 55 Backpack REVIEW - 3
The hipbelt sports two stretchy pockets (center). The one on the right side (left photo) is zippered and has a white lining for extra visibility. The one on the left side (right photo) is an unzippered sleeve. Both pockets are small and accommodate only smaller items.

A nice feature on the TrekLight AT 55 is zippered outside access to the main compartment. A 14.5-inch water-resistant zipper on the right side of the frontpanel extends to about the middle of the pack. However, the zipper opens from the top, which is under the top lid. It would be more convenient for it to open from the bottom.

Mountainsmith TrekLight AT 55 Backpack REVIEW - 4
A water-resistant zipper on the right side (left) provides direct access to the pack's interior. The AT 55 has 5 compression straps for load volume control (right), four on the sides and one on the top. There are tool loops at the upper side straps. All of the straps are extra long and have Velcro retainers so they can be rolled up.

Another nice feature is a removable top lid that converts to a fanny pack or day pack. The lid quickly detaches from the backpanel with two side-release buckles. With the straps extended, the lid can be worn as a fanny pack or as a shoulder pack. The straps are a little too short for use as a shoulder pack and it's a struggle to get it on and off. As a fanny pack it would work much better if one buckle was reversed where it attaches to the backpanel, which would make opposite buckles mate. As it is, only diagonal buckles mate.

Mountainsmith TrekLight AT 55 Backpack REVIEW - 5
The top lid converts to a day pack, which can be configured as a shoulder pack (left) that is a bit tight to get on and off, or a fanny pack (right) that is angled because only the diagonal buckles mate.

The TrekLight AT 55's frame is very different from the former Auspex. The Auspex had two vertical sleeved stays that could be removed and bent to fit the user. The AT 55 also has two flat stays, but they are arranged in an X pattern, with the ends secured in Hypalon sleeves. This arrangement discourages the user from removing the stays and bending them. I found the shape satisfactory as is, and did not feel the need to adjust it.

The AT 55 is available in regular torso and large torso versions (equivalent to size medium or large). To dial in the fit, the pack torso length is adjusted by raising or lowering the hipbelt on a Velcro pad, in combination with adjusting the shoulder strap length.

I found the AT 55's suspension system to be first class. The backpanel is thermo-molded firm EVA foam with extra padding in the lumbar region. The wide, thick shoulder straps and hipbelt are also firm EVA foam to increase their capability to carry heavy loads. The hipbelt is further reinforced with a thin external polyethylene sheet to increase stiffness and resist distortion.

Mountainsmith TrekLight AT 55 Backpack REVIEW - 6
The AT 55's shoulder straps, backpanel, and hipbelt (left) are molded double or triple density EVA foam, providing excellent weight dispersion and load-carrying ability. The frame (right) consists of two flat aluminum stays in an X configuration, which provides excellent flexibility at top and bottom and weight transfer to the hips.

On the trail, I found the AT 55 to be very competent. It easily carried 25 to 30 pound loads of lightweight gear with comfort. To see what the AT 55 would do, I loaded the pack with increasingly heavy loads up to 45 pounds, and found that its frame and suspension could handle anything that I could stuff in the pack. The limiting factor is how much weight you can carry without your knees buckling. For me (6 feet, 170 pounds), I found the AT 55 comfortable to carry all day with up to 35 pounds. The pack was capable of transferring all of the weight to my hips, and I tended to carry it without the sternum strap attached.


Overall, the TrekLight AT 55 is a well-designed lightweight internal frame backpack capable of carrying a sizeable load with comfort, and is very user-friendly. It quickly became my favorite medium volume internal frame pack. This lightweight pack is loaded with features! I really like the numerous outside pockets that provide plenty of places to organize my gear and make items needed on the trail readily available. Also it readily handles a bear canister; a Bearikade Weekender (9 inches wide x 10.5 inches high) easily slipped in and turned to horizontal at the bottom of the pack.

How does the Mountainsmith TrekLight AT 55 compare with the 2006 Osprey Aether 60? I found the AT 55 to be more user-friendly with more outside storage space, while the Aether 60's stretch-woven pockets were tight and would not hold very much. On the other hand, the Aether 60's peripheral frame rods and load lifters allowed the pack to pull in closer to my back and maintain a closer center of gravity. Bottom line, they are both very capable packs, and each has pros and cons, so I recommend trying both before making a decision.

Unfortunately, the AT 55 breaks the 4 pound limit we normally place on packs this size, as does the new Osprey Aether 60. I expect both packs to be very popular, but speaking on behalf of lightweight backpackers, I would like to see pack upgrades without a significant weight increase. Although I really like the improved suspension and features of these packs, I would like the changes to be weight neutral.

What's Unique

The new TrekLight AT 55 sets a new standard for a medium volume lightweight (not ultralight) internal frame backpack. Its design and materials are cutting edge, it's very user-friendly, load capable, and fits very well. It can easily become a good friend on a long trail, like the AT.

Recommendations for Improvement

Of course, we'd like to see it be lighter weight. Other than that, specific recommendations for improvements are:

  • Reverse the gender on one of the buckles of the top lid where it attaches to the backpanel. This will provide buckle ends that mate for easier use as a fanny pack
  • Reverse the direction on the zipper providing access to the main compartment, so that it opens from the bottom rather than the top
  • Eliminate the PU coating on the side stretch-woven pockets to save a little weight and allow items to slide in easier
  • Increase the size of the hipbelt pockets
  • Reduce weight by half a pound


"Mountainsmith TrekLight AT 55 Backpack REVIEW," by Will Rietveld. (ISSN 1537-0364)., 2006-04-04 03:00:00-06.